Forces and motion 3:
Friction
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Friction
The questions in this set probe pupils’ understanding of friction. Friction is the name
of the interaction between two surfaces when they move (or if an applied force tries to
make them move) relative to one another. It acts along the line of contact. A key
point to grasp about the friction force between two given objects is that its size
depends on the external applied force which causes it. For a given object on a given
surface, the friction force will increase in size to match the applied force that is trying
to make the object slide over the surface – up to a certain limit. If that limit is
exceeded, the object will move. Once it begins moving, the friction force is almost
constant, regardless of its speed.
In fact, the situation is more complicated than this: the applied force needed to start an
object moving is slightly bigger than the force needed to keep it moving. That is, the
starting friction force is greater than the moving friction force. In an introductory
exploration of friction, at Key Stage 3 level, however, it is better to avoid this point, as
even a simplified understanding of friction, as a force whose size matches the applied
force up to a limit, is quite challenging for pupils to grasp. Once they have grasped
this securely, the difference between starting friction and moving friction could be
explored – but this is not likely to be until after Key Stage 3. An understanding of the
difference between starting and moving friction is not required to answer any of these
questions.
The idea of friction is first introduced in the National Curriculum at Key Stage 2,
where pupils should be taught that friction is a force that slows moving objects and
may prevent objects from starting to move (Sc4/2c). This is then developed at Key
Stage 3 into an understanding of how frictional forces affect motion (statement
Sc4/2d). The basic idea of friction is introduced in the QCA Scheme of work for Key
Stages 1&2 in Unit 4E, Friction. It is then developed in the QCA Scheme of work for
Key Stage 3, in Unit 7K, Forces and their effects and Unit 9K, Speeding up.
Questions to probe understanding of friction inevitably bring in ideas about the
relationship of forces and motion – for example, knowing that when an object moves
at a steady speed, the forces on it add to zero (the total force is zero). These ideas are
probed by diagnostic questions in the set The link between force and motion. Several
questions in the set Identifying forces also probe understanding of when a friction
force arises. A selection of questions from these packs could, therefore, be used along
with those in this pack when teaching about friction.
Questions 1-2
These look at when there is, and is not, a friction force on an object. Some pupils may
think friction ‘switches off’ when the object moves. Others may think friction only
depends on the choice of surfaces, and so is present all the time. The situation of a
slope is not used in any of the subsequent questions because an analysis of the forces
acting involves vector ideas and is too difficult for Key Stage 3 (or 4).
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Questions 3-5
These probe understanding of friction as a responsive force, whose size depends on
the applied force, up to a limit – and is independent of speed, once an object is
moving. The two-tier structure of these questions helps to show the reasoning behind
the pupil’s choice of response. Four situations are considered:
• object not moving, with a small applied force;
• object not moving with a larger applied force;
• object moving at steady speed with an even larger applied force;
• object moving at an increasing speed with a still larger applied force.
The question becomes very complex if all four situations are included. So each of
these questions compares two situations. Q3 considers a smaller and larger force,
both of which are insufficient to make an object move. Q4 then compares the
situation where a small force does not cause motion, but a larger one makes the object
move at a steady speed. Q5 then considers two situations in which the object is
moving: at steady speed, and with increasing speed.
These questions are intentionally asked in a qualitative form, to probe understanding
without the distraction of numerical values. They require students also to understand
the relationship between force and motion (in particular, to appreciate that, when an
object moves at a steady speed, the resultant force on it is zero).
Question 6
Although this question looks very similar to Q4, it has one crucial difference. In Q4,
it is stated that the applied force is bigger in the second situation: the furniture
remover pushes harder. In Q6 nothing is said about the sizes of the forces, just about
the motion (or lack of it). In fact, there are two possible correct answers here. It is
possible that the box in Figure 1 is just on the point of beginning to move. If so, then
the friction force is almost at the limit of starting friction – in which case, the friction
force is smaller when it begins to move, and option 1 is the right answer. But it is also
possible that the push in Figure 1 is nowhere near the limit of starting friction, so that
the friction force in Figure 1 is much less than the moving friction force – making
option 2 the right answer. The second part of this question is therefore essential, in
order to probe the reasoning leading to the answer chosen.
Questions 7-10
These questions, in contrast to Q3-6, do use numerical values, though they do not
involve any calculations. Like earlier questions in this set, they require students also
to understand the relationship between force and motion. Q10(a) probes the quite
common misconception that the friction force on an object is directly related to its
weight. All of these, particularly Q7, would be suitable for small-group discussion.
Q8 is the only question in this set that uses a wheeled vehicle (a trolley) – largely to
introduce a little variety. In fact the forces on wheeled vehicles are very complicated
and it would not be wise to go into this in any depth. If you want more questions to
practise these ideas, questions similar to Q3-7 could be written for the situation in
which an object is dragged across a level surface.
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Questions 11-13
Students may have the idea that friction always opposes motion. In fact it opposes
relative motion of one thing past another – and this is sometimes essential to enable an
object to move at all. We could not walk if there were no friction, and bicycles and
cars could not move. Q11 looks at the direction of the friction force in a situation
where friction is necessary for an object (a crate on the back of a lorry) to move. The
situations in Q12-13 are similar. These are quite challenging questions.
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1
(a)
A box is sitting on a level floor. It is not moving.
box not moving
Is there a friction force acting on the box?
Tick ONE box ( )
(b)
yes
no
A furniture remover is pushing the box. But it is not moving.
box not moving
Is there a friction force acting on the box?
Tick ONE box ( )
(c)
yes
no
The furniture remover is now pushing the box harder. It is moving along
at a steady speed.
moving at a steady speed
Is there a friction force acting on the box?
Tick ONE box ( )
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yes
no
137
(d)
The furniture remover pushes even harder. The box moves along,
getting faster as it goes.
moving along, getting faster
Is there a friction force acting on the box?
Tick ONE box ( )
138
yes
no
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2
A box is sitting on a wooden plank. One end of the plank can be raised to
make a slope.
(a) At first, the plank is level. The box is not moving.
box not moving
Is there a friction force on the box?
Tick ONE box ( )
yes
no
(b) One end of the plank is then raised to make a gentle slope. The box is
still not moving.
box not moving
Is there a friction force on the box?
Tick ONE box ( )
(c)
yes
no
The slope is then made steeper. The box slides slowly, at a steady
speed.
moving at a steady speed
Is there a friction force on the box?
Tick ONE box ( )
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yes
no
139
(d) The slope is now made even steeper still. The box slides down, getting
faster and faster as it goes.
slides down, getting faster
Is there a friction force on the box now?
Tick ONE box ( )
140
yes
no
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3
box not
moving
A furniture remover is pushing on a box.
It is not moving.
Figure 1
still not
moving
He pushes harder.
But the box still does not move.
Figure 2
(a)
How does the size of the friction force in these two situations compare?
Tick ONE box ( )
The friction force on the box is bigger in Figure 1.
The friction force on the box is bigger in Figure 2.
The friction force on the box is the same size in both figures.
(b) How would you explain your answer?
Tick ONE box ( )
The total force on the box is zero each time.
It is the same box and the same surface.
When the pushing force is small, the box grips the surface better.
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4
box not
moving
A furniture remover is pushing on a box.
It is not moving.
Figure 1
moving at a
steady speed
He pushes harder.
Now the box is moving along
at a steady speed.
Figure 2
(a)
How does the size of the friction force in these two situations compare?
Tick ONE box ( )
The friction force on the box is bigger in Figure 1.
The friction force on the box is bigger in Figure 2.
The friction force on the box is the same size in both figures.
(b) How would you explain your answer?
Tick ONE box ( )
The total force on the box is zero each time.
It is the same box and the same surface.
In this situation, the friction force is big enough to stop the box moving.
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5
moving at
a steady
speed
Mai Lin is pushing a box.
The box is moving along
at a steady speed.
Figure 1
getting
faster
She pushes harder.
The box moves along,
getting faster as it goes.
Figure 2
(a)
How does the size of the friction force in these two situations compare?
Tick ONE box ( )
The friction force on the box is bigger in Figure 1.
The friction force on the box is bigger in Figure 2.
The friction force on the box is the same size in both figures.
(b) How would you explain your answer?
Tick ONE box ( )
The total force on the box is zero each time.
When the box is moving, the friction force on it does not depend on the
speed.
As the box moves faster, it grips the surface less and so the friction force
on it is smaller.
As the box moves faster, it rubs harder on the surface so the friction force
on it is bigger.
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6
box not
moving
Mai Lin is pushing on a box.
It is not moving.
Figure 1
moving at a
steady speed
Whitney pushes the same box.
It moves along at a steady speed.
Figure 2
(a)
How does the size of the friction force in these two situations compare?
Tick ONE box ( )
The friction force on the box is bigger in Figure 1.
The friction force on the box is bigger in Figure 2.
The friction force on the box is the same size in both figures.
It is not possible to say from this information.
(b) Explain your answer:
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7
Mohammed tries to push a box across a level floor.
(a) He pushes with a force of 50N. The box does not move. What is the size
of the friction force acting on the box?
The box does
not move.
Tick ONE box ( )
more than 50N
50N
50N
less than 50N
(b) Mohammed pushes harder, with a force of 100N. The box still does not
move. What size is the friction force acting on the box now?
Tick ONE box ( )
more than 100N
The box does
not move
100N
between 50N and 100N
100N
50N
less than 50N
(c) After taking a rest, he pushes harder still, with a force of 150N. Now the box
moves slowly, at a steady speed. What size is the friction force acting on
the box now?
Tick ONE box ( )
moves at a steady speed
more than 150N
150N
150N
just a little bit less than 150N
quite a lot less than 150N
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?
(d) After another rest, Mohammed pushes the
box even harder still, with a force of 200N.
How does the box move?
200N
Tick ONE box ( )
The box moves at a steady speed,
faster than in part (c).
The box accelerates, getting faster
and faster.
(e) When Mohammed pushes with a force of
200N, what size is the friction force acting
on the box now?
Tick ONE box ( )
200N
more than 200N
200N
just a little bit less than 200N
150N
less than 150N
(f) Mohammed stops pushing and the box sits on the level floor. What size is
the friction force acting on the box now?
Tick ONE box ( )
200N
150N
100N
50N
zero
another value (please write here): _____________________
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8
A shopper pushes a trolley across a level floor.
She pushes with a force of 40N. The trolley moves slowly, at a steady speed.
moves at a steady speed
(a) What is the size of the friction force on the trolley?
Tick ONE box ( )
more than 40N
exactly equal to 40N
a little bit less than 40N
a lot less than 40N
(b) She now pushes harder, exerting a force of 60N on the trolley. What will
happen?
Tick ONE box ( )
The trolley will continue to move across the floor at the same speed as
before.
The trolley will move across the floor at a faster steady speed.
The speed of the trolley will keep increasing, as long as she keeps
pushing with 60N.
The speed of the trolley will increase at first, then it will settle down at
a constant speed which is bigger than before.
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9
Mai Lin pushes a box across a level floor. The box moves slowly, at a steady
speed.
box moves at a
steady speed
(a)
While the box is moving at a steady speed, the horizontal force exerted
by the Mai Lin on the box is:
Tick ONE box ( )
equal to the weight of the box
greater than the weight of the box
exactly equal to the friction force on the box
a tiny bit bigger than the friction force on the box
a lot bigger than the friction force on the box
(b)
Mai Lin then doubles the horizontal force that she exerts on the box.
How does the box move now?
Tick ONE box ( )
With a constant speed that is double the speed in part (a).
With a constant speed that is bigger than the speed in part (a),
but not necessarily double.
With a continuously increasing speed.
With an increasing speed at first, then settling down at a higher
constant speed.
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10
Tom pushes a sled across some level ice. The sled moves slowly, at a steady
speed.
sled moves at a
steady speed
ice
(a)
While the sled is moving at a steady speed, the horizontal force exerted
by Tom on the sled is:
Tick ONE box ( )
equal to the weight of the sled
greater than the weight of the sled
exactly equal to the friction force on the sled
a tiny bit bigger than the friction force on the sled
a lot bigger than the friction force on the sled
(b)
Tom then doubles the horizontal force that he exerts on the sled. How
does the sled move now?
Tick ONE box ( )
With a constant speed that is double the speed in part (a).
With a constant speed that is bigger than the speed in part (a),
but not necessarily double.
With a continuously increasing speed.
With an increasing speed at first, then settling down at a higher
constant speed.
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(c)
Tom suddenly stops applying a horizontal force to the sled. How does
the sled move now?
Tick ONE box ( )
It immediately comes to a sudden stop.
It keeps on moving at a steady speed for a while and then slows
to a stop.
Its speed immediately starts to get less, and keeps on slowing
until it stops.
It continues at a constant speed.
Its speed keeps increasing for a while and then it gradually slows
down until it stops.
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11
A truck is travelling along a level road.
It is carrying a large crate.
The crate does not slide around on the truck because of the friction between
the bottom of the crate and the floor of the truck.
(a)
The truck is starting up from rest and is gaining speed.
Which of the following best describes the friction force acting on the crate?
Tick ONE box ( )
There is a friction force on the crate in the forward direction.
There is a friction force on the crate in the backward direction.
There is no friction force on the crate.
(b)
The truck is travelling at a steady speed.
Which of the following best describes the friction force acting on the crate?
Tick ONE box ( )
There is a friction force on the crate in the forward direction.
There is a friction force on the crate in the backward direction.
There is no friction force on the crate.
(c)
The truck is slowing down.
Which of the following best describes the friction force acting on the crate?
Tick ONE box ( )
There is a friction force on the crate in the forward direction.
There is a friction force on the crate in the backward direction.
There is no friction force on the crate.
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12
A large wooden block is sitting on a table, with a second smaller wooden block
on top of it.
blocks pulled along by a string
The two blocks are pulled across the table, by a string attached to the bottom
block. The top block does not slide on the bottom block. Both move together.
(a)
Which of the diagrams below best shows the friction force acting on the
top block, while the speed of the blocks is increasing?
(Circle one letter A, B or C to indicate your answer.)
no friction force
A
(b)
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B
C
Which of the diagrams below best shows the friction forces acting on the
bottom block, while the speed of the blocks is increasing?
(Circle one letter D, E, F, G, H, I to indicate your answer.)
D
E
F
G
H
I
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(c)
Which of the diagrams below best shows the friction forces acting on
the top block, while the blocks are moving at a steady speed?
(Circle one letter A, B or C to indicate your answer.)
no friction force
A
(d)
B
C
Which of the diagrams below best shows the friction forces acting on the
bottom block, while the blocks are moving at a steady speed?
(Circle one letter D, E, F, G, H, I to indicate your answer.)
D
E
F
G
H
I
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13
A large wooden block is sitting on a table, with a second smaller wooden block
on top of it.
blocks pulled along by a string
The two blocks are pulled across the table, by a string attached to the top
block. The top block does not slide on the bottom block. Both move together.
(a)
Which of the diagrams below best shows the friction force acting on the
top block, while the speed of the blocks is increasing?
(Circle one letter A, B or C to indicate your answer.)
no friction force
A
(b)
154
B
C
Which of the diagrams below best shows the friction forces acting on the
bottom block, while the speed of the blocks is increasing?
(Circle one letter D, E, F, G, H, I to indicate your answer.)
D
E
F
G
H
I
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(c)
Which of the diagrams below best shows the friction forces acting on
the top block, while the blocks are moving at a steady speed?
(Circle one letter A, B or C to indicate your answer.)
no friction force
A
(d)
B
C
Which of the diagrams below best shows the friction forces acting on the
bottom block, while the blocks are moving at a steady speed?
(Circle one letter D, E, F, G, H, I to indicate your answer.)
D
E
F
G
H
I
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Answers and discussion
Friction
In the discussion below, answer options are referred to by number (1, 2, 3 …), in the
order in which they appear in the question.
1
(a) no
(b) yes
(c) yes
(d) yes
Some students may think that there is only a friction force when the box is being
pushed but is not moving (‘yes’ to (b) only).
2
(a) no
(b) yes
(c) yes
(d) yes
This is similar to Q1. The reasoning involved is more difficult, though, as this is
a two-dimensional situation. When the box is sitting on a sloping surface, a
‘component’ of the gravity force on the box acts down the slope. This tends to
make the box slip – and friction forces then arise from the interaction between the
box and the surface of the slope.
3
(a) 2
(b) 1
Here the reasoning really goes from (b) to (a). In both cases, the box is not
moving, so the total force acting on it is zero. Friction matches the force exerted
by the workman.
Answer patterns 3/2 and 1/3 indicate other (incorrect) ways of thinking about this
situation, which might be worth following up.
4
(a)
(b)
2
1
Again, the reasoning goes from (b) to (a) – the total force is zero in both cases.
This requires an understanding of Newton’s First Law of motion.
Answer patterns 3/2 and 1/3 indicate other (incorrect) ways of thinking about this
situation, which might be worth following up.
5
(a)
(b)
6
For a student who has not been taught that there is a difference between starting
friction and moving friction (see Notes at the start of this question pack), the
correct answer is:
(a) 2
(b) The force exerted by Whitney in Figure 2 is bigger. In both cases, the
resultant (or total) force on the box is zero. So the friction force must also
be bigger in Figure 2.
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3
2
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However, if a student knows that the maximum value of starting friction is bigger
than moving friction, then the best answer is:
(a) 4
(b) In both cases, the friction force is equal to the force applied by the person
pushing. But it is not possible to say for certain whether this applied force is
bigger in Figure 1 or Figure 2. It is probable that Mai Lin is applying a
smaller force in Figure 1. But if the box in Figure 1 is just on the point of
moving, then Mai Lin might be applying a larger force in Figure 1 – because
the starting friction force will be at (or close to) its maximum value, which is
bigger than the value of the moving friction force.
7
(a) 2 (50N)
(b) 2 (100N)
(c) 2 (150N)
(d) 2
(e) 4 (150N)
(f) 5 (zero)
This question is probing understanding that the friction force matches the applied
force – up to a limit. If the applied force is bigger than this, the friction force
stays at its maximum value.
8
(a) 2
(b) 3
Part (b) also tests understanding of Newton’s first and second laws of motion.
9
(a) 3
(b) 3
Part (b) also tests understanding of Newton’s first and second laws of motion.
10 (a) 3
(b) 3
(c) 3
Parts (a) and (b) of this question are very similar to Q9. Parts (b) and (c) also test
understanding of Newton’s first and second laws of motion.
11 (a) 1
(b) 3
(c) 2
The key here is to think about the motion of the crate. In (a), the crate is
accelerating to the right. The only horizontal force on the crate is the force
exerted by the floor of the truck (friction). So this must be towards the right. In
(b), the crate is moving at a steady speed, so the resultant force on it is zero. In
(c), the crate is getting slower. So the resultant force on it is in the direction
opposite to its motion. Again the only horizontal force is the friction force, so
this must act towards the left.
If, however, air resistance were to be considered (and some pupils may feel that it
should be), there would also be a drag force acting on the crate in the backward
direction. For (a) and (c), the direction of the friction force on the crate would
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not be changed, but for (b) there would need to be a friction force acting on the
crate in the forwards direction to counter the drag force due to the air resistance.
12 (a) B
(b) D
As in Q11, start from the motion of the top block. It is accelerating to the right.
The only horizontal force on the top block is the friction force from its interaction
with the lower block. So this must be towards the right.
The other force of this interaction pair acts on the lower block – and must be on
the opposite direction, towards the left. There is also a friction interaction
between the lower block and the table. The friction force on the lower block is
towards the left.
(c) C
(d) H
When the blocks are moving at a steady speed, the resultant force on the top
block is zero. The friction forces between the lower block and the table are the
same as before, though.
13 (a) A
(b) F
The applied force tends to make the top block move to the right over the lower
block. So the friction force on the top block is towards the left.
The other force of this interaction pair acts on the lower block – towards the right.
There is also a friction interaction between the lower block and the table. The
friction force on the lower block is towards the left.
(c) A
(d) F
When the blocks are moving at a steady speed, the resultant force on the top
block is zero. So the friction force on the top block must be equal to the force
exerted by the string. The bottom block is also moving at a steady speed, so the
resultant force acting on it is also zero. The friction force exerted by the top
block on the bottom one is equal in size and opposite in direction to that on the
top block (they are an interaction pair). This is then balanced by the friction force
exerted by the table on the lower block – so that the resultant is zero.
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Acknowledgements
Some of the questions in this pack are based on probes used by science education
researchers to explore learners’ understanding of key ideas about forces and motion.
These have been modified, often quite substantially, to improve their clarity or to
make them easier to use in class. Others are new questions, written for EPSE Project
1: Using Diagnostic Assessment to Enhance Teaching and Learning in Science.
Questions which are knowingly based on previous probes are listed below, with an
indication of the original source. If precursors of any questions in this pack are not
acknowledged below, we would be grateful to have this pointed out, so that it can be
rectified in any subsequent publication.
Q2
Based on a question in:
Stead, K. & Osborne, R. (1980). Learning in Science Project Working Paper No.
19. University of Waikato: LISP Project.
Q9-10
Based on questions 25-27 in the revised version of the Force Concept Inventory (FCI)
(Halloun, I., Hake, R., Mosca, E. & Hestenes, D. 1995), reproduced in:
Mazur, E. (1997). Peer Instruction (pp. 47-58). Upper Saddle River NJ: Prentice
Hall.
Q12-13 The situation used in these questions is similar to (though not exactly the same as)
that used in probes by H. Caldas (1994), discussed in:
Viennot, L. (2003). Teaching Physics (pp. 70-71). Dordrecht: Kluwer Academic
Publishers.
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