Do Now 3.4.1
Physics Grade 8 (2021) – Dr Chantal Rudman
SORTING OUT TYPICAL MISCONCEPTIONS with forces
Name and Surname:……………………………………………………………
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1. TENSION FORCE:
A pulling force exerted by a string, cable, chain, or similar solid object on another
object.
Example 1.1:
Below is a tire wheel on a rope. The tyre
weighs 5kg. What is the tension in the
rope?
1. Draw the FBD of the tire and
name all the forces (Name NOT
the magnitude)
2. Write the equilibrium equation in
the y-direction.
3. What is the tension in the rope?
Example 1.2:
Two people are pulling on a rope as shown.
They are static . Both are pulling with 30N.
2. Draw the FBD of the right hand.
1. Draw the FBD of the rope.
3. Draw the FBD of the right hand.
4. What is the tension in the rope?
(a) 0 N
(b) 30 N
(c) 60 N
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2. NORMAL FORCE:
The normal force is the support force exerted upon an object that is in contact with another
object.
Example 2.1:
Rookie error: The Normal force is NOT always = m x g !!!!!!!!!
(a) A man is pushing down with (b) A man is pushing against
a force F=100N, onto an
a wall with F=200N, onto
object with m=25kg.
an object with m=25kg.
(c) A man is pushing up
against a ceiling with
F=300N,
onto
an
object with m=35kg.
Draw the FBD of the above
situation?
Draw the FBD of the above
situation?
Draw the FBD of the
above situation?
Write the equilibrium equation in
the direction of y.
Write
the
equilibrium
equation in the direction of
y.
Write
the
equilibrium
equation in the direction
of y.
What is the magnitude of the
Normal Force?
What is the magnitude of
the Normal Force?
What is the magnitude of
the Normal Force?
Is the Normal force = m x g ?
Is the Normal force = m x g ?
Is the Normal force = m x
g?
Conclusion: The Normal force is not always ……………………….……………………………………
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3. FORCE ON AN INCLINED PLANE:
Example 3.1:
A block is placed on an inclined plane. The
inclined plane has NO friction.
A block is placed on an inclined plane. The
inclined plane has friction.
Draw the FBD of the above situation?
Draw the FBD of the above situation?
Example 3.2:
The free-body diagram of a block being pushed up a frictionless ramp is best represented by
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4. DOES THE NORMAL FORCE CHANGE AS THE
INCLINED PLANE INCREASES OR DECREASES?
Example 4.1:
Step 1: Draw only the gravity force on both inclined planes. You can assume that 1cm = 10N.
Step 2: Decompose the gravity force into it’s “x” and “y” component in a different color.
Watch what Dr Chantal does as the “x” and “y” component is done differently as how you
have learnt up to now. Write down the values of the “x” and “y” component.
Step 3: Draw the normal force in another different color that will be exactly the same as the
“Y” component in each case. How do I know this? Well I know this because there is no other
forces in this direction and yet it is not moving up or down so something must balance it in the
other direction.
Does the normal force increase or decrease as the inclined plane becomes shallower? Explain
in a full paragraph and compare situation 1 with 2. Talk about how the “y” component
changes and therefore this is directly connected to the normal force.
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4.2 A teapot is at rest on a horizontal tabletop, then one end of the table is lifted slightly. The
normal force...
A. decreases
B. increases
C. is unaffected
D. is zero
4.3 Two identical polar bears are sliding down two different inclined planes. The one inclined
plane is shallow and the other one is steep. Which statement is true?
A. The normal force of the polar bear on the steep inclined plane is smaller than the one
of the shallow inclined plane.
B. The normal force of the polar bear on the steep inclined plane is larger than the one
of the shallow inclined plane.
D. The normal force of both are the same.
E. The normal force of both are zero.
5. OTHER rookie error type FORCES
5.1 A ball is thrown up in the air and the air resistance is negligible. Which free body diagram
is the correct picture?
5.2 A car is travelling to the right on a level highway at a constant velocity in a straight
line. Air resistance is negligible. Which of the following is the correct free body diagram
for the car?
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