Physics 221, January 26
Key Concepts:
•Newton’s 1st law
•Newton’s 2nd law
•Weight
•Newton’s 3rd law
Newton's first law, also called the law of inertia, defines a special
class of reference frames, called inertial frames. It states that, when viewed in an
inertial reference frame, an object at rest remains at rest, and an object in motion
continues in motion with constant velocity, unless it is acted on by an external net
force.
Note:
Newton's second and third laws are valid in all inertial reference frames
.
Newton's second law states that the
acceleration of an object is directly
proportional to the net force acting on
it, and inversely proportional to its
mass, F = ma.
Newton's third law states that for every force that an
object exerts on a second object, there is a force equal in
magnitude but opposite in direction exerted by the
second object on the first object.
Newton's third law is also called the law of action and
reaction.
As I apply the brakes in my car, books on the passenger seat
suddenly fly forward. That is most likely because
1. the car is not an inertial
reference frame.
2. the seat supplies a forward
push to make the books
accelerate.
3. there is a strong
gravitational
field generated by the
brakes.
4. there is a strong
magnetic field generated
by the brakes.
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Suppose an ice skater is moving on the surface of
a frozen lake at constant velocity. What is true
about the external (outside) forces acting on the
skater?
1. There are none.
2. There could be some but
they all cancel out.
3. Gravity can be ignored.
4. The all are perfectly
horizontal.
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Remember!
constant velocity

no acceleration

no net force
To cause a 25 kg object to experience an acceleration of 2 m/s2,
the net force that needs to be applied to the object is
1.
2.
3.
4.
5 N.
12.5 N.
50 N.
100 N.
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Remember!
a = ∆v/∆t
acceleration =
change in velocity/change in time
A change in speed and/or a change in direction
produces a change in velocity.
The positions of two blocks at successive 0.20-second time intervals are
represented by the numbered squares in the figure below. The blocks are
moving towards the right. Are the blocks accelerating?
1. No
2. Yes, both blocks are
accelerating.
3. Only the upper block is
accelerating.
4. Only the lower block is
accelerating.
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Extra Credit: The positions of two blocks at successive 0.20-second time
intervals are represented by the numbered squares in the figure below. The
blocks are moving towards the right. Do the blocks ever have the same speed?
1.
2.
3.
4.
5.
No
Yes, at instant 2
Yes, at instant 5.
Yes, at instants 2 and 5.
Yes, at some time during
the interval 3 to 4.
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Weight
• The force of gravity , Fg, acting on an object is
called its weight.
• Near the surface of Earth Fg = mg,
pointing downward.
• g = 9.8m/s2 pointing downward.
Suppose you are standing on a bathroom scale when you are
flying in a jet airplane. For a moment the scale reads less than
your actual weight. During that moment, the scale is exerting an
upward force on you that is
1.
2.
3.
4.
greater than your weight.
equal than your weight.
less than your weight.
zero.
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Suppose you are standing on a bathroom scale when you are
flying in a jet airplane. For a moment the scale reads less than
your actual weight. During that moment, you are
1.
2.
3.
4.
accelerating upward.
accelerating downward.
not accelerating.
accelerating horizontally.
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Extra Credit: Suppose you are standing on a bathroom scale
while riding in an elevator. For a moment the scale reads 155 lb
= 689 N while your actual is 175 lb = 778 N. What is your (and
the elevator’s) acceleration? (Let g = 10 m/s2.)
1.
2.
3.
4.
5.
-1.14 m/s2
-1.29 m/s2
-10 m/s2
-20 m/s2
-89 m/s2
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Solution:
Your actual weight: 778 N
Your mass: 77.8 kg
Let the upward direction be the positive direction.
Net force acting on you: Ffloor – mg = 689 N – 778 N = -89 N
a = Fnet/m = -89 N/77.8 kg = -1.14 m/s2
A woman exerts a constant horizontal force on a
large box. As a result, the box moves across a
horizontal floor at a constant speed v0. The constant
horizontal force supplied by the woman
1.
2.
3.
4.
5.
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has the same magnitude as the
weight of the box.
is greater than the weight of the
box.
has the same magnitude as the total
force which resists the motion of the
box.
is greater than the total force which
resists the motion of the box.
is greater than either the weight of
the box or the total force which
resists its motion.
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If the woman in the previous question doubles the
constant horizontal force that she exerts on the box,
and the force resisting the motion of the box does
not change, the box then moves
1.
2.
3.
4.
5.
with constant speed, that is double
the speed v0 in the previous
question.
with a constant speed that is greater
than the speed v0 in the previous
question, but not necessarily twice
as great.
for a while with a speed that is
constant and greater than the speed
v0 in the previous question, then
with a speed that increases
thereafter.
for a while with increasing speed,
then with decreasing speed.
with constantly increasing speed.
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If the women suddenly stops applying a horizontal
force to the box, then the box will
1.
2.
3.
4.
5.
immediately come to a stop.
continue moving at a constant
speed for a while and then slow
to a stop.
immediately start slowing to a
stop.
continue at a constant speed.
increase its speed for a while
and then start slowing to a stop.
constantly increasing speed.
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A car has a maximum acceleration of 3.0 m/s2. What would be its
maximum acceleration while towing a second car twice its mass?
1.
2.
3.
4.
5.
2.5 m/s2
2 m/s2
1.5 m/s2
1 m/s2
0.5 m/s2
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Extra Credit: If you push on a friend and he is accelerating away from you,
how will the force you exert on your friend compare to the force your friend
exerts on you?
1.
2.
3.
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You push harder, since the friend
is moving away from you.
Your friend pushes harder, he
has to push himself off.
The forces are equal in
magnitude.
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