**CP PHYSICS KEY**
**NEWTON’S LAWS OF MOTION REVIEW**
1. What are the names given to Newton’s Laws?
1^{st}: Law of Inertia
2^{nd}: Law of Acceleration
3^{rd}: Law of Action-Reaction
2. State what each of the laws means:
1^{st}: An object at rest tends to stay at rest and an object in motion tends to stay in motion with a constant speed and in a straight line unless acted upon by a net force.
2^{nd}: When a net force acts on an object, the object accelerates such that the product of
the mass and the acceleration equal the net force. (F = ma)
3^{rd}: Whenever one body exerts a force on a second body, the second body exerts an
equal force in the opposite direction on the first body.
3. What unit is used for all forces? Newton (N)
4. What is the relationship between mass and weight?
Weight is the force of gravity acting on a mass.
No matter where you are in the universe, mass is constant.
Weight depends on your location in the universe and the force of gravity
at that location.
5. What is the relationship between mass and acceleration?
mass resists acceleration, inverse proportion
when mass increases, the acceleration will decrease
6. What is the relationship between force and acceleration?
forces cause accelerations, direct proportion
when the force increases, the acceleration increases
7. What happens to the acceleration of an object if more force is applied?
the object’s acceleration increases
8. What happens to the acceleration of an object if the mass increases?
the object’s acceleration decreases
9. What velocity must a falling object have to maintain an orbit around Earth?
8 km/s
10. What distance from Earth’s surface do most satellites have to be located?
150 km (any lower and the satellite would burn up due to friction with
Earth’s atmosphere)
**CP PHYSICS, NEWTON’S LAWS OF MOTION REVIEW, page 2**
11. What two factors affect the air resistance experienced by an object?
1) surface area – greater surface area results in greater air resistance
2) velocity – the faster the object falls, the greater its air resistance
12. What is meant by the net force that acts on an object?
The net force acting on an object is the sum of all the forces acting on an
object. If all the forces cancel each other out, then the net force on the
object is zero.
13. If the forces exerted on an object are 50 N in one direction and 30 N in the opposite
direction, what is the net force exerted on the object?
50 N – 30 N = 20 N
14. Forces of 10 N and 20 N in the same direction act on an object. What is the net
force on the object? 10 N + 20 N = 30 N
15. How much air resistance acts on a 100 N bag of nails that falls at its terminal
velocity? 100 N
16. If you hit a wall with a force of 200 N, how much force is exerted on you?
200 N, action-reaction forces
17. What is free fall?
Free fall occurs when there is no air resistance acting on the falling object and
it falls at 9.8 m/s^{2}.
18. What occurs during non-free fall?
During a non-free fall, air resistance acts on the falling object. Therefore, the
object will fall at an acceleration less than 9.8 m/s^{2}. As the object falls, the
air resistance pushing upward on it will increase until it is the same value as
its weight (force of gravity). At that point the object stops accelerating. It
has reached its terminal velocity.
19. For the following, tell whether the statement matches Newton’s 1^{st}, 2^{nd}, or 3^{rd} Law.
1^{st} a. Things tend to keep doing what they are doing.
1^{st} b. When the road is covered with ice, cars tend to keep going in a straight line
rather than making a curve.
2^{nd} c. Using aluminum instead of steel makes it possible to trim 477 lbs. off a typical
3500 lb. car. The less weight (or mass) of a car, the less fuel it takes to make it
go.
3^{rd} d. One reason it is so painful to stub your toe against a rock is that the rock hits you
back.
3^{rd} e. To every action there is an equal and opposite reaction.
2^{nd} f. Forces tend to accelerate things while mass tends to resist acceleration.
**CP PHYSICS, NEWTON’S LAWS OF MOTION REVIEW, page 3**
1^{st} g. Boxers often experience hemorrhaging because their brain slams into their skull
repeatedly during matches.
1^{st} h. Whiplash may occur when sitting in a car which is hit from behind, especially if
headrests are not in place.
2^{nd} i. F = ma
2^{nd} j. Law of acceleration
3^{rd} k. action-reaction
2^{nd} l. It takes more force to accelerate a semi truck than a compact car.
3^{rd} m. Forces always occur in pairs.
1^{st} n. An astronaut pushed out of a spacecraft will continue to move in a straight line at
constant speed forever.
3^{rd} o. If you blow up a balloon and release it, the balloon flies away.
20. The following statements are FALSE. Change the incorrect term to make the
statement correct!
a. It would easier to lift a car on the moon as it is to lift the same car on Earth.
b. A baseball bat hits a ball with a force of 100 N. The bat slows down a bit because
the ball exerts a resisting force against the bat of 100 N.
c. The acceleration of an object is inversely proportional to its mass.
d. A ball falling from a great height will reach terminal velocity when the force of air
resistance equals its weight.
e. Newton’s first law deals with two forces acting on one object.
f. If a light object and a heavy object are dropped together in a vacuum, both hit the
bottom at the same time.
g. When an object is at rest, its inertia tends to make it stay at rest.
h. If there is no force acting on an object, its motion will be one with zero
acceleration.
**CP PHYSICS, NEWTON’S LAWS OF MOTION REVIEW, page 4**
i. Assume that you are driving down a straight road at constant speed. A small ball is
tied on the end of a string hanging from the rearview mirror. When you apply the
brakes, the ball will swing forward.
j. The Newton is the unit for force in the metric system.
21. If a car starts from rest and reaches a velocity of 20 m/s in 5.0 seconds,
what is its average acceleration?
∆v = 20 m/s a = ∆v/t
t = 5.0 s = 20/5.0
a = 4 m/s^{2}
22. What is the force needed to accelerate a 205 kg object at 3.91 m/s^{2}?
m = 205 kg F = ma
a = 3.91 m/s^{2} = (205)(3.91)
F = 802 N
23. What is an object’s acceleration if it has a mass of 120 kg and a force of
2350 N acts on it.
m = 120 kg F = ma
F = 2350 N __2350__ = __120a__
120 120
a = 19.6 m/s^{2}
24. A man whose mass is 118 kg is taken to a planet where his weight is 2900 N.
What is the planet’s acceleration due to gravity?
m = 118 kg Wt = mg
Wt = 2900 N __2900__ = __118g__
118 118
g = 24.6 m/s^{2}
25. What is the weight of a 688 kg bookcase?
m = 688 kg Wt = mg
g = 9.8 m/s^{2} = (688)(9.8)
Wt = 6742 N
26. A woman whose mass is 80 kg on Earth travels to another planet where the
acceleration due to gravity is 3.4 m/s^{2}. What is the mass of this woman on
the other planet?
Her mass would be 80 kg. Mass is constant no matter where in the
universe one is located.
27. A care package weighing 225 N reaches terminal velocity as it falls to the
ground. What is the force of air resistance on the package?
at terminal velocity, weight = air resistance, so air resistance = 225 N
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