Answer Key
Review - Newton’s Laws
Terms:
inertia
air resistance
force
terminal velocity
net force
projectile
Newton’s 1st Law - Law of __Inertia___
Newton’s 3rd Law - Law of _Action-Reaction
trajectory
vertical velocity
horizontal velocity
Newton’s 2nd Law - Law of __Acceleration
I. Problems:
1.
What is the weight of an object whose mass is 4.2 kg?
F=ma
F=mg
F=4.2 (9.8) = 41.16 N
2.
If the object in #1 is pushed across the floor with a net force of 20 N, what will the
acceleration be?
F across = m a (across)
3.
a = 4.8 m/s2
20 N = 4.2 (a)
Where would a sack of flour land horizontally from the dropping point if it were dropped
out of a plane flying horizontally at 60.0 m/s at an altitude of 300.0 m?
Vertical
vI=0
a=9.8
d=300.0
Horizontal
v=60.0
d=?
V=d/t
D = vt = 60.0(7.8) =
1 2
at
2
2d
2(300)
t

 7.8s
a
9.8
d
469m
4. While skiing, Ellen encounters an unexpected bump that she leaves travelling horizontally at 12.0 m/s.
(a) How far out from her starting point will she land if she falls from a height of 7.00 m? (b) What
is the final vertical velocity?
Vertical
vI=0
a=9.8
d=7.00
Horizontal
d=?
v=12.0
a) t 
2d
2(7)

 1.19s
a
9.8
vt=d=12(1.19)= 14.3m
5.
b)
v f  vi  2ad
2
2
v f  2(9.8)(7)
=11.7 m/s
A “g” is an acceleration of 9.80 m/s2. How much force is needed to accelerate a 0.0090 kg
object at 3.0 “g’s”?
“g” = 3.0 x 9.8 = 29.4 m/s2
F=ma = 0.0090(29.4) = 0.26 N
6. What force is required to stop a 1200 kg in 7.0 s if the car is traveling at 22 m/s?
m = 1200 kg
t = 7.0 s
v f  vi 0  22
F=?
vf = 0
a

= -3.14 m/s2
t
7.0
a = _____
vI = 22 m/s
Plug this into F=ma
a=?
F = 1200(-3.14) = 3800 N back
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7.
Answer Key
a. What is the acceleration of a falling skydiver (mass 72 kg including the parachute)
when the upward force of air resistance is equal to ¼ of her total weight?
Fair = 706/4 = 176 N
Fwt = 706 N
Fnet = big – little
Fnet = 706-176 = 530
N
Fnet = ma
a = F/m = 530/72 = 7.4 m/s 2
b. Shortly after opening her parachute, the skydiver descends to the ground at constant
velocity. What is the air resistance on her now?
Fnet=0 so Fair = Fst = 710 N (rounded to 2 digits)
8.
A stone is thrown horizontally at 8.0 m/s from the edge of a cliff 78 m high. How far
from the base of the cliff does the ball land?
Horizontal
Vertical
v=d/t
2d
v = 8.0 m/s
d = 78 m
d = vt
= 4.00 s
t
a
d=?
a = 9.8 m/s2
d=8.0(4.00) = 32 m
t = _____
vi = 0
t = ______
Plug this in to
horizontal
II. The following statements are false. Tell what is wrong with each or change it
to a true statement.
1.
An object always moves accelerates in the direction of the net force applied to it.
2.
A horse must pull a cart harder than the cart pulls back on the horse to move forward.
The horse must push back on the ground to move the cart.
3.
Action-reaction forces cancel out so that neither object can accelerate. They don’t cancel
because they act on different objects.
4.
When a car hits a bug, the bug hits back with the same force and both car and bug
accelerate equally. The bug accelerates more because it has less mass (inertia).
5.
If everyone inside a car pushes forward on the car, they can make the car speed up. No –
you must have an external force.
6.
To hit a distant target with an arrow, Robin Hood should aim directly at the target. Aim
above it because gravity will pull it down as it moves horizontally.
7.
A feather and a hammer will fall with the same acceleration in a vacuum because there is
no gravity air in a vacuum. There is gravity! You won’t fall without it.
8.
The real reason for #7 is that gravity pulls the feather and the hammer with the same
force acceleration. They have different weights.
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Answer Key
9.
When the human cannonball leaves the barrel of the cannon, his horizontal motion is
accelerated uniform and his vertical motion is uniform accelerated.
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