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PHYSICS – CHP. 5 HOMEWORK
Pg. 115-116 C.R. #3,8,13,14,16
Problems #2,4,5,7,8,10,12
Concept Review
3.
a.
False. Example I gave in class: pushing against a wall. Wall has too much mass to
move. Example: Standing. When you stand on the ground, it does not move. The Earth’s
mass is too much to overcome. Static equilibrium – no movement but present forces.
b.
False. An object will continue at a constant velocity until acted upon by an
outside force.
c.
True. An object will continue to move in a straight line unless a force is applied.
8.
A Newton is the force required to accelerate a 1 kg object at a rate of 1 m/s2.
13.
Force will always have the same sign as acceleration. If force is positive, the acceleration
will be positive. If acceleration is negative, force will be negative.
14.
Both are in equilibrium. The net force of both objects will be zero.
16.
This question should NOT state “how does the force you FEEL during the push…your
cousin FEELS.” You do not necessarily “feel” the force of gravity on your body when
you stand on the ground, though the force is there. There is a force present or there is not
a force present. The question should ask, “How does the magnitude of your force exerted
compare to the force of the cousin’s force exerted back to you?”
The forces are the same size but in opposite directions. Because you have a larger mass
than your cousin, the cousin will accelerate at a higher rate.
Problems
2. *Do not get acceleration confused with force. 1 N = 1 kg*m/s2. 1 N ≠ 1 kg.
a.
16 N (1 lb/4.48 N) = 3.57 lb ≈ 3.6 lb
b.
7 lb (4.48 N/1 lb) = 31.4 N ≈ 30 N
c.
3 kg (9.8 N/1 kg) = 29.4 N ≈ 30 N
d.
12 N (1 kg/9.8 N) = 1.224 kg ≈ 1.2 kg
4.
Its acceleration is reduced to 1/3 of its initial value.
5.
The acceleration triples.
7.
F = m*a = (72 kg)(3.25 m/s2) = 234 N
8.
The bowling ball will exert and equal but opposite force back on the girl. When dealing
with this problem as one whole system, either the ball or girl will have a positive
acceleration and force. The other object will have a negative acceleration and force. I did
not take off points if you did not do this. Just be aware of it.
a.
a = F/m = 75 N / 32 kg = 2.34 m/s2
b.
Only use the mass of the bowling ball.
a = 75 N / 7 kg = 10.71 m/s2
10.
a.
The ball and bat strike each other with equal but opposite forces, so the force
applied on the bat by the ball will also be 500 N.
b.
c.
12.
a = 500 N / 0.15 kg = 3.333 x 103 m/s2
v = a*t = (3.333 x 103 m/s2)(0.2 s) = 666.6 m/s
Making the right direction positive:
∑F = F2 + F1 = 8.0 N + (–2.0 N) = 6.0 N
a = 6.0 N / 2 kg = 3 m/s2