Chapter 5 Review Questions
pp.71-72
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1. Distinguish between the relationship that defines the
acceleration and the relationship that states how it is produced.
• Definition: acceleration = change in velocity
divided by the change in time
• How is acceleration produced? Acceleration =
the net force divided by the mass
2. What is meant by the net force that acts on an object?
• The net force is equal to the sum of all the
forces acting on the object.
• The net force is the resultant of all the applied
forces.
3. Suppose a cart is being moved by a certain net force. If the net
force is doubled, by how much does the car’s acceleration change?
• The acceleration will double.
• Acceleration is directly proportional to the net
force.
• As forces increases acceleration increases.
4. Suppose a cart is being moved by a certain net force. If a load is
dumped into the cart, so its mass is doubled, by how much does the
acceleration change?
• The acceleration will be cut in half.
• Mass is inversely proportional to acceleration.
• So, if more mass is added, the acceleration
slows down.
5. Distinguish between the concepts directly proportional and
inversely proportional. Support your statement with examples.
• Directly: as one variable increases the other
increases by the same factor.
• Inversely: as one variable increases the other
decreases by the same factor.
• Examples: the force applied to a cart doubles,
therefore the acceleration doubles
• Example: the mass of a cart doubles,
therefore the acceleration decreases by half.
6. State Newton’s second law in words and then in the form of
an equation.
• The acceleration produced by a net force on
an object is directly proportional to the
magnitude of the net force, is in the same
direction as the net force and is inversely
proportional to the mass of the object.
7. How much force does a 20 000 kg rocket develop to
accelerate 1 m/s2?
• 20, 000 N
8. What is the cause of friction, and in what direction does it act
with respect to the motion of a sliding object?
• Friction is caused when two surfaces come
into contact with one another. The
irregularities on the surfaces rub against one
another and friction results.
• Friction opposes motion. That is to say,
friction acts in the direction opposite to
motion.
9. If the force of friction acting on a sliding crate is 100 N, how much
force must be applied to maintain a constant velocity? What will be
the net force acting on the crate? What will be the acceleration?
• 100 N of force must be applied to maintain
constant velocity.
• The system is in equilibrium so the net force
acting on the crate is zero.
• There is no acceleration because there is
constant velocity and a net force of zero.
10. Distinguish between force and pressure.
• Force, simply stated, is the amount of push or
pull on a system.
• Pressure is the amount of force encountered
over a certain area.
11. Which produces more pressure on the ground, a person
standing up or the same person lying down?
• The person standing up produces more pressure
on the ground because when you stand on two
feet the surface area in contact with the ground is
less. So, if the same amount of force is applied
to less area the pressure is greater.
• When you are lying down there is more surface
area in contact with the ground. So the same
amount of force is applied to a greater area, and
the pressure is less.
12. The force of gravity is twice as great on a 2-kg rock as on a 1-kg
rock. Why does the 2-kg rock not fall with twice the acceleration?
• Neglecting air resistance, a 2 kg rock will not
fall with twice the acceleration because twice
the force acts on twice the mass.
13. Why do a coin and a feather in a vacuum tube fall with the
same acceleration?
• Neglecting air resistance, they have the same
weight to mass ratio.
• a = F/M
• a = f/m
14. Why do a coin and a feather fall with different accelerations
in the presence of air?
• Because air resistance has more of an effect
on the feather, thus causing it to reach
terminal velocity sooner. The air resistance
builds up quickly and counteracts its tiny
weight.
15. How much air resistance acts on a 100-N bag of nails that
falls at its terminal speed?
• Remember that air resistance is a fluid
frictional force.
• 100 N
• When an object reaches terminal speed the
net forces are equal to zero.
16. How do the air resistance and the weight of a falling object
compare when terminal speed is reached?
• Air resistance and weight are equal to one
another when terminal speed is reached
17. All other things being equal, why does a heavy sky diver have a
terminal speed greater than a light sky diver? What can be done so
that the terminal speeds are equal?
• Terminal speed is when the air resistance acting
on an object equals the weight of the object.
Lighter objects obtain terminal speed sooner
than heavier objects.
• A greater weight is more effective at “plowing
through” the air.
• Terminal speeds can be equalized if the heavier
skydiver orientates his/her body to encounter
more air resistance. Spreading out your body’s
surface area (wing suit flying).
18. What is the net force acting on a 25-N freely falling object? What
is the net force when the object encounters 15 N of air resistance?
When it falls fast enough to encounter 25 N of air resistance?
• 25 N downward
• 10 N downward
• Zero, terminal velocity is reached