Chapter 3
Railroad car A rolls at a certain speed and makes a perfectly elastic collision with car B of the same mass. After the collision, car A is observed to be at rest. How does the speed of car B compare with the initial speed of car A?
Car B's speed after the collision equals car A's initial speed.
If the equally massive railroad cars of Question 11 couple together after colliding inelastically, how does their speed after the collision compare with the initial speed of car A?
Car B's speed after the collision is half of car A's initial speed.
When a ball is hit with a given force, why does contact over a long time impart more speed to the ball than contact over a short time?
Change in momentum, and speed, is proportional to impulse, which is force times time.
When is energy most evident?
Energy is most evident when it is being transferred or transformed.
Which of the following equations is most useful for solving a problem that asks for the distance a fast-moving crate slides across a factory floor in coming to a stop?
Fd = Δ1/2 mv 2
Cite an example in which a force is exerted on an object without doing work on the object.
Holding a weight overhead.
A lunar vehicle is tested on Earth at a speed of 12 km/h. When it travels at the same speed on the Moon, is its momentum greater, less, or the same?
Its momentum is the same
What becomes of energy when efficiency is lowered in a machine?
More energy is lost as heat.
Can we correctly say that hydrogen is a relatively new source of energy? Why or why not?
No, hydrogen is the most common atom in the universe created billions of years ago.
When a cannonball is fired, momentum is conserved for the system of cannon + cannonball. Would momentum be conserved for the system if momentum were not a vector quantity? Explain.
No, momentum is conserved only if the directions of momenta are taken into account.
Select the equation that shows that when a 3.0-kg book is lifted 1.0 mm, its increase in gravitational potential energy is 30 J.
PE=mgh
An apple hanging from a limb has potential energy because of its height. If the apple falls, what becomes of this energy just before the apple hits the ground? When it hits the ground?
Potential energy becomes kinetic energy; work is done on the ground and apple, heating them both.
What is the relationship between work and power?
Power is work done per unit time.
In boxing, why is it advantageous to roll with the punch?
Rolling with a punch increases the time over which the impulse is applied; this decreases the force.
Why is it a good idea to extend your bare hand forward when you are getting ready to catch a fast-moving baseball?
So you can pull your hand back, increasing the time to slow the ball thus deceasing the force.
The main sources of energy on Earth are
Solar and nuclear
Two cars are raised to the same elevation on service station lifts. If one car is twice as massive as the other, how do their potential energies compare?
The more massive car has twice as much potential energy.
Which has more momentum after firing, the cannonball or the recoiling cannon?
They are both the same
What does it mean to say that momentum (or any quantity) is conserved?
When momentum does not change it is conserved.
What is the relationship between the gain in kinetic energy and the work when work is done?
Work is equal to kinetic energy gain.
Why is it poor judgment to have the back of your hand up against the outfield wall when you catch a long fly ball?
You cannot move your hand backward. This means that the time over which you slow the ball is short and the force is high.
What will be the kinetic energy change of the ram of a pile driver when it suddenly undergoes a 10 kJ decrease in potential energy?
a 10 kJ increase
If a machine multiplies force by a factor of 4, what other quantity is diminished, and by how much?
distance by 1/4
When the speed of an object is doubled, its momentum
doubles
What is the ultimate source of the energy supplied by fossil fuels, dams, and windmills?
fusion of hydrogen in the Sun
Which has greater momentum: an automobile at rest or a moving skateboard?
moving skateboard
If a moving car speeds up until it is going three times as fast as it was, how much kinetic energy does it have compared with its initial kinetic energy?
nine times
According to the law of energy conservation, energy can be
only transferred or transformed
What is the ultimate source of geothermal energy?
radioactive decay of isotopes in the Earth
The impulse-momentum relationship is a direct result of Newton's
second law
A car is raised a certain distance in a service station lift, and therefore it has potential energy relative to the floor. If it were raised twice as high, how much potential energy would it have compared with what it had in the first case?
twice as much
Raising an auto in a service station requires work. Raising it in half the time requires
twice the power
Raising an auto in a service station requires work. Raising it twice as high requires
twice the work
Select the option that shows that when an impulse produced by a 12-N force acts over 2.0 s on an ice puck initially at rest on an air table, the change in momentum is 24 kg⋅m/s.
Δmv=Ft=(12N)(2.0s)=24N⋅s=24kg⋅m/s.
What is the efficiency of a machine that miraculously converts all the input energy to useful output energy?
100%
Compared with the work that the brakes must supply to stop a car moving at its original speed, how much more work must the brakes supply to stop that car if it is moving four times as fast as the original speed? How do the respective stopping distances compare?
16 times as much work; 16 times the distance
In an ideal pulley system, a woman lifts a 100-N crate by pulling a rope downward with a force of 25 N. For every one-meter length of rope she pulls downward, the crate rises
25 centimeters
Calculate the kinetic energy of a 80 kg scooter moving at 8.0 m/s.
2560 J
If a ball has the same speed just before being caught and just after being thrown, in which case does the ball undergo the greatest change in momentum: (1) when it is caught, (2) when it is thrown, or (3) when it is caught and then thrown back?
3
In which of cases (1), (2), and (3) in Question 7 is the greatest impulse required?
3
When 100 J are put into a device that puts out 40 J, the efficiency of the device is
40%
Can a machine multiply input force, input distance, and/or input energy? (If your three answers are the same, seek help, because this is an important question.)
A machine can multiply force and distance but not energy.
In karate, why is it advantageous to apply a force for a short time?
An impulse applied for a short time maximizes the force on the object that is struck.
