Chapter 4 Mastering Physics

By what fraction did the kinetic energy change from its initial value KEinitial to its final value KEfinal? Express your answer using two significant figures.

0.5

If the skater starts from rest at position 1, rank, in increasing order from least to greatest, the kinetic energy of the skater at the five positions shown. Rank from smallest to largest. To rank items as equivalent, overlap them.

1 3 5,2 4

Place the skater on the track 7 mm above the ground, and look at the resulting motion and the Bar Graph showing the energetics. Match the approximate numerical values on the left with the energy type categories on the right to complete the equations. Assume that the mass of the skater is 75.0 kgkg and that the acceleration of gravity is 9.8 N/kgN/kg . Drag the appropriate numerical values to their respective targets.

1. total energy at initial position= 5145 J 2. potential energy at initial position= 5145 J 3. kinetic energy at initial position= 0 J 4. total energy at the bottom of track= 5145 J 5. potential energy at bottom of track= 735 J 6. kinetic energy at the bottom of track= 4410 J

One common application of conservation of energy in mechanics is to determine the speed of an object. Although the simulation doesn't give the skater's speed, you can calculate it because the skater's kinetic energy is known at any location on the track. Consider again the case where the skater starts 7 mm above the ground and skates down the track. What is the skater's speed when the skater is at the bottom of the track? Express your answer numerically in meters per second to two significant figures.

11

Exert 100J in 50s, and your power output is

2 W

Suppose we repeat the experiment from the video, but this time we use a rocket three times as massive as the one in the video, and in place of water we use a fluid that is twice as massive (dense) as water. If the new fluid leaves the rocket at the same speed as the water in the video, what will be the ratio of the horizontal speed of our rocket to the horizontal speed of the rocket in the video after all the fluid has left the rocket? (Ignore air resistance.)

2/3

If the skater started from rest 4 mm above the ground (instead of 7mm), what would be the kinetic energy at the bottom of the ramp (which is still 1 mm above the ground)?

2205 J

Rank PEs from greatest to least at each point A-E. Rank from greatest to least. To rank items as equivalent, overlap them.

A E C B D

What is the definition of an elastic collision?

A collision in which colliding objects rebound without lasting deformation or the generation of heat

What is the definition of an inelastic collision?

A collision in which the colliding objects become distorted, generate heat, and possibly stick together

Why does the bowling ball behave the way it does the first time Dr. Hewitt lifts the bowling ball near his teeth and lets go?

All of the initial energy of the ball was converted completely back to potential energy when the ball returned.

Rank them from greatest to least for momentum. Rank from largest to smallest. To rank items as equivalent, overlap them.

B 1000kg A 800 kg C 90 kg

Rank them from greatest to least for kinetic energy. Rank from largest to smallest. To rank items as equivalent, overlap them.

C 8 m/s B 2 m/s A 1 m/s

Rank them from greatest to least for work done to bring them up to their respective speeds from rest. Rank from largest to smallest. To rank items as equivalent, overlap them.

C 8 m/s 90kg B 2 m/s 1000kg A 1 m/s 800kg

Rank KEs from greatest to least at each point A-E. Rank from greatest to least. To rank items as equivalent, overlap them.

DBCEA

Rank speeds from greatest to least at each point A-E. Rank from greatest to least. To rank items as equivalent, overlap them.

DBCEA

What quantity does it change?

Energy

How does impulse differ from force?

Force is a push or pull, while impulse is the product of force and time.

Imagine that you replace the block in the video with a happy or sad ball identical to the one used as a pendulum, so that the sad ball strikes a sad ball and the happy ball strikes a happy ball. The target balls are free to move, and all the balls have the same mass. In the collision between the sad balls, how much of the balls' kinetic energy is dissipated?

Half of it

How does Dr. Hewitt break the piece of wood?

He causes a change in momentum of his hand over a short period of time.

How is Dr. Hewitt able to break a piece of wood in his demonstration?

He moves his hand very quickly.

In what unit POWER is measured?

In Watt

What is the law of energy conservation?

In the absence of external work input or output, the energy of a system remains unchanged . Energy cannot be created or destroyed.

What is the law of energy conservation?

In the absence of external work input or output, the energy of a system remains unchanged. Energy cannot be created or destroyed.

What would Dr. Hewitt need to have done to exert an even greater force than he did in his karate demonstration?

Increase the change in momentum, and decrease the time duration.

What does it mean to say that a quantity is conserved?

It means that the quantity remains unchanged in a process.

Suppose our experimenter repeats his experiment on a planet more massive than Earth, where the acceleration due to gravity is g=30 m/s2g=30 m/s2. When he releases the ball from chin height without giving it a push, how will the ball's behavior differ from its behavior on Earth? Ignore friction and air resistance. (Select all that apply.)

It will take less time to return to the point from which it was released.

For which type of collision is momentum conserved?

Momentum is conserved in both elastic and inelastic collisions.

Which has a greater momentum −− a heavy truck at rest or a moving skateboard?

Moving skateboard.

Is it possible to design a machine that has an efficiency greater than 100%?

No because a machine that has an efficiency greater than 100% outputs more energy than it uses, therefore, the machine creates additional energy. It violates the law of energy conservation.

Can you produce a net impulse on a car by sitting inside and pushing on the dashboard?

No, because your push is internal. To change the automobile's momentum, an external force must be applied.

Now, consider the collision between two happy balls described in Part A. How much of the balls' kinetic energy is dissipated?

None of it

What happens the second time Dr. Hewitt lifts the bowling ball near his teeth and gives it a push?

The ball leaves Dr. Hewitt and returns to him, going past the point where it was released.

What happens the first time Dr. Hewitt lifts the bowling ball near his teeth and lets go?

The ball returns to Dr. Hewitt, stopping almost exactly at the point where it was released.

Consider a baseball that is caught and thrown at the same speed. Which case illustrates the greatest change in momentum?

The baseball being caught and then thrown back

Consider a baseball that is caught and thrown at the same speed. Which case requires the greatest impulse?

The baseball being caught and then thrown back.

What has become of this energy?

The energy goes into thermal energy.

Why does the bowling ball behave as it does when Dr. Hewitt lifts it and gives it a push?

The extra energy from the push is converted into kinetic energy, which is then converted into more potential energy at the end of the motion than the ball had when it was released.

For the same force, which cannon impacts the greater speed to a cannonball - a long cannon or a short one?

The long cannon

For the same force, which cannon imparts the greater speed to a cannonball −− a long cannon or a short one?

The long cannon.

How can a huge ship have an enormous momentum when it moves relatively slowly?

The ship has enormous momentum due to huge mass.

What is the source of energy that powers a hydroelectric power plant?

The sunlight that evaporated water that eventually falls as rain and fills the reservoir.

What is (are) the way(s) in which the impulse exerted on something can be increased?

Two ways to increase impulse are to increase force or increase the time the force is exerted.

What do we call the quantity (Force) x (Distance)?

Work

What do we call the quantity force X distance?

Work

In what units are work and energy measured?

Work and energy are measured in joules.

Now observe the potential energy bar on the Bar Graph. As the skater is skating back and forth, where does the skater have the most potential energy?

at its maximum value at the locations where the skater turns and goes back in the opposite direction.

Click on Bar Graph, and observe the kinetic energy bar as the skater goes back and forth. You can select Slow Motion below the track for a more accurate observation. Where on the track is the skater's kinetic energy the greatest?

at its maximum value at the lowest point of the track.

Based on the previous question, which statement is true?

equal to the amount of potential energy loss in going from the initial location to the bottom.

When the skater starts 7 mm above the ground, how does the speed of the skater at the bottom of the track compare to the speed of the skater at the bottom when the skater starts 4 mm above the ground?

higher, but less than twice as fast.

When an increase in speed doubles the momentum of a moving body, its kinetic energy.

more than doubles

When an increasing in speed doubles the momentum of moving body, its kinetic energy

more than doubles

Can a machine multiply input energy?

no

Which has a greater momentum - a heavy truck at rest or a moving skateboard?

the moving skateboard

Because we are ignoring friction, no thermal energy is generated and the total energy is the mechanical energy, the kinetic energy plus the potential energy: E=K+UE=K+U. Observe the total energy bar on the Bar Graph. As the skater is skating back and forth, which statement best describes the total energy?

the same at all locations of the track.

True or false: One watt is the unit of power equivalent to 1 joule per second.

true

You lift a barbell a certain distance from the floor. If you lift it twice as high, its potential energy is

twice as great

Can a machine multiply input distance?

yes

Can a machine multiply input force?

yes