AP Physics Chap 7 Test Conceptual Questions

In Example 7.7, we calculated the final speed of a roller coaster that descended 20 m in height and had an initial speed of 5m/s downhill. Suppose the roller coaster had had an initial speed of 5 m/s uphill instead, and it coasted uphill, stopped, and then rolled back down to a final point 20 m below the start. We would find in that case that it had the same final speed. Explain in terms of conservation of energy.

According to the law of conservation of energy, the loss of gravitational potential energy of the coaster equals the gain in kinetic energy because there if NO Friction.

A spark of static electricity, such as that you might receive from a doorknob on a cold dry day, may carry a few hundred watts of power. Explain why you are not injured by such a spark.

Although the power is high, the spark only lasts for a tiny fraction of a second, so it contains very little energy.

Describe the energy transfers and transformations for a javelin, starting from the point at which an athlete picks up the javelin and ending when the javelin is stuck into the ground after being thrown.

Chemical energy in his muscles transfers into increased GRAVITATIONAL POTENTIAL ENERGY. When he runs PE is converted to KE. Once it is thrown, it loses KE because of friction and gravity.

Consider the following scenario. A car for which friction is not negligible accelerates from rest down a hill, running out of gasoline after a short distance. The driver lets the car coast farther down the hill, then up and over a small crest. He then coasts down that hill into a gas station, where he brakes to a stop and fills the tank with gasoline. Identify the forms of energy the car has, and how they are changed and transferred in this series of events. (See Figure 7.34.)

Coasting downhill....loses PE and gains KE and some energy is lost due to friction. When is coasts over the bump, loses KE and gains PE When brakes are applied KE turns to heat energy.

Most electrical appliances are rated in watts. Does this rating depend on how long the appliance is on? (When off, it is a zerowatt device.) Explain in terms of the definition of power.

Doesn't depend on how long it is used.

List four different forms or types of energy. Give one example of a conversion from each of these forms to another form.

Electrical ....electric fan converts electrical energy to mechanical energy Mechanical ... turbine converts mechanical energy to electrical enegy Solar....solar panel converts solar energy to electrical energy Chemical ....battery converts chemical energy to electrical energy

What is the difference between energy conservation and the law of conservation of energy? Give some examples of each.

Energy conservation is reducing the amount of energy used. Example riding bike vs driving car. Law of conservation means you cannot destroy energy.

If the efficiency of a coal-fired electrical generating plant is 35%, then what do we mean when we say that energy is a conserved quantity?

Energy is stored with the kinetic energy and remain constant

List the energy conversions that occur when riding a bicycle.

Energy to push the pedals comes from nutrients in the body. Chemical energy used by the body turns into KE to push the pedals. Pedal gain rotational KE. Bike moves forward gaining translational KE.

What is a conservative force?

Force of gravity and force of a Spring

Give an example of something we think of as work in everyday circumstances that is not work in the scientific sense. Is energy transferred or changed in form in your example? If so, explain how this is accomplished without doing work.

Formula for Work done by force is W = F*S*cos(theta) A traffic police standing at the signal wthout moving he clears all the trafic from morning to evening, but displacement S= 0m

The person in Figure 7.33 does work on the lawn mower. Under what conditions would the mower gain energy? Under what conditions would it lose energy?

Gain: horizontal force is GREATER than friction Lost: horizontal force is LESS than friction

Give an example of a situation in which there is a force and a displacement, but the force does no work. Explain why it does no work.

If a person is moving with a constant velocity carrying a heavy box then there is NO WORK done because the force is at the right angle to the bottom of the box. Force is applied in an upward direction equal to the weight of the box. Force is perpendicular to the motion so the work done is zero.

When solving for speed in Example 7.4, we kept only the positive root. Why?

In Kinetic Energy speed is ALWAYS positive

Do devices with efficiencies of less than one violate the law of conservation of energy? Explain.

Less than one DO NOT violate the law of conservation of energy it just transfers into forms of LESS useful energy.

Define mechanical energy. What is the relationship of mechanical energy to nonconservative forces? What happens to mechanical energy if only conservative forces act?

ME= KE + PE Kinetic + Potential Non-Conservative Force = A force, such as friction, that performs work over a distance that is dependent on the path taken between the initial and final positions

Do you do work on the outside world when you rub your hands together to warm them? What is the efficiency of this activity?

No, very little work on outside world Due to friction, heat is generated and your hands warm up The heat dissipates to the rest of your body. Very little goes to warming up the environment.

Explain, in terms of the definition of power, why energy consumption is sometimes listed in kilowatt-hours rather than joules.

Power consumption is the rate at which energy is used fo E=Pt

Shivering is an involuntary response to lowered body temperature. What is the efficiency of the body when shivering, and is this a desirable value?

Shivering helps to warm up the body. While shivering efficiency gets low as the body tries to get back to optimal value.

Describe a situation in which a force is exerted for a long time but does no work. Explain.

W= F*d Work depends on 3 things: displacement, force and angle between force and displacement. If the displacement is 0 then the work done is 0. Example, trying to push a heavy object but it does not move. Person will be tired and there will be a loss of muscular energy, but the work done is 0.

Does the work you do on a book when you lift it onto a shelf depend on the path taken? On the time taken? On the height of the shelf? On the mass of the book?

W=mgh Path = NO Time = NO Height= YES Mass= YES

Work done on a system puts energy into it. Work done by a system removes energy from it. Give an example for each statement.

Work done ON a system...person pushing a mower puts work ON, creating energy Work done BY a system... once moving friction causes it to lose energy

The force exerted by a diving board is conservative, provided the internal friction is negligible. Assuming friction is negligible, describe changes in the potential energy of a diving board as a swimmer dives from it, starting just before the swimmer steps on the board until just after his feet leave it.

Work done by conservative forces is independent of the path followed. Total energy is conserved in the presence of conservative forces conserved. The potential energy of the diving board provides, gravitational potential energy. Now he kumps from the board this gravitational potential energy transformed into kinetic energy to the swimmer.

What is the relationship of potential energy to conservative force?

Work is done by a force, and some forces, such as weight, have special characteristics. A conservative force is one, like the gravitational force, for which work done by or against it depends only on the starting and ending points of a motion and not on the path taken.

Discuss the relative effectiveness of dieting and exercise in losing weight, noting that most athletic activities consume foodenergy at a rate of 400 to 500 W, while a single cup of yogurt can contain 1360 kJ (325 kcal). Specifically, is it likely that exercise alone will be sufficient to lose weight? You may wish to consider that regular exercise may increase the metabolic rate, whereas protracted dieting may reduce it.

You need exercise AND diet to lose weight. Diet alone will reduce metabolic rate.

Explain why it is easier to climb a mountain on a zigzag path rather than one straight up the side. Is your increase in gravitational potential energy the same in both cases? Is your energy consumption the same in both?

on a longer path, less force is needed to move up. Straight up has more gravitational pull. Increase in GPE is the same in both cases Energy consumption depends on total distance so the longer one will consume more energy.