Exam 2 Physics Concepts

Which pulls harder gravitationally, the Earth on the Moon, or the Moon on the Earth? Which accelerates more?

Both the same; the Moon.

Earth's orbit around the Sun is slightly elliptical. Thus, Earth actually gets closer to the Sun during part of the year. What happens to Earth's orbital speed when it is closer to the Sun?

Earth's orbital speed is greater when it is closer to the Sun than when it is farther from the Sun.

At what point does the ball have the most energy? Ignore air resistance

Everywhere! it is equal throughout!

Two blocks of ice, one four times as heavy as the other, are at rest on a frozen lake. A person pushes each block the same distance d. Ignore friction and assume that an equal force F is exerted on each block. What is true about the kinetic energy of the heavier block after the push?

It is equal to the kinetic energy of the lighter block The work-energy theorem states that the change in kinetic energy of an object equals the net work done on that object. The only force doing work on the blocks is the force from the person, which is the same in both cases. Since the initial kinetic energy of each block is zero, both blocks have the same final kinetic energy.

An object is lifted from the surface of a spherical planet to an altitude equal to the radius of the planet. As a result, what happens to the mass and weight of the object?

Mass remains the same; weight decreases

When a force acts on an object, what does the object's change in momentum depend upon?

The change in momentum depends upon the change in the velocity of the object. The change in momentum depends upon the magnitude of the force. The change in momentum depends upon the mass of the object. The change in momentum depends upon the time interval during which the force acts.

While driving face around a sharp right turn, you find yourself pressing against the car door. What is happening?

The door is exerting a rightward force on you.

A block of mass m is pushed up against a spring with spring constant k until the spring has been compressed a distance x from equilibrium. What is the work done on the block by the spring?

The equation for work presented in this problems requires that the force be constant. Because the force exerted on an object varies with the spring's displacement from equilibrium (F = kx) you cannot use W = Fdcosθ to find the work done by a spring. In actuality the work done by a spring is given by the equation Wspring = -1/2 kx^2

Two objects attract each other gravitationally. If the distance between their centers decreases by a factor of 2, how does the gravitational force between them change?

The gravitational force increases by a factor of 4.

Now assume that both blocks have the same speed after being pushed with the same force F. What can be said about the distances the two blocks are pushed?

The heavy block must be pushes 4 times farther than the light block. Because the heavier block has four times the mass of the lighter block, when the two blocks travel with the same speed, the heavier block will have four times as much kinetic energy. The work-energy theorem implies that four times more work must be done on the heavier block than on the lighter block. Since the same force is applied to both blocks, the heavier block must be pushed through four times the distance as the lighter block.

What represents the impulse of the force in a graph of force versus time?

The impulse is equal to the area under the curve.

Two identical balls are thrown vertically upward. The second ball in thrown with an initial speed that is twice that of the first ball. How does the maximum height of the two balls compare?

The maximum height of the second ball is four times that of the first ball.

An object moves in a circular path at a constant speed. What is the direction of the net force acting on the object?

The net force is directed toward the center of the circular path.

If the net work done on an object is positive, what can you conclude about the object's motion?

The object is speeding up

If the net work done on an object is zero, what can you determine about the object's kinetic energy?

The object's kinetic energy remains the same.

Neglecting air resistance, which statement is not true regarding the energy of the ball?

The potential energy decreases while the ball is going up

The following statements are true...

The total mechanical energy of a system is constant only if conservative forces act. Mechanical energy can be dissipated to nontechnical forms of energy.

Two objects are moving at equal speed along a level, frictionless surface. The second object has twice the mass of the first object. They both slide up the same frictionless inline plane. Which object rises to a greater height?

The two objects rise to the same height.

An object moves in a circular path at a constant speed. What is the relationship between the directions of the object's velocity and acceleration vectors?

The velocity and acceleration vectors are perpendicular.

The work-energy theorem states that a force acting on a particle as it moves over a ____ changes the ____ energy of the particle if the force has a component parallel to the motion.

distance/kinetic It is important that the force have a component acting in the direction of motion. For example, if a ball is attached to a string and whirled in uniform circular motion, the string does apply a force to the ball, but since the string's force is always perpendicular to the motion it does no work and cannot change the kinetic energy of the ball.

To illustrate the work-energy concept, consider the case of a stone falling from xi to xf under the influence of gravity. Using the work-energy concept, we say that work is done by the gravitational ____, resulting in an increase of the ____ energy of the stone.

force / kinetic

The Moon does not crash into the Earth because:

it is freely falling but it has a high tangential velocity

A motorcycle drives up a steeply inclined ramp. The work done on the motorcycle by the Earth's gravitational force is ____.

negative. When an object's displacement have a component in a direction opposite that of a given force, that force does negative work on the object.

To calculate the change in kinetic energy, you must know the force as a function of ____. The work done by the force causes the kinetic energy change.

position

This process happens in such a way that total mechanical energy, equal to the ____ of the kinetic and potential energies, is ____.

sum / conserved

During a certain time interval, the net work done on an object is zero joules. We can be certain that ____.

the object's final speed was the same as its initial speed. When the net work done on an object is zero, there is no overall change in the object's kinetic energy.

Two identical cars are driving in opposite directions at the same speed. Their kinetic energies have ____.

the same magnitude and sign. The kinetic energy of a moving object is always positive, regardless of the direction it is moving in, or the coordinate system used.

Compared to the speed of the heavier block, what is the speed of the light block after both blocks move the same distance d?

twice as fast Since the kinetic energy of the lighter block is equal to the kinetic energy of the heavier block, the lighter block must be moving faster than the heavier block.

A graph depicts force versus position. What represents the work done by the force over the given displacement?

The work done is equal to the area under the curve

Why does a satellite in a circular orbit travel at a constant speed?

There is no component of force acting along the direction of motion of the satellite.

The planet Earth travels in a circular orbit at constant speed around the Sun. What is the net work done on the Earth by the gravitational attraction between it and the Sun in one complete orbit? Assume that the mass of the Earth is given by Mc, the mass of the Sun is given by Ms, and the Earth-Sun distance is given by Res.

W = 0 An object undergoing uniform circular motion experiences a net force that is directed in toward the center of the circle; this net force is called the centripetal force. This force is always perpendicular to the distance the object moves and therefore never does any work on the object.

Rather than ascribing the increased kinetic energy of the stone to the work of gravity, we now (when using potential energy rather than work-energy) say that the increased kinetic energy comes from the ____ of the ____ energy.

change / potential

You have just moved into a new apartment and are trying to arrange your bedroom. You would like to move your dresses of weight ,500 N across the carpet to a spot 5 m away on the opposite wall. Hoping to just slide your dresser easily across the floor, you do not empty your bother out of the drawers before trying to move it. You push with all your might but cannot move the dresser before becoming completely exhausted. How much work do you do on the dresser?

W = 0 J Remember that to a physicist work means something very specific, and since you were unable to move the dresser, d = 0 and therefore W = 0. However, you got tired and sweaty trying to move the dresser, just as you do when you go to "work out" at the gym. Your muscles are not static strips of fibrous tissue. They continually contract and expand a slight amount when you exert them. Chemical energy from food is being transformed in to the energy needed to move your muscles. Work is being done inside your muscles, but work is not being done on the dresser.

A box of mass m is sliding down a frictionless plane that is inclined at an angle θ above the horizontal. What is the work done on the box by the force due to gravity w, if the box moves a distance d?

W = wdcos(90 - θ) The angle given to you in a problem is not always the same angle that you use in the equation for work!

A ball is thrown straight upward. How does the sign of the work done by gravity while the ball is traveling upward compare with the sign of the work done by gravity while the ball is traveling downward?

Work done by gravity is negative while the ball is traveling upward and positive while the ball is traveling downward.