Unit 4 Physics

How many joules of energy are used by a 1.0 hp motor that runs for 1.0 hr? (1 hp = 746 W)

2.7 ×106 J

A 400-N box is pushed up an inclined plane. The plane is 4.0 m long and rises 2.0 m. If the plane is frictionless, how much work was done by the push?

800 J

A worker does 25 J of work lifting a bucket, then sets the bucket back down in the same place. What is the total net work done on the bucket?

0 J

A driver, traveling at 22 m/s, slows down her 2000 kg car to stop for a red light. What work is done by the friction force against the wheels?

-4.84×105 J

If the mass in a horizontal mass-spring system was doubled, the elastic potential energy in the system would change by a factor of

0 (no change)

A 50-N object was lifted 2.0 m vertically and is being held there. How much work is being done in holding the box in this position?

0 J

A child moving at constant velocity carries a 2 N ice-cream cone 1 m across a level surface. What is the net work done on the ice-cream cone?

0 J

A pendulum of length 50 cm is pulled 30 cm away from the vertical axis and released from rest. What will be its speed at the bottom of its swing?

1.4 m/s

An object hits a wall and bounces back with half of its original speed. What is the ratio of the final kinetic energy to the initial kinetic energy?

1/4

A toy rocket, weighing 10 N, blasts straight up from ground level with a kinetic energy of 40 J. At the exact top of its trajectory, its total mechanical energy is 140 J. To what vertical height does it rise?

10 m

. A skier, of mass 40 kg, pushes off the top of a hill with an initial speed of 4.0 m/s. Neglecting friction, how fast will she be moving after dropping 10 m in elevation?

15 m/s

A roller coaster starts from rest at a point 45 m above the bottom of a dip (See Fig. 6-2). Neglect friction, what will be the speed of the roller coaster at the top of the next slope, which is 30 m above the bottom of the dip?

17 m/s

A roller coaster starts at a point 30 m above the bottom of a dip with a speed of 25 m/s (See Fig. 6-2). Neglect friction, what will be the speed of the roller coaster at the top of the next slope, which is 45 m above the bottom of the dip?

18 m/s

A roller coaster starts with a speed of 5.0 m/s at a point 45 m above the bottom of a dip (See Fig. 6-2). Neglect friction, what will be the speed of the roller coaster at the top of the next slope, which is 30 m above the bottom of the dip?

18 m/s

A brick is moving at a speed of 3 m/s and a pebble is moving at a speed of 5 m/s. If both objects have the same kinetic energy, what is the ratio of the brick's mass to the rock's mass?

25 to 9

Ball A has triple the mass and speed of ball B. What is the ratio of the kinetic energy of ball A to ball B

27

A 1.0-kg ball falls to the floor. When it is 0.70 m above the floor, its potential energy exactly equals its kinetic energy. How fast is it moving?

3.7 m/s

A 60-kg skier starts from rest from the top of a 50-m high slope. If the work done by friction is -6.0×103J, what is the speed of the skier on reaching the bottom of the slope?

34 m/s

The kinetic friction force between a 60.0-kg object and a horizontal surface is 50.0 N. If the initial speed of the object is 25.0 m/s, what distance will it slide before coming to a stop?

375 m

If the displacement of a horizontal mass-spring system was doubled, the elastic potential energy in the system would change by a factor of

4

A 0.200-kg mass attached to the end of a spring causes it to stretch 5.0 cm. If another 0.200-kg mass is added to the spring, the potential energy of the spring will be

4 times as much.

Car J moves twice as fast as car K, and car J has half the mass of car K. The kinetic energy of car J, compared to car K is

4 to 1

An 800-N box is pushed up an inclined plane. The plane is 4.0 m long and rises 2.0 m. It requires 3200 J of work to get the box to the top of the plane. What was the magnitude of the average friction force on the box?

400 N

A 10-N force is needed to move an object with a constant velocity of 5.0 m/s. What power must be delivered to the object by the force?

50 W

A cyclist does work at the rate of 500 W while riding. How much force does her foot push with when she is traveling at 8.0 m/s?

63 N

At what rate is a 60.0-kg boy using energy when he runs up a flight of stairs 10.0-m high, in 8.00 s?

735 W

In which of the following scenarios is work done?

A car decelerates while traveling on a flat stretch of road

In which of the following scenarios is no net work done?

A car travels at constant speed on a flat road.

Work is done when

a the displacement is not zero.

Gravitational potential energy is always measured in relation to

a zero level

Which of the following are not units of power?

J

An acorn falls from a tree. Compare its kinetic energy K, to its potential energy U

K increases U decreases

A ball falls from the top of a building, through the air (air friction is present), to the ground below. How does the kinetic energy (K) just before striking the ground compare to the potential energy (U) at the top of the building?

K is less than U

Which of the following formulas would be used to directly calculate the kinetic energy of an object with mass m bouncing up and down on a spring with spring constant k?

KE= 1/2mv^2

What are the units for a spring constant?

N/m

Can work be done on a system if there is no motion?

No, because of the way work is defined.

In which of the following sentences is work used in the scientific sense of the word?

Sam and Rachel pushed hard, but they could do no work on the car.

A truck weighs twice as much as a car, and is moving at twice the speed of the car. Which statement is true about the truck's kinetic energy compared to that of the car?

The truck has 4 times the kinetic energy of the car

Which of the following is a true statement about the conservation of energy?

Total energy is always conserved.

What is the common formula for work? Assume that W is the work, F is a constant force, ∆v is the change in velocity, and d is the displacement.

W = Fd

You throw a ball straight up. Compare the sign of the work done by gravity while the ball goes up with the sign of the work done by gravity while it goes down.

Work is - on the way up and + on the way down.

Is it possible for a system to have negative potential energy?

Yes, since the choice of the zero of potential energy is arbitrary.

What is the correct unit of power expressed in SI units?

a kg m2 /s3

On a plot of Force versus position (F vs. x), what represents the work done by the force F?

area under curve

Describe the energy of a car driving up a hill.

both kinetic and potential

A 4.0-kg mass is moving with speed 2.0 m/s. A 1.0-kg mass is moving with speed 4.0 m/s. Both objects encounter the same constant braking force, and are brought to rest. Which object travels the greater distance before stopping?

both travel same distance

If the net work done on an object is negative, then the object's kinetic energy

decreases.

To accelerate your car at a constant acceleration, the car's engine must

develop ever-increasing power

The quantity 1/2 kx2 is

he elastic potential energy of the object.

. If the net work done on an object is positive, then the object's kinetic energy

increases

A force does work on an object if a component of the force

is parallel to the displacement of the object.

What is the correct unit of work expressed in SI units?

kg m2 /s2

Which of the following energy forms is associated with an object in motion?

kinetic energy

The main difference between kinetic energy and potential energy is that

kinetic energy involves motion, and potential energy involves position

A ball drops some distance and gains 30 J of kinetic energy. Do not ignore air resistance. How much gravitational potential energy did the ball lose?

more than 30 J

A container of water is lifted vertically 3.0 m then returned to its original position. If the total weight is 30 N, ho w much work was done?

no work was done

Which of the following energy forms is associated with an object due to its position?

potential energy

Which of the following is the rate at which energy is transferred?

power

If you push twice as hard against a stationary brick wall, the amount of work you do

remains constant at zero.

If the sign of work is negative,

the displacement is in the direction opposite the force.

The quantity mgh is

the gravitational potential energy of the object.

The quantity 1/2 mv2 is

the kinetic energy of the object

The quantity Fd/t is

the power supplied to the object by the force.

Consider two masses m1 and m2 at the top of two frictionless inclined planes. Both masses start from rest at the same height. However, the plane on which m1 sits is at an angle of 30° with the horizontal, while the plane on which m2 sits is at 60°. If the masses are released, which is going faster at the bottom of its plane?

they are going same speed

Of the following, which is not a unit of power?

watt/second

The area under the curve, on a Force versus position (F vs. x) graph, represents

work

If you walk 5.0 m horizontally forward at a constant velocity carrying a 10-N object, the amount of work you do is

zero