### a woman carries a heavy box across a room at a constant speed. how much work does she do on the box while walking?

none

### when a rifle is fired it recoils as the bullet is set in motion. The rifle and bullet ideally acquire equal

amounts of energy but opposite amounts of momentum

### the total momentum of a flock of identical birds could be zero only if the birds are

flying in different directions

### if you push for a half hour or a whole hour against a stationary wall

no work is done on either case

### an open freight car rolls friction free along a horizontal track in a pouring rain that falls vertically. as water accumulates in the car, the car's speed

decreases

### two identicial arrows, one with twice the speed of the other, are fired into a hay bale. the faster arrow will penetrate

four times as far as the slower arrow

### two identical arrows, one with twice the kinetic energy of the other, are fired into a hay bale. the faster arrow will penetrate

twice as far as the slower arrow.

### if you do work on an object in one-third the usual time, your power output is

three times the usual power output.

### a car moves 4 times as fast as another identical car. compared to the slower car the faster car has

16 times the kinetic energy

### if you find that twice as much work is needed to perform a task but it takes as much time, the amount of power required is

unchanged

### a 1000-kg car and a 200-kg car are hoisted the same distance. raising the more massive car requires

twice as much work

### which has a greater kinetic energy, a car traveling at 30 km/hr of a car of half the mass traveling at 60 km/hr

the 60 km/hr car

### both a 50-kg sack is lifted 2 meters from the ground and a 25-kg sack is lifted 4 meters in the same time. the power expended in raising the 50-kg sack is

the same

### a 2-kg mass is held 4 m above the ground. what is the approximate potential energy of the mass with respect to the ground

80 j

### it takes 40 J to push a large box 4 m across a floor. assuming the push is the same direction as the move, what is the magnitude of the force on the box

10 N

### a machine puts out 100 watts of power for every 1000 watts put into it. the efficiency of the machine is

10%

### a diver who weighs 500 N steps off a diving board that is 10 m above the water. the diver hits the water with kinetic energy of

5000 J

### a 2 kg mass has 40 J of potential energy with respect to the ground. Approximately how high is it above the ground

2 m

### using 1000 J of work, a toy elevator is raised from the ground floor to the second floor in 20 seconds. the power needed to do this job was

50 w

### a pully system raises a 1000-N load 2 meters with 100 N of input force. The efficiency of the system is

not enough information given

### an object lifted 10 meters gains 200 J of potential energy. if the same object is lifted 20 meters its potential energy gain is

twice as much

### two identical 5000-kg bulldozers roll toward each other on a level road without friction (one at 1m/s, the other at 2m/s) just before they collide, one has a kinetic energy of 2500 J and the other 10000J this instant their total kinetic energy is

12,500 J

### a 2500 N pile driver ram falls 10 m and drives a post 0.1 m into the ground. the average impact force on the ram is

250,000 N

### a bow is drawn so that it has 40 J of potential energy. when fired, the arrow will ideally have a kinetic energy that is

40 J

### a car moving at 50 km/hr skids 20 m with locked brakes. how far will the car skid with locked brakes if it were traveling at 150 km/hr

180 m

### a jack system will increase the potential energy of a heavy load by 1000 J with a work input of 2000 J. the efficiency of the jack system is

50%