Work
Work depends on __________ and __________.
Force and distance
Both kinetic energy and gravitational potential energy depend on _________________.
mass
The _________ energy of a system is always __________.
total, constant
total, conserved
total, the same
The total energy of a system is 100 J. When the kinetic energy is 20 J, then the gravitational potential energy is _____ J.
E = KE + PE
100 = 20 + PE
PE = 80 J
An efficient device produces more useful/wasted energy and less useful/wasted energy.
useful, wasted
A person tries to push a wall, but it does not move. What is the work done on the wall? Explain your answer.
The work done is 0 J, because W = Fd and d=0m.
What is the gravitational potential energy of a 50-kg book if it is on a shelf 6 m from the ground? Include units in your answer.
3000 J
See question #1
C, A
See question #5
KE = 0 J (it's not moving, so v = 0 m/s)
E = 40,000 J (it's the sum of KE and PE)
800 J
A dog uses 50N to pull a sled 45m. How much work does the dog do? Include units with your answer!
W = Fd
W = 50*45
W = 2250 J
What is the total mechanical energy of a 0.4-kg ball rolling on the floor at 1.7 m/s? Include units in your answer!
0.58 J
See question #2
Gravitational potential energy transforms into kinetic energy.
See question #6
KE = 15,000 J (The total is 40,000 J and 25,000 J of it is potential. The remainder, 15,000 J, is kinetic.)
E = 40,000 J (Total energy at A was 40,000 J and total energy stays constant.)
See question #10
0.6 or 60%
A box weighs 30 N and is at rest on a table. A person pushes the box horizontally across the table with a force of 50 N a distance of 1.2 m. What is the work done on the box? Include units with your answer!
W = Fd
W = 50*1.2
W = 60 J
What is the total mechanical energy of a 75-kg person running at 2.1 m/s if she is on a hill 8 m high? Include units in your answer!
6165 J
See question #3
Kinetic energy transforms into potential energy.
See question #7
20,000 J and 20,000 J
See question #11
Decrease the friction between the rope and the pulley by using smoother ropes/surfaces or lubricating them. This way, less energy is transformed into heat and more energy is transformed into gravitational potential energy.
A student lifts a 5.2-kg box directly upward by 0.8m. What is the work done by the student? Include units with your answer!
F = mg
F = 5.2*10
F = 52 N
W = 52*0.8
W = 41.6 J
A golf ball has a kinetic energy of 25 J. What is its velocity if its mass is 0.5 kg? Include units in your answer!
10 m/s
See question #4
The 2,000 J transformed into heat and sound as the cart rolled on the track. This energy is now in the track and in the air.
See question #8
20 m/s
See question #12
Eout is the potential energy the box has after it is lifted up, so Eout = mgh = 5*10*3 = 150 J.
eff = Eout/Ein = 150/200 = 0.75
%eff = 0.75*100% = 75%