Question: A force is exert on an object without doing work on the object

Answer: so if no distance moved, there is no work done.

Ex. pushing on a wall

W=Fd

Question: If both sacks in the preceding question are lifted their respective distances in the same time, how does the power required for each compare?

Answer: Since the work done is the same, power is the same. P=w/t P=Fd/t

Question: Two cars are raised to the same elevation on service-station lifts. If one car is twice as massive as the other, how do their pontential energies compare?

Answer: the car with twice the mass will have twice the potential energy. PE=mgh PE~m

Question: A moving car has kinetic energy. If it speed up until it is going four time as fast, how much kinetic energy does it have in comparison?

Answer: KE~V^2

If the new V=4 times, V=4, KE=(4)^2=16 times the original KE.

Question: Compared with some original speed, how much work must the brakes of a car supply to stop a car that is moving four times as fast? How will the stopping distance compare?

Answer: a)

, W=1/2m(V)^2, W~V^2, W=(V)^2

if new V=4 times, W=(V)^2=(4)^2= 16 times the original work.

b)

,d=(V)^2=(4)^3=16 times the original distance.

Question: An apple haging from a climb has potential energy because of its height. If it falls, what becomes of this energy just before it hits the ground? When its hit the ground?

Answer: As the apple falls, PE becames KE

when it hits the ground KE becomes force.

(W=KE, W=Fd, KE=1/2mv^2, Fd=1/2mv^2)

Question: Can a machine multiply input force? Input distance? Input energy?

Answer: A machine can multiply input force and distance but not energy.

Question: A force of 50N is applied to the end of a lever, which is moved a certain distance. If the other end of the lever moves one-third as far, how much force can it exert?

Answer: Fd=fD, d=1, d=1/3, f=50N, F=?

F=fD/d=50N(1)/1/3=150N

Question: What is the efficiency of a machine that miraculously converts all the input energy to useful output energy?

Answer: An automobile.

Efficiency=output/input=100/100*100= 100% efficient.

Question: If a moving object doubles its speed, how much more momentum does it have? How much more energy?

Answer: a)momentum=mv, momentum~velocity if v doubles, momentum double.

b) KE=V^2=(2)^2=4 times the original KE.