forces exerting?

Question

What force is Catalina exerting on her bike, if she pushes it 15 meters for 30 seconds, and transfers 40 watts of power to the bike?

1. 20 newtons
2. 40 newtons
3. 60 newtons
4. 80 newtons

What is the mass of a garbage truck moving at 2 meters per second and having a kinetic energy of 5,000 joules?

1. 1,250 kilograms
2. 2,500 kilograms
3. 5,000 kilograms
4. 20,000 kilograms

At what height on a bookshelf is a science book which has a mass of 5 kilograms and a potential energy of 98 Joules?

1. 1 meter
2. 2 meters
3. 3 meters
4. 4 meters

Answer 1

For your first one, there are two equations to know: power (P = W/t) and work (W = Fd). You have enough information to use both of these to find the force applied on the bike:

P = W/t
40 W = W/30 s
W = 1200 Nm

W = Fd
1200 Nm = F(15 m)
F = 80 N

For your second one, the equation for kinetic energy is KE = 0.5mv^2:

KE = 0.5mv^2
5000 J = 0.5m(2 m/s)^2
5000 J = m(2 m^2/s^2)
m = 2500 kg

For your last one, the equation for potential energy is PE = mgh:

PE = mgh
98 J = (5 kg)(9.8 m/s^2)h
h = 2 m