1. A boy pushes a car to slow it down while the car is rolling horizontally down the road. If the boy pushes backwards on the car with 10N of force as the car rolls forward 3.0m, the amount of work done by the boy is:

Question

a)-30
b)0
c)3.3 J
d)30 N•m
e)300J
my answer: a

2. what is the speed of a 5.0 kg ball if its kinetic energy is 40J?
a)2.8 m/s
b)4 m/s
c)8 m/s
d)16 m/s
e)200 km/h
my answer: b

3. The amount of energy required to raise a 20 kg child onto a counter top that is 1.25 m off the ground is:
(a) 30 J
(b) 0.25 N
(c) 245 J
(d) 30 N
(e) 0 J
my answer: c

4. If mechanical energy is conserved, then
(a) the sum of the kinetic energy and gravitational potential energy remains constant.
(b) the amount of kinetic energy is constant.
(c) thermal energy losses due to friction are constant.
(d) the amount of gravitational potential energy is constant. (e) none of the above.
my answer: a

5. A cyclist on a bike has 4 100 J of energy. If the bicycle and cyclist weigh 82 kg the velocity of the cyclist is:
(a) 100 km/h
(b) 50 km/h
(c) 100 m/s
(d) 10 m/s
(e) 0 m/s
my answer: d

Answer 1

1. My answer is zero, because the car did not move backwards. However, you should get a 2nd opinion.

2. KE = 0.5*m*V^2 = 40 Joules
KE = 0.5*5*V^2 = 40
2.5V^2 = 40
V^2 = 16
V = 4 m/s.

3c.

4a. PE + KE = Constant

5. I don't know what you mean by 4 100 J
If you mean 400 J. the answer is d.