Here are some practice problems I found that you might use to help prepare
for the Energy Test when we return to school. I would suggest working the
problem out yourself then checking your answer after.
Work= Force x distance
(Unit is a Newton-meter but is also called a Joule, when we are referring to energy)
Power = Work divided by time
SOOOOO… Nm (Joules) divided by seconds so the unit for power is Watts.
1. A student was rolling a pencil on the table when startled by the teacher calling their name.
They pushed it off and it fell to the floor. Calculate the work done by a 47 N force pushing a
pencil 0.26 m across the table.
Work = force * distance
W = 47 N x 0.26 m
W = 12.22 Nm
47
x 0.26
282
+940
12.22
2. Calculate the work done by a 47 N force left pushing a 0.025 kg pencil 0.25 m against a
sliding frictional force of 23 N. (What is the net force? You must figure net force before you
calculate for work)
Net force =
Net=
47 – 23 = 24
24 N, left
Work = Force x distance
W = 24 N * 0.25 m
W = 6.0 Nm
24
x 0.25
120
+ 480
.600
3. Tommy hated tuna fish. When opening his sack lunch he discovered that his dad had
packed tuna for his lunch! In disgust, he pushed the sandwich to his friend Carl. Calculate
the work done by a 2.4 N force pushing a 400 g sandwich across a table 0.75 m wide.
W=Fxd
W = 2.4 N * 0.75 m
W = 1.8 Nm
2.4
x 0.75
120
+ 1680
1.800
4. How far can a mother push a baby carriage, using a force of 62 N, if she can only do 2930
J of work? (Hint: 1 Joule is 1 Newton-meter; use your work formula to discover distance)
Work = F x d
2920 Nm = 62 N * d
45.781 m = distance
64 2930.000
-256
370
-320
500
- 448
5. David’s job requires him to stack bags of potatoes at the store. For each bag of potatoes,
how much work is it to lift a 20 N sack of potatoes vertically 6.5 m?
W=F*d
W = 20 N * 6.5 m
W = 130 Nm
20
x 6.5
130
6. A girl pushes her little brother on the sled with a force of 300 N for 750 m. If he pushed her
the same distance and his force was 200 N, how much work did they each perform? (10 pts)
(a) 300 N?
(b) 200 N?
W = 300 N * 750 m
W = 200 N * 750 m
W = 225,000 Nm
W = 150,000 Nm
7. A 75.0 kg teenager pushes on a 500,000 N wall for 250 s but it does not move. How much
work does he do on the wall?
Work = force * distance
Work = 0 Nm
because the wall did not move. The distance would be zero, and anything multiplied by zero is
zero.
8. Samantha uses a force of 5.0 N moves a block along a rough floor for 10 meters at a
constant speed of 2.5 m/s.
(a) Calculate how much work is done by Samantha.
W=F*D
W = 5 N * 10 m
W = 50 Nm
(b) Calculate power being used in the 2.5 secs (Write your answer as Watts)
P=W/T
P = 50 Nm / 2.5 s
P = 20 Watts
9. While training for breeding season, a 0.38 kg male squirrel does 32 pushups in 50 seconds,
displacing its center of mass by a distance of 8.5 cm for each pushup. Determine the total
work done on the squirrel while moving upward (32 times).
0.38kg * 9.8 m/s/s (force of gravity)
3.724 N (force of squirrel)
8.5 cm needs to be changed to meters = 0.085 meters
W=F*D
W = 3.724 N * 0.085 m
W = 0.316.4 Nm or j
* This is the work required for 1 pushup… he did 32, so we need to multiply the work by 32.
Work for 32 pushups = 0.31654 Nm * 32
Total Work = 10.12928 Nm
Rounded to about 10 Nm
10. Calculate the power the squirrel in the problem above uses to complete 32 pushups.
P=W/T
P = 10 Nm / 50 sec
P = 0.2 Watts
P = 0.2 Watts
11. A new conveyor system at the local packaging plan will utilize a motor-powered
mechanical arm to exert an average force of 890 N to push large crates a distance of 12
meters in 22 seconds. Determine the power output required of such a motor.
P = (F * D) / T
P = (890 * 12) 22
P = 485.45 Watts