Accelerated Physics Work, Energy and Power Review Problems
Name______________________________________________ Date ________
Energy Formulas
W = F d Cos
P = W/t = Fav v
KE = 1/2mv2
PEgrav = mgh
Fa = kx
PEspring = 1/2 k x2
PE + KE = WNC
1HP = 746W
Practice
1. The express elevator in the Sears Tower in Chicago averages a speed of 1800 ft/min in
its climb to the 103rd floor, 1340 ft above the ground. Assuming a total load of 1.0 x 103
kg, what average power must the lifting motor supply? Put your answer in Watts (W) and
horsepower (hp). Always use SI units.
Note: 3.281 ft = 1 m
2. A 5000 kg roller coaster train is winched up the first hill, which is 80 m high. What is I) the
speed of the train over the top of the second hill 70m off the ground and ii) the average
breaking force needed to stop the train if it has 100 m to pull into the station?
3. A coyote, whose mass is 20.0 kg, is chasing the roadrunner when the coyote accidentally
runs off the edge of a cliff and plummets to the ground 30.0 m below. What average force
does the ground exert on the coyote as he makes a coyote-shaped dent 0.42 m deep in
the ground?
4. The average food energy intake for humans is 2000 Calories/day. One dietary Calorie is
equivalent to 4186 J. How high would a 55 kg person have to climb in order to “burn off”
all the calories they consume in one day? (note; this calculation will ignore the large
amount of energy the body continually loses to heat)
5. A student pushes a 0.5 kg mass along a horizontal ramp according to the F-d graph
shown below. a) How much work did the student do on the mass? b) What speed did the
mass leave the student’s hand? c) If a constant friction force of 1N opposes the mass
after it leaves his hand, how far did it slide along the ramp? d) If the mass leaves the
student’s hand twice as fast, how far would it slide now? e) Does your answer to d)
depend on the mass of the object being slid? Explain!
3
F (N)
d (m)
1
6.
A “hot wheels” toy car launcher uses a spring powered device to launch a 30 g toy car
down a flat track on the floor and then up and over a loop before coming back down to
the track on the floor again. A spring with a force constant of 60 N/m is used in the
launcher and is compressed 10cm before firing the car.
a)
b)
Determine the speed of the car when fired from the launcher?
If the car goes over the top of the loop at 2.7 m/s, how high is the top of the loop above
the ground?
7.
A 70.0 kg base runner begins his slide into second base while moving at a speed of 4.0
m/s. The coefficient of friction between his clothes and Earth is 0.70. He slides so that his
speed is zero just as he reaches the base.
a) How much mechanical energy is lost due to friction acting on the runner?
b) How far does he slide?
8.
Calculate the power required of a 1400-kg car that climbs a 100 hill at a steady 80 km/h.
Assume that a friction force of 700 N acts on the car as it climbs.