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Energy Unit Review Sheet
Physics
Work
Know the definition of work - W = Fd (when F and d point along the same direction)
Be able to recognize situations where work is being done
Be able calculate the work done by any force in a given situation
Know how changes in the force or distance will effect work output
Be able to calculate net work in cases where friction plays a role
Energy
Know the definition of energy - Energy is the ability to do work
Be able to explain how work effects energy - Work changes an object’s energy. When work is done on the
object, the object gains energy, when the object does work it loses energy
Be able to calculate the change in any type of energy based on the amount of work done in a situation
Be able to calculate the amount of work done based on the change in energy in any situation
Be able to calculate the change in kinetic energy from the work-kinetic energy theorem: Wnet = KE. In
other words: Wnet = Kf-Ki or Wnet = (½)mvf2 - (½)mvi2
Know and be able to identify the different types of energy: potential (elastic and gravitational), kinetic &
thermal
Be able to identify object that have potential energy (any kind), kinetic energy or thermal energy
Be able to calculate the gravitational potential energy of a particular object - GPE = mgh
Know how changing an object's mass or height will affect GPE
Be able to calculate the kinetic energy of a given object at a given speed - KE = (½)mv2
Know how changing an object's mass or velocity will affect KE
Conservation of Energy
Be able to state the Law of Conservation of Energy in your own words
Be able to explain how conservation of energy applies to a given situation
Be able to use conservation of energy with kinetic energy and gravitational potential energy to find the height
or speed of an object at any point.
Be able to use conservation of energy to find height or speed of an object at any point along a hill when
friction is involved.
Be able to calculate the work done by friction to find the thermal energy produced by the friction.
Know how work changes an object’s energy and still conserves energy
Power
Know the definition of power - P = W/t
Be able to explain the difference between Work and Power
Be able to calculate the power that is output in a given situation
Know how changes in the work or time will effect power output
Review Questions:
1. A construction worker lifts a 4.5 kg cement block 1.2 m off the ground and then carries is 7.3 m to where it
is needed.
a) How much work is done by the person in this process?
b) How much work is done by gravity in this process?
2. A boat pulls a parasailer 500 m forward at constant elevation by pulling with a force of 1000 N at 40 degrees
below horizontal.
a) How much work does the boat do on the parasailer?
b) How much work does gravity do on the parasailer?
3. A pitcher pushes the 0.145 kg ball 97 N for 2.5 m while throwing a pitch. If the ball starts out at rest, how
fast is it moving when the pitcher releases it?
4. A horse draws a 200 kg sleigh across the snow by pulling directly forward on it against 392 N of friction. If
the horse accelerates the sleight from rest to 5 m/s while the sled pulls forward 10 m, how hard does the
horse have to pull?
5. The pilot of a 500 kg motorboat speeds the boat from 3 m/s to 10 m/s. The engine exerts 5,000 N of force
while traveling 65 m during the acceleration. How strong is the force of drag (water resistance) against the
boat’s motion?
6. A truck hauls an 800 kg boat out of the water and 100 m along a 15° ramp. Assume that friction is
negligible.
a) How much work does the truck do hauling the boat up the ramp?
b) How much gravitational potential energy does the truck gain as it goes up the ramp?
7. In a game of dodgeball, Danielle throws a 0.33 kg ball at Erica. The ball hits Erica with 375 N of force
while it compresses and gets 12 cm smaller before it comes to a complete stop.
a) How much elastic potential energy does the ball have when it is fully compressed?
b) Assuming nothing is lost to thermal energy, how much kinetic energy did the ball have to have before it ran
into Erica?
c) How fast was the ball moving before it hit Erica?
8. A trebuchet launches a 5,000 kg rock from a height of 60 m going 100 m/s.
a) If the top of its arc carries it 400 m off the ground, how fast is it moving at the top of its arc?
b) How fast will it be moving when it is just about to hit the ground?
c) If the rock slides 500 m on the ground as it skids to a stop, how big is the force of friction on it?
9. A rope swing hangs from a tall branch on a cliff over a small lake. Ryan, 60 kg, swings from the cliff 10 m
above the surface of the lake.
a) If they let go of the rope at the bottom of the swing, when they are 2 m above the surface of the lake, how
fast will they be going when they let go?
b) If they let go of the rope when they are halfway up the far side of the swing (how high above lake?), how
fast will they be going when they let go?
10. Chantelle becomes ski jumper. During one run, she starts and rest and lets her 80 kg body (with gear) drop
down the 105 m high (above the ground below) slope. She slides down the 45° ramp and launches into the
air 73 meters off the ground.
a) Assuming that the skis are really well waxed (friction is negligible), how fast will she be going when she
launches off the ski jump?
b) If the jumper experiences 55 N of friction as they slide down the ramp, how fast will they be going when
they launch off the ski jump?
11. Calvin and Hobbes ride in their wagon down a 20° sloped hill. The top of the hill is 50 m above the ground
below. If the mass of the two friends and their wagon is 50 kg and the coefficient of friction against the
wagon is 0.15, how quickly will they be going when they reach the bottom of the hill (if they reach the
bottom of the hill)?
12. Alissa, with 60 kg of mass, goes bungee jumping off a 30 m tall bridge with an 8 m long bungee cord. As
she reaches the bottom of the jump, the cord has stretched to 28 m long.
a) How much energy does the bungee cord have when it is stretched? What type? (Where did it come from?)
b) How much force does Alissa exert on the bungee cord exert to make it stretch?
c) As cord pulls the jumper back up from the lowest point, what types of energy transformations occur? (In
other words, what types of energy does the bungee cord’s energy become?)
d) When Alissa is 10 m off the ground, how fast is she moving?
e) How fast will Alissa be moving when the bungee cord returns to its original length and stops pulling her
upward?
13. In a war zone, a tank fires a shell at an abandoned Jeep sitting in the middle of the road. The 7 kg shell is
going 1600 m/s when it hits the 1200 kg Jeep and has an inelastic collision.
a) How fast is the Jeep going after it gets hit with the shell? [Hint: what about the shell?]
b) How much thermal energy is created in the collision between the shell and the Jeep?
14. Kelly is doing some carpentry to fix up her
porch. She has a 4 m long board, which she
balances on a sawhorse until she needs it. She
ends up resting a 5 kg box of nails on one end
of the board and putting 5 kg of bricks on the
other end to keep it balanced. As she stops to
take a break, she puts her 2 kg hammer down
on top of the box of nails, upsetting the balance. How fast is her hammer moving when it reaches the
ground? [hint: What else is moving? How fast is it moving?]
15. Sari pushes the lawnmower while cutting a 50 m long strip of grass in 18 seconds. If she pushes with 90 N
to keep the lawnmower going at a constant speed, how much power does she have to produce?
16. A portable space heater uses 1500 W.
a) How much energy does it take to run the heater for 1 hour?
b) How much energy does it take to run the heater for 1 day?How much work does a
17. A 1400 kg Porsche Carrera GT is rated to produce 605 horsepower (451,148.4 W). Assuming that its
power output is constant, how long will it take to accelerate from rest to 26.8 m/s (60 mph)?
18. A team of huskies exerts 1000 N of force forward on a sled to haul it at a constant speed of 5 m/s over a
distance of 10.5 km.
a) How much work will they do on the sled?
b) How much work will friction do on the sled?
c) What is the net work on the sled?
d) How much does the sled’s kinetic energy increase?
e) How much power do the dogs put out (Hint: you have to solve for time!)