Energy Review Answer Key
Practice:
1. What are the two categories of energy?
a. ___Potential______________ and _____Kinetic______________
2. If an object is moving, can it have both potential and kinetic energies?
a. __Yes_ (Yes/No) because ______The object could be moving down a hill, then it has potential to fall
more, so it has potential energy too.___________________
3. What are the units for kinetic energy?
a. Joule (J)
4. What are the units for potential energy?
a. Joule (J)
5. If a person riding a bike has a mass of 200 kg and is moving at a velocity of 5 m/s, what is their kinetic energy?
KE =
KE = 2500 J
6. If you pick up a 10 N book a distance of 2 meters, how much gravitational potential energy did you give it?
GPE = weight x height
GPE = 10 x 2
GPE = 20 J
Practice:
1. If a ball rolling down a hill has a GPE of 200 J and its KE is 300 J, what is the mechanical energy?
ME = GPE + KE
ME = 200 J + 300 J
ME = 500 J
2. Use the picture below to solve for the following questions:
a. Find the ME at point A.
So … GPE = 500 x 10
GPE = 5000 J
This skier is at the top of the hill and isn’t moving yet, so his kinetic energy = 0 J.
To find ME, add together the GPE + KE.
ME = 5000 J + 0 J
ME = 5000 J (Don’t forget the units!)
b. GPE at Point B = _____________________
Mechanical energy is what we are trying to solve for, but you aren’t given PE or KE! You need to
calculate these using the formulas you used in the first practice section. Here is how you would do it:
First write out what you do know for point B.
This point, only gives you one thing, height! But, would the weight of the skier change? No! So you know
that the skier has a weight of 500 N and that he is 5 m high. From here you can calculate how much GPE
he has. Remember that GPE = weight x height.
So … GPE = 500 x 5
GPE = 2500 J
Practice:
1. What energy transformations occur when you start a fire?
Chemical
Radiant
Mechanical
Thermal
Chemical is first because the wood has stored energy in it. The fire gives off light, so that is why radiant is there,
fires can also give off sounds, so that is why mechanical is there, and finally thermal because you can feel the
warmth coming from the fire. The last three did not have to be in the same order!
2. What energy transformations occur when you start your hair dryer?
Electrical
Mechanical
Thermal
Hair dryers are plugged into the wall, and that is why electrical is first. The hair dryer causes sound, so that is
why mechanical is involved, and you can feel the warmth again from the thermal energy.
3. What energy transformations occur when you toast a piece of bread?
Electrical
Thermal
Chemical
The toaster is plugged into the wall, and that is why electrical is first. The toaster uses thermal energy to heat up
the bread, and the bread changes color, which is a clue that a chemical reaction has taken place.
4. What energy transformations occur when you turn on speakers to play music (assume the speakers work using a
battery)?
Chemical
Electrical
Mechanical
Thermal
The speaker works by using a battery, which is chemical energy. Every time a battery is used, this is chemical
energy. The battery sends energy through electricity to get to the speaker, so electrical energy is second. The
electricity is changed into sound waves, which are mechanical waves and finally thermal energy is involved
because every energy transformation involves thermal energy due to friction (have you ever touched a battery
after it’s been used… it’s hot!)