INDEX CARD

Kinetic energy (joule) = KE = 0.5 mV²
work (joule) = energy transfered in a system = force(N) x distance (m) (if force is // displacement, no work is done)
change in energy kinetic = work done
Potential energy (joule) = PE = mgh
weight (N) = mg with g = 9.8m/s/s
Power (watt) = energy delivered in one second = energy/time = work / time
Energy is conserved if there is no friction: KE = PE .
Heat is a form of energy. Energy can change form like PE to KE and heat.
CONVERT to meters, seconds, kilograms


LAB: ENERGY IN A PEANUT

INTRODUCTION: REVIEW 6th GRADE SCIENCES

1) In middle school you learned the different form of energy : thermal energy , kinetic energy (energy of motion or energy of movement),  potential energy (energy due to position), nuclear energy (energy locked in the atoms), electrical energy ,
chemical energy (energy stored in atomic bonds) ... Use the previous forms to fill the blanks:
A) Suppose you are pushing a block. You cause the speed of the block to increase. Some of your ________ energy is transformed in __________ energy (the block moves faster). The work you are doing is a measure of how much energy is being transfered.
Your work causes the block's ____________ to change..

B) Inside the sun, fusion reactions take place and this releases a lot of energy. The plants use the ____________ energy of the sun to transform it in _________ energy. When we eat the plant, we get also _________ energy. When we move around, this energy is transformed (by burning some fat ) in __________ energy (you move) and in __________________ energy (you get warmer).

C) If you lift a book, you are doing work against the weight of the book. Your ___________ energy is being transformed into gravitational ____________energy.

D) At the top of  a hill, a rock has more ______________ than at the bottom. If the rock rolls  down all its _____________energy  is being transformed into _____________ energy.

E) In an electrical plant coal is used to heat water. The water vapor rises and turn a magnet in to a huge circuit (generator). When the magnet turns, electricity is produced. Billion years ago trees transformed the  _____________ energy of the sun into  __________energy.. When they died, they were buried in swamps and didn't decade. They were changed into coal over the years. The coal was  still containing the chemical energy of the plant. When coal is burned in a plant the ___________ energy is transformed into ___________energy . The water changes state and the vapor makes the wheel turns. The ___________ energy is then transformed into ______________energy . Electricity is produced. So Kinetic energy has been transfered into _____________ energy.
Conclusion: The electrical energy used in your home comes from the Sun ! of course a lot of the original energy was lost because transformed into thermal energy. The electric wires get hot, the wheel gets hot... no machine is 100% efficient transforming energy (input)  into another form (output). Any machine produces  heat (lost energy for us).

2) Observe the table and comment: (from Physics for presidents, Richard A. Muller)
1 food Calorie = amount of energy it takes to raise one kg of water by one degree celcius. = 4,200 J
1 calorie (used in chemistry) = amount of energy it takes to raise one g of water by one degree celcius.
1 can of soda (coka) = 200 food Calories  = 840 , 000 J = 840 KJ
You need 10J to lift 1 kg  1 meter.

object	energy in 1 g cal	Joules	compared to TNT
bullet (1000ft/s)	0.01	40J	0.015
battery (flashlight)	0.01		0.015
TNT	0.651	2,723J	1
Choocolate cookie	5	21KJ	8
butter	7	29KJ	11
gasoline	10	42KJ	15
CH4 (methane)	13	54KJ	20
fuel cell (hydrogen gas)	26	110KJ	40
asteroid (30km/s)	107	450KJ	165
uranium-235	20 million	84 billion J	30 million

- How do you extract gasoline energy in a car ?
- Why can't you use a chocolate cookie to blow off an old building ?
- What is the ratio between mass of gasoline and mass of battery (for the same energy delivered ) ?
that means: You can carry 100 lb of gasoline in a car. You will need 5 tons of batteries to get the same energy.

3) Did you know ? KWh is another unit for enery. (look at your electricity bill )
A) CAn you convert 1 KWh in joule ? ( E (J) = power (W) x time (s) )  1KWh = ___________________

B)  horse power was the unit for power in the 19th century. (when the first automobiles were made)
It is the amount of work a horse can do in one second. 1 horsepower = 745W (about 1000W) = 745 J /s
You can illuminate your house (medium one) brightly with 10 bulbs of 100 Watts during 1 second,
How many horse do you need to do the same job in one second ? (take 1 horsepower = 1,000 Watt= 1,000 J/s)

C) optional:  The Sun delivers 1KW/m² . If you want to use the energy of the Sun to go green and to produce
clean energy, you have to use cells. You will produce 150W per m². (not 100% efficient).
A nuclear plant can produce 50 GW. How large must be the cell area to produce the same energy ?
(in km²) (hint :1 km² = 1,000,000 m² = 10⁶ m²)

KINETIC ENERGY  (if you are a Math person , try without the hints)

Kinetic energy is the energy of motion. The formula is: Kinetic energy = KE = 0.5 m V²
Energy in joule, mass in kg and speed in m/s.
(write that on an index card)

For the kinetic energy of an object to increase, work has to been done on it.
THat is you need to apply a force F on the object over a distance d.
 W = ∆KE or F x d = ∆KE   ∆KE is the change in energy.
∆KE - KE2 - KE1 = final kinetic energy - initial kinetic energy
F in newton, d in meter and ∆KE in joule.
(write that on an index card)

2) A 1600kg car travels at a speed of 12.5 m/s. What is its kinetic energy?

3) Toni has a mass of 45kg and is moving with a speed of V1 = 10.0m/s.
A) find Toni's kinetic energy. Call it KE1 = _____________
B) Toni's speed changed to V2 = 5.0m/s. Now what is her new kinetic energy ? Call it KE2 = _________
C) what is the ratio of the kinetic energies in A and B ? Explain the ratio ?
hint:  ratio = KE1/  KE2  compare to (V1 / V2)

4)  Shawn and his bike have a total mass of 45.0kg. Shawn rides his bike d = 1.80km in t= 10.0 min at a constant velocity V. What is Shawn 's kinetic energy?
hint: first convert minutes to seconds and kilometers to meter. check your kinematics index cards to find the speed V  using d and t. then compute KE.

5) In the 1950s, an experimental train that had a mass of m = 2.50 10⁴kg was powered across a level track by a jet engine that produced a trust of F = 5.00 10⁵ N for a distance of  d = 509m.
A) Find the work done on the train
hint: use W = F x d
B) Find the change in kinetic energy ∆KE ?
hint: .work done on the train = change in energy ∆KE. Use A)

C) Find the final kinetic energy of the train if is started from rest.
hint: change in energy computed in C = final energy KE2 - initial KE1
D) Find the final speed of the train if there were no friction.
KE2 = 0.5 m V2²

6) A 14 700N car is traveling at 25m/s. The brakes are applied suddenly, and the car slides to a stop. THe average braking force between the tires and the road is 7100N.  Can you answer this question :How far will the car slide once the brakes are applied? Follow the steps if you need help:
A) What is the change in energy kinetic of the car ? KE2 - KE1. with V2= 0m/s and V1 = 25m/s. Use the formula.

B) The change in kinetic energy is equal to the work done by the brake. There is a transfer of energy between the car and the road. (All the kinetic energy is being transformed into thermal energy) So ∆KE = work done by the brakes = F x d. Solve for d.

E) Suppose now that the initial speed is twice as much. Find the new distance.

F) conclusion : if you multiply your speed by 2, the distance it takes for the car to come to a stop is multiplied by ___________.
If you multiply your speed by 3, the distance is multiplied by ___________
THis is why, you should be very very careful not exceeding speed limit and keeping a good distance with the car in front of you.

7) A 15.0kg cart is moving with a velocity of 7.50m/s down a level hallway. A constant force of -10N acts on the cart, and its velocity becomes 3.20m/s.
A) What is the change in kinetic energy ? ∆KE = __________

B) How much work was done on the cart ? W = __________

C) How far did the cart move while while the force acted ? _________
hint: ∆KE = W   solve for d

8) REmember the formula: F t = m ∆ V
It is not uncommon during the service of a professional tennis player for the racket to exert an average force of 150.0N
on the ball. If the ball has a mass of 0.060kg and is in contact with the strings of the racket for 0.030s, what is the kinetic
energy of the ball as it leaves the racket ? Assume that the ball starts from rest.

hint: Use F t = m ∆ V  with  ∆ V = V2 - V1. V1 = 0m/s. Solve for V2. Then Find ∆KE = KE2 = KE1.

GRAVITATIONAL POTENTIAL ENERGY

9)  The increase in potential energy of any system is equal to the work done on the system. If you lift  a  50kg  mass  from the grounf to a height of 10m, you are doing work against the weight of the mass.
WEight of the 50kg = _________.       Force you exert on the weight to balance the weight  =___________
So work done by you = W = ____________. You increase the energy of the mass . Some energy has been transfered from you to the mass.

The increase in gravitational potential energy of an object is equal to the work done in placing the object  in its final position:
∆PE = PE2 - PE1 = W = Fd
If the gravitational energy PE1 is equal to zero when the object is on the EArth's surface, the gravitational potential energy of the object is
PE = mgh
h is the height  above the ground.

10)  A  5.0kg bowling ball is lifted from the floor to a height of 1.50m . What is its increase in gravitaional potential energy?

11) The 200kg hammer of a pile driver is lifted 10.0 m. Find the gravitaional  potential energy of the system when the hammer is at this height.

12)  A 60kg shell is shot from a cannon to a height of 4.0  10² m.
A) What is the gravitational potential energy of the eart-shell system when the shell is at this height ?

B) What is the change in potential energy of the system when the shell falls to a height of 2.00 10² ?

13) A person weighing  630N climbs up a ladder  to a height  of  5.0m
A) what work does the person do ?

B) What is the increase in the gravitational potential of the person at this height ?

C) Where does the energy  comes from to cause this increase in the gravitational potential energy?

CONSERVATION OF ENERGY

LAB conservation of energy

The total emergy of a closed, isolated system is constant. WIthin the system, energy can change form, but the total amount of energy doesn't change.  A system get energy if work is done on it. The work done is a measure of the amount of energy transfered.

1) A) If you lift a 45kg book to a height of 10m (above ground)  the work done on the book (against its weight) is
W = _________ J . (work = force x distance, force = weight of the book).
B) In this case, we are considering the system EArth-book.
We ignore friction and we say that the system is : funny ? isolated ? conserved ? angry ?
The total amount of energy: decreases ? stays the same ? increases ? don't know ?
C) The work done on a book increases the ____________ energy of the book. PE = ______J
D) If you drop the book, the potential energy is transformed in ______________ energy.
If we ignore firction PE = ∆KE = _________ J
E) Find the final speed of the book using conservation of energy (see D)
. V2 = _________ m/s. (V1 = 0m/s , the book is at rest at t= 0s)

2) THe KE and PE of a block freely sliding down a ramp are shown in only one place in the sketch. Fill the missing values. 



3)Fill the blancks.



4)


5)

6)

 

7)





8) A 2kg mass is placed on a frictionless track at point A and releases from rest.
(assume the gravitational potential energy of the system is zero at point E.

1) Find the total mechanical energy of the mass. Use g = 10m/s/s
hint: focus on point A
2) Find the PE at point B
3) From 1)and 2) Find the KE at point B
4) From 3) Find the speed of the mass at point B.
5) If you want to find the speed at point C, do you have to go over all the omputations again ?
6) Find the speed at E
7) Find the speed at F

9)



In the above diagram, a toy car having a mass of 4.00 E-2 kg starts from rest at point A and travels 3.60meters along a uniform track until coming to rest at point B. (can't go higher)
A) How can you tell that the track is not frictionless? (there is friction between the car and the road)
B) Suppose the PE is 0 at ground level. Find the PE at point A and at point B: PEA = ________________
and PEB = ____________________
C) Find the energy loss between these two points . (Find PEA - PEB)
D) the energy lost has been transfered into thermal enery because of the work of the force of friction. (work = force x distance)
If the car travels a distance of 3.60 meters along the track, find the magnitude of the fircitonal force acting on the car.
hint: energy lost = work done by friction

10) worksheet