Energy

Chapter 3pp. 53-75

Mechanical Energy-

• Potential energy –– energy stored in an object due to its

position

• Kinetic energy- – energy in motion

Potential Energy-

• Equation-– PE = mgh – m - mass– g - acceleration. of gravity– h - height

Kinetic energy-

• Equation– KE = ½ mv2

•m – mass•v - velocity

Conservation of Energy-

TE=PE+KE

As they fall potential energy decreases and kinetic energy increases!

The total amount of energy always stays the same it just changes forms!

Momentum- •the product of the mass of

an object and its velocity

Equation-mv (momentum

= mass x velocity)

is called mew. We use it so there is not two m’s in the equation

1.) Does a parked car have momentum?

• No, there is no velocity.

2.) What has greater momentum a truck at 30 mph or a car’s at 30 mph?

• A truck because it has more mass

Apply to egg drop!

• What is the potential energy of a 0.05 kg egg at a height of 4 meters?

No calculation required!

• What is the potential energy of a 0.05 kg egg at a height of 4 meters?

• Based on conservation of energy what is the kinetic energy of the egg just before it hits the ground (height=0)?

Tough algebra, you can do it!• What is the potential energy of a

0.05 kg egg at a height of 4 meters?• Based on conservation of energy

what is the kinetic energy of the egg just before it hits the ground (height=0)?

• What is the speed of the egg just before it hits the ground?

Finally momentum!• What is the potential energy of a 0.05

kg egg at a height of 4 meters?• Based on conservation of energy

what is the kinetic energy of the egg just before it hits the ground (height=0)?

• What is the speed of the egg just before it hits the ground?

• What is the momentum of the egg just before it hits the ground?

Impulse-

• The product of force acting on an object and the time during

which it acts.

Equations-• I = Ft = mv

– (Impulse = Force x time)– (Impulse = change in mass x velocity)

• I = µ– Impulse = change in momentum

Impulse- (cont.)

• Impulse = µ and is therefore constant• The force is dependant on the time

applied• Short time means large force

– I = F t• Long time means small force

– I = F t

Review(you should have all these equations)

Work-•Equation-

– W = fd – (Work = force x

distance)

– More weight or more movement means more work was done

1.) How much work did a mover do if he lifted a 500 N box to a shelf that was 4 m high?

•W = fd– = 500 N X 4 m– = 2000 N-m– = 2000 J (Joule)

•A joule is a measurement of energy just like calories. (the amount of energy in food)

Power- The time rate of work

• Equation-– P = w/t– Power = work ÷

time

Lets compare some athletes.

ALWAYS USE A SPOTTER WHEN LIFTING!

At nearly 7 feet 9 inches tall, with size 19 feet, Chinese basketball player Sun Mingming dreams of being in the NBA.

If 2 people do the same job (the same amount of work), and one finishes faster, the person who did the job faster has more power.

8.) Mr. Foster and a small student run a 5K. Mr. Foster slows up so they cross the finish line in a tie. Who has more power?

Mr. Foster, Of course!

9.) Why?

Mr. Foster has more junk in the trunk and therefore did more work.

4.) Which of the following car crashes have a greater force?

A.

B.

Answer: Collision A5.) Why?• The car was stopped faster in

crash A because a pole is less forgiving than another car.

• The force in B was carried out over a longer period of time.

Conservation of momentum-

Net momentum before collision = Net momentum after collision

mV MvMomentum before collision = Momentum after collision

The velocity changes because the mass changes making it hard to see that momentum is conserved

Collisions- • Elastic Collision-

– colliding objects rebound without lasting deformation or generation of heat.

• Inelastic Collision – deformation and generation of heat

or both during a collision

6.) Are these collisions elastic or inelastic?

7.) Are these collisions elastic or inelastic?

3.) What is the momentum of a 80 kg jogger that is running 1 m/s?

• Answer:

mv• m = 80 kg• v = 1 m/s• 80 x 1 = 80• 4.) Kg-m/s