Energy

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Energy

• Chapter 3
• pp. 53-75

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Mechanical Energy-

• Potential energy
• energy stored in an object due to its position
• Kinetic energy-
• energy in motion

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Potential Energy-

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

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Kinetic energy-

• Equation
• KE ½ mv2
• m mass
• v - velocity

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Conservation of Energy- TEPEKE
The total amount of energy always stays the same
it just changes forms!
As they fall potential energy decreases and
kinetic energy increases!
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Momentum-

• the product of the mass of an object and its
  velocity

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Equation-

• m mv (momentum mass x velocity)

m is called mew. We use it so there is not two
ms in the equation
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1.) Does a parked car have momentum?

• No, there is no velocity.

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2.) What has greater momentum a truck at 30 mph
or a cars at 30 mph?

• A truck because it has more mass

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Apply to egg drop!

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

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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 (height0)?

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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 (height0)?
• What is the speed of the egg just before it hits
  the ground?

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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 (height0)?
• 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?

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Impulse-

• The product of force acting
• on an object and the time during which it
  acts.

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Equations-

• I Ft mv
• (Impulse Force x time)
• (Impulse change in mass x velocity)
• I µ
• Impulse change in momentum

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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

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Review(you should have all these equations)

•  

 
 
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Work-

• Equation-
• W fd
• (Work force x
• distance)
• More weight or more
• movement means
• more work was done

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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)

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Power- The time rate of work

• Equation-
• P w/t
• Power work time

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Lets compare some athletes.
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ALWAYS USE A SPOTTER WHEN LIFTING!
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At nearly 7 feet 9 inches tall, with size 19
feet, Chinese basketball player Sun Mingming
dreams of being in the NBA.
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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.
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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.
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4.) Which of the following car crashes have a
greater force?
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A.
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B.
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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.

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Conservation of momentum-

• Net momentum before collision Net
  momentum after collision

mV
Mv
Momentum before collision Momentum after
collision The velocity changes because the mass
changes making it hard to see that momentum is
conserved
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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

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6.) Are these collisions elastic or inelastic?
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7.) Are these collisions elastic
or inelastic?
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3.) What is the momentum of a 80 kg jogger that
is running 1 m/s?

• Answer
• m mv
• m 80 kg
• v 1 m/s
• 80 x 1 80
• 4.) m 80_________?

Kg-m/s
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