energy chapter 3 pp. 53-75. mechanical energy- potential energy – –energy stored in an object...
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Energy
Chapter 3pp. 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-
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!
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Momentum- •the product of the mass of
an object and its velocity
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Equation-mv (momentum
= mass x velocity)
is called mew. We use it so there is not two m’s 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 car’s 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 (height=0)?
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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 (height=0)?
• 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 (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?
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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 MvMomentum 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:
mv• m = 80 kg• v = 1 m/s• 80 x 1 = 80• 4.) Kg-m/s
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