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Lec 07: Energy
• Energy, Work, Power
• Mechanical Energy
• Conservation of Energy
• Types of Energy
Energy
• Things moving must have energy
• Kinetic Energy (Joules)
KE = (1/2) (mass) (velocity)2
Example: A 20,000-kg train is moving at 22
m/s (about 50 mph). How much is its kinetic
energy?
Work
• Amount of Energy moved by a force (Watt)
Work = (Force ) (Distance )
W=Fd
Work
Example: You are lifting 10 books from the floor to
the bookshelf, 1 m above. Each book weighs 10.0 N.
How much work do you do in lifting them?
Energy and Work
Example: If you carry your 5-kg backpack from the
parking lot to the fourth floor of the building (15 m
above), how much is the work done by the
gravitational Force?
Work = - (mg) x (15 m)
Example: How much is the work when you
carry your 5-kg backpack back to the parking
lot?
Work = (mg) x (15 m)
Question: How much is the total work?
Power
• How fast the Work is being done
• Power
Power = (Work) / (Time)
Example: You leave a 25-W light bulb on for
10 minutes. How much energy in Joules has it
consumed?
Potential Energy
PE = (mass) (g) (change in elevation)
PE = mgh
Example: How much is the PE of the ball?
Puzzles
P1. You do work when pushing a cart with a constant force.
If you push the cart twice as far, how much is the work?
P2. How much is the Kinetic Energy of a 2-kg object moving
at 3.0 m/s?
P3. You run a 100-W light bulb on for 1 hour. How much
energy have you consumed?
P4. What costs more to run: a 100-W light bulb on for 1 day
or a 1,000-W hair-dryer run for 10 minutes?
P5. A typical grade school pitcher can throw a baseball at 80
km/h, but only a few professional athletes have the
extraordinary strength needed to throw a baseball at twice that
speed because they need to impart the ball much more energy.
By what factor?
Conservation of Energy
Energy moves and is transformed from one
form to another but never appears or
disappears “unaccountably”
Example: You drop a 100-g rock from height of 2 m.
How much is its potential energy at the top?
• How much is its potential energy at the bottom?
• How much is its kinetic energy at the bottom in
the absence of friction?
Conservation of Energy
Energy moves and is transformed from one
form to another but never appears or
disappears “unaccountably”
Example: You drop a 100-g rock from height of 2 m.
By the time it reached ground, the force of air
resistance removed 1.0 J. How much is the kinetic
energy just before the ground?
• How much is the speed of the rock?
Ordered vs. Disordered Energy
Question: Which of the following is an example
of ordered energy?
• The energy of a hurricane?
• Energy of steam coming of hot pot
• Energy delivered by the electrical company?
Simple machines: RAMP
(FORCE) x (distance) = (force) x (DISTANCE)
Simple machines: PULLEY
The arrangement allows the load to be lifted
by half as much force!
Simple machines: Efficiency
Efficiency =
(useful energy output)
(total energy input)
Example: A certain machine is 30% efficient. If
the amount of energy you got out of it was 30
Joules, you paid for
(a) 30 J (b) 70 J (c) 100 J (d) Don’t know
Puzzles
P1. You throw a 10-g pebble upward with velocity of 6 m/s.
How high will it reach in the absence of air resistance?
P2. You throw a 10-g pebble upward with velocity of 6 m/s.
If the air-resistance takes about 0.05 J of its energy, how
high will it go?
P3. You are lifting a 5-kg object using a pulley.
With how much force do you need to pull?