Transcript Energy - Troxel

WORK
 Work is done on an object when a force moves an object through a
distance
 Work = force x distance
 Work is measured in Joules (Newton-meter)
 Examples:
 Pushing a swing
 Lifting a box
 Carrying a box – no work is done! If 90° angle, between force and
motion
PROBLEMS
You push a refrigerator with a horizontal force of 100 N. If you move the
refrigerator a distance of 5 m which pushing, how much work do you do?
Work = force x distance
W = fd
W = (100 N)(5 m)
= 500 J
PROBLEMS
A couch is pushed with a horizontal force of 80 N and moves a distance of 5 m
across the floor. How much work is done on the couch?
Work = force x distance
W = fd
W = (80 N)(5 m) = 400 J
PROBLEMS
How much work do you do when you lift a 100 N child 0.5 m?
Work = force x distance
W = fd
W = (100 N)(0.5 m)
= 400 J
PROBLEMS
The brakes on a car do 240,000 J of work in stopping the car. If the car travels a
distance of 40 m while the breaks are being applied, how large is the average
force that the brakes exert on the car?
W = fd
F = W/d
F = (240,000 J)/(40 m)
= 400 J
ENERGY
 Is defined as the ability to do work or cause a change
 Work is the transfer of energy
 Measured in Joules
 System: anything around which you can imagine a boundary
TYPES OF ENERGY
Kinetic energy
 Energy of motion
 Depends upon mass and velocity
 Kinetic energy (in J) = ½ mass (in kg) x [velocity (in m/s)] 2
KE = ½ mass x velocity2 = mass x velocity2
2
KE ↑ as mass ↑
KE ↑ as velocity ↑
POTENTIAL
 Stored energy
 It has the potential to do work
 Elastic PE – Energy that is stored by compressing or stretching an object
 Chemical PE – Energy due to chemical bonds
 Gravitational PE – depends on height
Mass (kg) x gravity (N/kg) x height (m)
GPE = mgh
where g = 9.8 N/kg
PROBLEMS
A jogger moving forward with a mass of 60.0 kg is moving forward at a speed
of 3.0 m/s. what is the jogger’s kinetic energy from this forward motion?
KE = ½ (mass x velocity2 )
KE = ½ (60.0 kg) (3.0 m/s)2
KE = ½ (60.0 kg)(9.0 m2/s2)
= 270 J
PROBLEMS
A baseball with a mass of 0.15 kg is moving at a speed of 400 m/s. What is
the baseball’s kinetic energy from this motion?
KE = ½ (mass x velocity2 )
KE = ½ (0.15 kg) (40.0 m/s)2
KE = ½ (0.15 kg)(1600 m2/s2)
= 120 J
PROBLEMS
A 4.00 kg ceiling fan placed 0.25 m above floor. What is the gravitational
potential energy of the Earth-ceiling fan system relative to the floor?
GPE = mass x gravity x height
GPE = mgh
GPE = (4.00 kg)(9.8 N/kg)(2.5 m)
= 98 N•m = 98 J
PROBLEMS
An 8.0 kg history textbook placed on 1.25 m high desk. How large is the
gravitational potential energy of the textbook-Earth system relative to the
floor?
GPE = mass x gravity x height
GPE = mgh
GPE = (8.0 kg)(9.8 N/kg)(1.25 m)
= 98 N•m = 98 J
CONSERVATION OF ENERGY
 Energy cannot be created or destroyed.
 Energy can only be converted from one form to another or transferred
from one place to another.
 Mechanical energy is the sum of the kinetic and potential energy of the objects in
a system
 Example: running water, swing
 Swing