Chapter 8 Energy - LCPS

Chapter 8 Energy

Conceptual Physics

Objectives: The student will be able to:

? Define and describe work ? Define and describe power ? Define potential and kinetic energy ? State the law of conservation of energy ? Describe simple machines and mechanical advantage

8.1 Work

Work the quantity force x distance

Two things enter into every case where work is done:

(1) The application of force (2) The movement of something by that force

=

2

= 2 ? = 2 = ()

The resultant unit of work is N-m or joule.

Example Problems: 1.) Calculate the work done when a 20 N force pushes a cart 3.5 meters.

2.) Calculate the work required to lift a 30 N crate a vertical distance of or 4 meters.

Calculate the work required to lift the 90 N block of ice as shown:

Ice: 90 N

Height: 3 m

Force: 54 N

Ramp length: 5 meters

8.2 Power

Power is the rate at which work is done. It equals the amount of work done divided by the time interval during which work is done.

=

=

The unit of power is joule per second, also known as watt. One watt is expended when one joule of work is done in one second.

=

2 2

=

2 3

=

()

Example Problem: Calculate the work done in lifting a 500 N barbell 2.2 meters above the floor. What is the power expended if the lift is done in 2 seconds?

8.3 Mechanical Energy The two most common forms of mechanical energy are potential energy and kinetic energy. Potential Energy: the energy due to ___________________ Kinetic Energy: the energy due to ___________________

8.4 Potential Energy

Gravitational Potential Energy = weight x height

U=mgh

2

= ? 2 = 2 =

2m

2m

2m

A

B

C

Which scenario above requires more work? Why? How does the potential energy differ in each scenario?

Example Problems: 1.) A 4 kg box is at a height of 3 meters relative to the ground. What is its potential energy?

2.) At what height does a 5 kg mass have a potential energy of 100 joules relative to the ground?

8.5 Kinetic Energy

=

1 2

?

2

=

1 2

2

2 2 ? = 2 =

30

10 m Skid

60

120

Explain Why? Example Problem: Calculate the kinetic energy of a 3 kg toy cart that moves at 4 m/s.

Calculate the kinetic energy of the same cart at twice the speed.

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