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Work. (Pay special attention to words in BLACK) What is “Work”? According to physics… Work is a force applied for a certain distance. W=F D x. Work. A 50 Kg car is moved 40 m by a 30 N force. How much work is done to the car? 1200 Nm 1Nm = 1 Joule = 1 J So, W = 1200 J.

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Work

(Pay special attention to words in BLACK)

What is “Work”?

According to physics…

Work is a force applied for a certain distance.

W=FDx

Work

A 50 Kg car is moved 40 m by a 30 N force. How much work is done to the car?

1200 Nm

1Nm = 1 Joule = 1 J

So, W = 1200 J

Substitute: W= maDx = m (Vf2-Vi2)/2

So: W = 1/2m Vf2-1/2mVi2

1/2mv2 has a special name:

Kinetic Energy!

Kinetic Energy

Kinetic energy is another way that physics looks at motion:

KE = 1/2mv2

Looking back… W=DKE

So: Work is the change in Kinetic Energy.

Kinetic Energy

Reconsidering the car problem:

If the 50 Kg car began at rest and experienced 1200 J of work how fast is the car traveling after the work is done?

Vf =6.9 m/s

Potential Energy

Potential energy is anything that has the potential to do work to an object.

A rock on top of a hill, a stretched slingshot, a hydrogen/Oxygen mixture etc.

(What types of potential energy are each of the examples?)

Gravitational Potential Energy:

PEg = mgh

m = mass, g = 9.8 m/s2, h = height

Gravitational Sample
• A 2000 Kg boulder sits 43m above the ground on top another larger boulder. What is the potential energy of the top boulder?

PEg = 842,800 J

Elastic Potential Energy

PEe= 1/2kx2

k = spring constant = F/x (Hooke’s Law)

X= displacement from the relaxed position

F = Force

A 45 Kg block is against a spring with spring constant 950 N/m. The spring is compressed 10 cm from the relaxed position. How much energy is stored in the spring?

PEe = 4.8 J

Conservation of Energy

Energy cannot be created nor destroyed.

The total mechanical energy initially must equal the total mechanical energy finally.

SMEi =SMEf

KEi+PEgi + PEei = KEf + PEgf + PEef

Conservation of Energy

KEi+PEgi + PEei = KEf + PEgf + PEef

Spring?

Moving?

Up?

Spring?

Moving?

Up?

KE is the Energy of “MOVING”

PEg is the energy of “UP”

PEe is the energy in a “SPRING”

A 2000 Kg boulder sits 43m above the ground on top another larger boulder. It falls! How fast is the boulder moving just before hitting the ground?

Vf = 29 m/s

A spring with a spring constant of 35 N/m

is compressed 0.23 m from its relaxed

position. Resting against this spring is a

0.08 Kg character. When the character is

released, what is his maximum velocity?

4.8 m/s

Pendulum
• A pendulum has a 5.0 Kg mass attached to a 1.2 m long string. If the pendulum is brought back through an angle of 25o, what will the maximum velocity be?

1.4 m/s