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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


W = FDx = maDx

But, Vf2=Vi2+2aDx

Rearrange: aDx= (Vf2-Vi2)/2

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

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 energy1
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

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
Gravitational Potential Energy:

PEg = mgh

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


Gravitational sample
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
Elastic Potential Energy

PEe= 1/2kx2

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

X= displacement from the relaxed position

F = Force


PEe = 4.8 J


Conservation of energy
Conservation of Energy N/m. The spring is compressed 10 cm from the relaxed position. How much energy is stored in the spring?

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 energy1
Conservation of Energy N/m. The spring is compressed 10 cm from the relaxed position. How much energy is stored in the spring?

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”


Vf = 29 m/s


A spring with a spring constant of 35 N/m larger boulder. It falls! How fast is the boulder moving just before hitting the ground?

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
Pendulum larger boulder. It falls! How fast is the boulder moving just before hitting the ground?

  • 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


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