Work & Energy

Work & Energy

What determines if work is done? What exactly is defined as energy? How are these two topics related to each other?

Slide 3

Slide 22

Slide 23

Open Ended

What do the terms with Joules as their unit all have in common?

Multiple Choice

Jesus is applying 25 N of Force to his couch. It moves in the direction he is pushing.

Is work being done?

• yes, the force and direction are the same.

• no, work is not being done.

Multiple Choice

Jesus is applying 30 N of Force to a chair while his brother sits in it. Although he continues to try, It will not move. He is using alot of energy in the attempt to move the chair and his brother. Is he doing work?

• yes, he is doing work

• No, he is not doing work because the chair did not move.

Multiple Choice

Is this person doing work?

• yes

• no

Multiple Choice

Is the woman doing work holding her baby up with 1 N of force as she walks forward 2 m?

• yes

• no

Multiple Choice

As this baby is applying a force down on the skateboard with his weight, the skateboard moves to the right .5 m. Is this baby doing work?

• yes

• no

Multiple Choice

The man pushes up with a force of 255 N and moves his bar 2 m. Is he doing work?

• yes

• no

Slide 3

Multiple Choice

W=Fxd

Miguel pushed a grocery cart with 10 N of force across a distance of 30 m. How much work did he do?

• 3 J

• 30 J

• 300 J

Slide 3

Now Lets Look at Kinetic Energy

• We use energy to do the work

• We will be able to calculate how much energy is used when work IS done.

• We can also predict the velocity of an object when we already know how much energy was used to complete the work.

Kinetic Energy is the energy involved with objects already in motion.

The energy comes from stored "Potential Energy".

Lets Practice calculating how much Kinetic Energy is used during its movement...

Multiple Choice

A 164kg skateboarder is near the bottom of a really tall hill. They are traveling at their max velocity of 50 m/s. How much total Kinetic Energy do they have just before they reach the bottom?

• 410,000 J

• 4100 J

• 205,000 J

Potential Energy is the stored energy due to an objects Position away from the surface of the earth.

Multiple Choice

At what point is the girl reaching her maximum stored Potential energy?

• A

• B

• C

• D

• E

Multiple Choice

Pe= mgh

jimmy drops a ball with a mass of 5 kg. It falls 1m to the ground. How much potential energy did the ball have, before he released the ball? (g = 9.8 m/s^2)

• 49 J

• 50 J

• 1.96 J

Energy of a Spring.

• k=the spring constant is a measurement of how resistant the spring is to change in compression or stretching.

• x=displacement (how far has the edge of the spring moved from its original position?)

Spring displacement (x) in meters.

(a) is the equilibrium position (at rest)

(b) is the stretched displacement

(c) is the compressed displacement

Multiple Choice

PE_(e)= 1/2 kx².

A force of 5N is used to compress a spring -.3m. The spring has a constant resistance to change (k) of 18 N/m. Determine how much Potential energy is stored on the spring before it is released.

• -5.4 J

• -1.62 J

• -0.81 J

• 0.81 J