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Reflection Question. Is energy lost when a car burns gasoline? Explain why or why not. Work Section 5.1. Have you done any work today?. What is Work?. Work – the transfer of energy that occurs when a force makes an object move

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reflection question
Reflection Question
  • Is energy lost when a car burns gasoline? Explain why or why not.
work section 5 1

WorkSection 5.1

Have you done any work today?

what is work
What is Work?
  • Work – the transfer of energy that occurs when a force makes an object move
  • The motion of the object must be in the same direction as the applied force on the object.
  • Two conditions must be met in order for work to be done:
    • Object must move
    • Force must be applied in the direction of movement
example
Example

When you pick up a pile of books from the floor the books upward in the direction of the force you are applying.

What if you just hold the books in your arms without moving them. Are you doing work?

There is still an upward force being applied by you to keep the books from falling, but there is no movement.

calculating work
Calculating Work
  • Work = force x distance
  • W = Fd
    • Force in Newtons
    • Distance in meters

Example #1 – You move a 75kg refrigerator 35m. This requires a force of 90N. How much work, in joules, was done while moving the refrigerator?

slide6

Example #1 – You move a 75kg refrigerator 35m. This requires a force of 90N. How much work, in joules, was done while moving the refrigerator?

W = Fd

= (90)(35)

= 3150J

you try
You try 

Example 2: A book weighing 1.0 newton is lifted 2 meters. How much work was done?

Example 3:A book weighing 1.0 kg is lifted 2 meters. How much work was done? (use Fw=mag) to find the force before calculating work)

you try1
You try 

Example 5: 250J of work are needed to push a box up a ramp. The force needed to push the box up the ramp is 10 Newtons. How far up the ramp was the box pushed?

Example 6: If 56J of work energy is required to lift a dumbell to a height of 7meters, what is the weight of the dumbell? The mass?

when do you do work
When do you do work?
  • When you push a book and it slides 1 m, you only do work on the book while your hand is in contact with it!
  • Work is done on an object ONLY when a force is being applied to the object.
power
Power
  • Power – the amount of work done in a certain time----in other words: it is the rate at which work is done
  • Power = work / time
  • P = W/t
  • Work in joules
  • Time in seconds
  • Power measured in Watts (W)
example 2
Example #2
  • It took 150 seconds to move a refrigerator. You did 3150 joules of work in the process. How much power was required to move the refrigerator?

P = W/t

= 3150 / 150

= 21W