Chapter 12 Work and Energy
What is work? Work- when a force causes a change in the motion of an object. Equation: W = F x d W is work F is force d is distance
What is work? If you apply a large force to pick up a car but the car does not move, have you done any work? Discuss with a partner for one minute. You might apply a large force, but if the distance that the car moves is equal to zero, the work done is also equal to zero.
Is there any work being done here?
What is work? The unit for work is newtons times meters, N·m = joules (J) A joule (J) can equal 1kg·m²/s²
Example problem A crane uses an average force of 5200N to lift a girder 25m. How much work does the crane do on the girder? W = F = d = Equation used: Show work:
Example problem - Advanced A mechanic uses a hydraulic lift to raise a 1200kg car 0.5m off the ground. How much work does the lift do on the car? First step: we need to change kg into N. Remember w = mg. w = weight (force of gravity on an object) m = mass g = gravity
Example Problem 2 A mechanic uses a hydraulic lift to raise a 1200kg car 0.5m off the ground. How much work does the lift do on the car? We only have weight and not force (N). We have to use the weight equation from last chapter. w = mg Once we get weight it will be in Newton unit (N), so we can put it in with force (N).
Example problem - Advanced w = ? m = 1200kg g = 9.8 m/s² w = 1200kg x 9.8 m/s² w = 11760 kg· m/s² W = F x d W = 11760 kg· m/s² x 0.5m = 6000 J or 6000 kg·m²/s²
Practice Problems Page 379 in your book 2-4 on your own.
What is power? Power is the rate at which work is done, that is, how much work is done in a given amount of time. Power Equation: P = W P = power t W = work t = time
Look at the Power equation !!! Running up the stairs takes less time than walking. How does reducing the time in the Power equation affect the power if the amount of work stays the same? If time decreases, then power will increase keeping the amount of work constant.
What is power? Power is measured in unit called watts (W). A watt is the amount of power required to do 1J of work in 1 sec. This is equal to lifting an apple over your head in 1 second. Be careful with the abbreviations for work (W) and watts (W).
Power practice problems While rowing across the lake during a race, John does 3960 J of work on the oars in 60.0 sec. What is his power output in watts? P = W = t = Equation used: Show work:
Power practice problems Page 381 2 and 3 on your own. We will do #5 together. https://www.youtube.com/watch?v =JfjSnIK055s
Simple Machines Which is easier; lifting a car yourself or using a car jack? Using a car jack is easier, but does it reduce the amount of work on the car? No!!! Using a jack requires the same amount of work. It will increase the distance over which the force is applied.
Simple Machines Redistribute the work on an object Change the direction of an input force Change the distance over which the force is applied
Simple Machines W = F x d If work stays the same, then force decreases when distance increases.