Work and Power

Work and Power

Mr. Hawboldt

Open Ended

What does it mean to "do work" in a physical sense?

Work is...

• "adding or removing energy from an open system by exerting a force over a displacement"

• Pushing/Pulling harder is more work than pushing/pulling softly

• Pushing/Pulling over a longer distance is more work than pushing/pulling over a shorter distance

• measured in Joules (J)

•  $W=F\Delta x$  

Multiple Choice

You exert a force of 20 Newtons to pick up your backpack and put it on your shoulders (approximately) 2 meters from the ground. How much work did you do

• 2 Joules

• 20 Joules

• 10 Joules

• 40 Joules

Open Ended

To mow the grass, you take the lawnmower out of the garage, walk a total distance of 2000 meters then put the mower back in the garage. While mowing you exert a constant force of 1000 Newtons. How much work did you do beginning to end of this chore?

The Work-Energy Theorem

• "Doing work on an open system changes the energy to the system by the same amount of work done"

• as an equation this is  $W=\Delta E$  

• the energy added can be any kind (kinetic or gravitational potential for us)

Open Ended

Choose kinetic or gravitational potential energy. Describe how exerting a force on an object could give that object your choice of energy

Multiple Choice

A golf ball is sitting at rest on the ground when it is struck by a golf club. The club exerts a force of 1000 Newtons over a displacement of 0.1 meters. Ho much kinetic energy does the golf ball have after being struck?

• 1 Joules

• 1000 Joules

• 100 Joules

• 10 Joules

Multiple Choice

A motor attached to an elevator does 98000 Joules of work by lifting a 1000 kg elevator car. What height does the elevator car reach?

• 1 meter

• 10 meters

• 100 meters

• 1000 meters

Open Ended

Give an example of something that is physically powerful. Describe why that thing has a lot of physical power.

Open Ended

Which is more powerful? A toddler pushing a toy truck 1 meter or his father pushing a real truck 1 meter? Explain

Open Ended

Which has more power: You push your car down your driveway or you push your car down your whole street? Explain

Power

• "the rate that work is done"

• if we combine this with the definition of work the definition of power would be

• "the rate at which energy is added to the system"

• both definitions require a change over a certain amount of time

•  $P=\frac{W}{t}$  

• measured in Watts (W)

Multiple Choice

An engine's output of energy is 2250 Joules every 3 seconds. How much power did the engine develop in that time?

• 3 Watts

• 750 Watts

• 250 Watts

• 2250 Watts