Unit 5 Summary

WORK AND POWER Work=force X distance →Work is measured in joules, and the force and distance must be parallel in order for work to be done So walking up the stairs is doing work, but holding an object above your head (force upwards) while walking across the room is not doing work, since the force and distance are in different directions. Power=joules/seconds Measured in watts →time is a factor in power, unlike time If you push a 20N box for 10m in 5 seconds, how much work and power are you generating?

Check out our video! WORK AND KINETIC ENERGY Energy is the ability to do work, which requires both mass and speed Work = change in kinetic energy

Kinetic energy is energy that requires movement. Remember that... KE final- KE initial= change in KE Potential energy is energy that does NOT require movement, but rather height (position of object). Here are the formulas for the two…

A 100kg car travels 10 meters in 5 seconds. Find the kinetic energy

Unfortunately, the car falls off a cliff and falls 200 meters to the ground. Find the potential energy of the car.

CONSERVATION OF ENERGY Change in kinetic energy = change in potential energy Work = f x d Work in = work out (Force in)(Distance in)=(Force out)(Distance out) energy in= energy out Say it takes 500 joules of energy for a roller coaster to run. From start to finish, the roller coaster will have 500 joules, because energy cannot be created or destroyed. However, energy shifts from potential (height) to kinetic (movement) depending on the position of the cars.

As you can see in the diagram, when the roller coaster is higher up on the tracks, it has a higher potential energy because it has height. Similarly, it has more kinetic energy at the bottom because it is in motion, and has less height. Why do airbags in cars keep us safe? When in a car crash, you go from moving to not moving despite how you are stopped. So whether you hit a tree, a brick wall, or a person, your change in kinetic energy is going to be the same. Remember… Ke= 1/2mv^2 Change in KE= Ke final- Ke initial If the change in KE is the same (which it is) so is the work. The airbag will increase the distance that the force acts on you, so the force will be smaller. Less force= less injury

MACHINES The purpose of a machine is to increase the distance at which an object moves, while decreasing the force used to move it. It is important to remember that machines do not change the amount of work done, they just make it easier since you don’t have to use such a great force to accomplish your task. (insert diagram) Work= fd (insert ramp diagram) work out (small distance) work in (big distance) work in= work out no change in work Say you have to lift a 20N refrigerator 5 meters onto a moving truck.

The work out is always the smaller distance, and the work in is the larger distance.

Remember that machines do not change the amount of work done. They just redistribute the amount of force you exert the object over a certain distance. —>So even though energy is conserved, you can never get more work out of a machine than you put in. In fact, it is impossible for a machine to be 100% efficient. This is because some energy escapes through the form of sound and heat. Efficiency= workout/work in x 100