Unit 5 Summery

In this unit we discussed work, power, kinetic energy, the conservation of energy, and machines.

Work

·      Work= Force x distance

·      It transfers energy

·      Work is measured in Joules

Work can only be done when the force and distance are parallel. 

However, work cannot be done when the force and distance are perpendicular. For example, if you carry a box weighing 20N up the same flight of stairs- the force and distance are not parallel and therefor no work is done.

If you exert a force onto a wall and the wall does not move, no work is done. This is because you did not move any distance though you exerted the force.

Power

·      Power= work/ time (the amount of work done per unit of time)

·      Work is responsible for power

·      Power is measured in watts (J/s)

·      746 watts= 1 Horsepower

So, you weigh 200N and go up the stairs to third Lawrence, which is 10m and it takes you 40 seconds to do this. How much work was done? How much power was generated?

Work= F x d                 

Work= 200 x 10

Work=2000J

Power=work/time

Power=2000/ 40

Power= 50 watts

Kinetic Energy

·      It is defined as the measurement of movement.

·      There is no KE when something is at rest.

·      KE= ½ mv²

·      Change in KE= work

·      Change in KE= KE final- KE initial

In terms of energy, why do airbags keep us safe?

           -When you crash, you go from moving to not moving regardless of what stopped you. If KE doesn’t change neither does work. The airbag increases the distance to stop and since work is conserved and remains the same the F is compensated and decreased.

No airbag= FORCE x distance

Airbag= force= DISTANCE

Potential Energy

·      PE= mgh

·      PE= (mass) (gravity) (height)

·      Defined as the energy of a certain position.

·      Unlike KE, this can occur whether or not an object is moving.

·      PE when falling will convert to and from KE

·      (for more info watch the link)

Conservation of Energy

     

No energy can be created nor destroyed- only converted.

·      Work requires energy!

Machines

·      Machines increase the amount of distance and decrease the amount of force needed (this could be an inclined plane).

 Because work remains constant and the distance is increased, the force must compensates and decreases.

For example: If Janice and Stitch were just you know, chillin’, and they needed to move their grandma’s 300N piano into her new home, it would be easier for them to use a ramp than to lift straight up 2m. Because then it would take…

work= F x d

Work= 300N (2m)

Work= 600J

But if they used a 4m ramp then…

Work= F x d

600= F x 4

F=150N

Thank you!