The change in energy

How much the energy changes

If a car has 100J of Ek when the brakes are applied, and has 20J of Ek after, the brakes have done 80J of work on the car

If an object has 100J of GPE at the top of a hill and 70J of KE when it gets to the bottom friction did 30J of work on the car

A force must be applied over distance!◦If we push a stationary object and it doesn’t move, there is no work done

Work = Force * distanceThe units of work are Joules1J of work is defined as 1N of force applied over a distance of 1m

The circle is where we keep track of how much work is done

The object can gain energy or lose energy◦A force causing an object to speed up adds energy to the object

◦A force causing an object to slow down removes energy from the object

A 25kg car at rest speeds up to 60m/s.

Draw an LOL graph

How much work was done?

How much force did the engine apply if this was accomplished over a distance of 50m?

Work = Change in energy = ½ * 25kg (60m/s)2 -OJ =12.5kg 3600m2s2

= 45,000J

Work = Force *distance 45,000J = F * 50m F = 900N

A 5kg ball moving 10m/s comes to a hill. It comes to a stop at a height of 4m on the hill.

Draw an LOL graph

How much work did friction do on the ball?

If the force of friction was 5N, how much distance did the ball travel while rolling uphill?

Only KE to start ½ * 5kg * (10m/s)2 = 250J When the ball stops, only GPE 5kg*10N/kg * 4m = 200J The object lost 50J of energy = 50J of work 50J = F * d 50J = 5N * d d= 10m