Work = Force x Distance

Work is done only if there is motion in the same direction as the force.

Force Motion

Work = Force x Distance

Joe pushes on the car with a force of 900 Newtons and the car

moves forward 2 meters. How much work was done on the car?

Work = Energy

How much energy did Joe expend in moving the car?

Power = Work / Time

Power is the rate at which work gets done.

If Joe pushes the car for 3 seconds, what is his power output?

Note:1 horsepower = 750 watts

• Joe pulls a sled 100 m with a force of 50 N

using a rope which is 30 degrees from

horizontal. How much work is done?

Work = Force x Distance x Cos ө

How much work is done on this car

if it moves 50 m?

If the person in the figure

were to stop pushing, the

car would quickly stop

moving.

Positive work done by a

force increases the

energy, while negative

work decreases it.

Calculating Work

The work done by friction acts in the direction

opposite that of motion—at an angle of 180°.

Because cos 180° = −1, the work done by friction is

negative. Negative work done by a force exerted by

something in the external world reduces the kinetic

energy of the system.

Gravitational Potential Energy = mass x gravity x height

Gravitational Potential Energy is the

energy an objects has because of its

mass and position.

The higher the paint climbers, the more potential

energy he has.

Gravitational Potential Energy = mgh

Bill, the painter, carries a 5 kg can of paint up the ladder 15 meters.

What is the potential energy of the paint can?

Potential Energy = Work

How much work is done on the paint can?

Power = Work / Time

If Bill carries the can up the ladder in 30 seconds, what is his power

output?

Other examples of PE

Kinetic Energy = ½ mass x velocity2

Bill accidentally drops the can and it falls. Prior to

landing, the 5 kg can reaches a velocity of 17.14 m/s.

Calculate the kinetic energy of the can.

What has more affect on KE? Mass or velocity?

If we double the mass of a vehicle….how is the KE changed?

If we double the speed of a vehicle…how is the KE changed?

Work -Energy Theorem

Work done

A 105-g hockey puck is sliding across the ice. A player exerts a

constant 4.50-N force over a distance of 0.150 m. How much

work does the player do on the puck?

What is the change in the puck’s energy?

Assuming there is no friction, what is the velocity of the puck?

Total Mechanical = Potential Energy + Kinetic Energy

Energy

The Conservation of energy says that the total amount of energy

in a system does not change. Potential energy is converted to

kinetic energy, and kinetic energy is converted to potential energy.

Ball slows down Ball speeds up

Energy Conversions

Potential Energy transforms to Kinetic Energy

PE =1000 kJ

KE = 800 kJ

KE = 0 kJ

PE =200 kJ

1000kg

What is the speed of the car as it hits the

bottom of the track?