Download Work = Force x Distance

Work = Force x Distance
Work is done only if there is motion in the same direction as the force.
Motion
Force
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
Work = Force x Distance x Cos ө
• 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?
How much work is done on this car
if it moves 50 m?
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.
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.
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 = 0 kJ
PE =200 kJ
KE = 800 kJ
1000kg
What is the speed of the car as it hits the
bottom of the track?