Work & Power
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Work & Power
The unit of work is the Joule –Force x Distance = Work –1 Newton x 1 meter = 1 Newton meter • 1 Newton meter = 1 Joule
Example 1: • A high jumper weighs 700 newtons. What work does the jumper perform in jumping over a bar 2.0 meters high? • Answer: W = F x d • W = 700N x 2.0 m • = 1400 nm • = 1400 Joules
No work is done if there is no distance! • The statue of liberty has been holding up her torch for an awfully long time. How much work has she done? • Answer: 0! Although it takes a force to hold the torch against the force of gravity, there is no motion so no work is done.
Which does more work?
The net work is the force (weight) of the cart x the vertical distance. This is the same in all three cases. In the first, the force is less, but distance is greater to reach the same vertical height.
Example 2 • A force of 200N is required to push a lawn mower. If 4000 J of work is performed on the lawnmower, how far does it move? • Answer: W__ • F d • 4000J__ • 200N d • 4000Nm ÷ 200N • = 20m
Power • Work done per unit of time
• Power = •
• Power = •
Work Time Force x distance Time
The unit of power is the watt
• Power =
Work
•
• 1 watt •
Time
=
1 Joule Second
Example 3 • A crane lifts a car into a junk pile in 10 seconds. What is the crane’s power if 120,000 J of work are performed? • Answer: Power = Work • • • •
Time 120,000J 10 sec
= 12,000 Watts
Example 4 • A 750 N diver does a somersault off a 10m platform. It takes her 1.5 seconds to hit the water. What is her power? • Since work is Force x distance, the power formula can be written as
• Power •
=
Force x distance Time
Answer: •
Power =
•
• Power
=
Time 750N x 10m
=
1.5 sec 7500N m
•
• Power • •
Force x distance
1.5 sec = 5000 Watts
Other units of power • 1 kilowatt = 1000 watts • 1 horsepower = 750 watts • In the previous example, how many kilowatts power was generated? How much horsepower?
Answer: • 5000 watts ÷ 1000 kw/w = 5 kilowatts • 5000 watts ÷ 750 hp/w = 6.7 horsepower
The unit of work is the Joule –Force x Distance = Work –1 Newton x 1 meter = 1 Newton meter • 1 Newton meter = 1 Joule
Example 1: • A high jumper weighs 700 newtons. What work does the jumper perform in jumping over a bar 2.0 meters high? • Answer: W = F x d • W = 700N x 2.0 m • = 1400 nm • = 1400 Joules
No work is done if there is no distance! • The statue of liberty has been holding up her torch for an awfully long time. How much work has she done? • Answer: 0! Although it takes a force to hold the torch against the force of gravity, there is no motion so no work is done.
Which does more work?
The net work is the force (weight) of the cart x the vertical distance. This is the same in all three cases. In the first, the force is less, but distance is greater to reach the same vertical height.
Example 2 • A force of 200N is required to push a lawn mower. If 4000 J of work is performed on the lawnmower, how far does it move? • Answer: W__ • F d • 4000J__ • 200N d • 4000Nm ÷ 200N • = 20m
Power • Work done per unit of time
• Power = •
• Power = •
Work Time Force x distance Time
The unit of power is the watt
• Power =
Work
•
• 1 watt •
Time
=
1 Joule Second
Example 3 • A crane lifts a car into a junk pile in 10 seconds. What is the crane’s power if 120,000 J of work are performed? • Answer: Power = Work • • • •
Time 120,000J 10 sec
= 12,000 Watts
Example 4 • A 750 N diver does a somersault off a 10m platform. It takes her 1.5 seconds to hit the water. What is her power? • Since work is Force x distance, the power formula can be written as
• Power •
=
Force x distance Time
Answer: •
Power =
•
• Power
=
Time 750N x 10m
=
1.5 sec 7500N m
•
• Power • •
Force x distance
1.5 sec = 5000 Watts
Other units of power • 1 kilowatt = 1000 watts • 1 horsepower = 750 watts • In the previous example, how many kilowatts power was generated? How much horsepower?
Answer: • 5000 watts ÷ 1000 kw/w = 5 kilowatts • 5000 watts ÷ 750 hp/w = 6.7 horsepower
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