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The Work-Energy TheoremPhysics 6(A)
Learning Objectives• Explain how work is done on an object• Describe the relationship between the work
done on an object and the kinetic energy of the object
• Calculate the quantities associated with work and the work-energy theorem
Work-Energy Theorem
• Work (W ) – quantity describing when a force (F ) is applied to an object over a distance (d )– Measured in joules, J
• Force and distance must be in the same direction– Work is done by lifting a barbell overhead
• Force is in the vertical direction• Distance is in the vertical direction
– No work is done while walking down the hall wearing backpack• Force is in the vertical direction• Distance is in the horizontal direction
Defining Work
Work Calculation
Ex) A sled is pulled 11 m across a patch of ice by a force of 5.0 N. What is the total work done on the sled?
1. Write unknown and givens
2. Identify the formula and rearrange, if needed
3. Convert units and find intermediates, if needed
4. Plug in and solve
5. Make sure the answer is reasonable
Work Calculation
Ex) The sled is let go and eventually slides to a stop on the ice due to friction. If the force of friction is 4.0 N and the work done on the sled by the ice is 24 J, how far did the sled go before it came to a stop?1. Write unknown and
givens
2. Identify the formula and rearrange, if needed
3. Convert units and find intermediates, if needed
4. Plug in and solve
5. Make sure the answer is reasonable
• Kinetic energy (KE ) – the energy of an object due to its motion
• Work-Energy theorem – states that the work done on an object is equal to the change in the object’s kinetic energy
The Work-Energy Theorem
Work-Energy Calculation
Ex) A car moving with a kinetic energy of 12,500 J slows to a stop as it approaches a stop sign. How much work is done on the car to bring it to a stop?1. Write unknown and
givens
2. Identify the formula and rearrange, if needed
3. Convert units and find intermediates, if needed
4. Plug in and solve
5. Make sure the answer is reasonable
Work-Energy Calculation
Ex) A second car approaches the same stop sign, but does not slow to a complete stop before proceeding. The car has a kinetic energy of 13,000 J as it begins to slow down. The brakes do 9,000 J of work on the car. How much kinetic energy does the car have when it begins to proceed through the stop sign?1. Write unknown and
givens
2. Identify the formula and rearrange, if needed
3. Convert units and find intermediates, if needed
4. Plug in and solve
5. Make sure the answer is reasonable
Work-Energy Calculation
Ex) A child pulling a sled is joined by a friend. Combined, they do 200 J of work on the sled causing it to move with a final kinetic energy of 350 J. What was the kinetic energy of the sled when only the first child was pulling it?1. Write unknown and
givens
2. Identify the formula and rearrange, if needed
3. Convert units and find intermediates, if needed
4. Plug in and solve
5. Make sure the answer is reasonable
Learning Objectives• Explain how work is done on an object• Describe the relationship between the work
done on an object and the kinetic energy of the object
• Calculate the quantities associated with work and the work-energy theorem
Work-Energy Theorem
