Energy and Power

P2: Work, energy and power

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Work

• relate work done to the magnitude of a force and distance moved in the direction of the force

• W = Fd

• Unit: Joule (J)

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WORK DONE = ENERGY TRANSFERRED​

Energy

Conservation of Energy

​the law of conservation of energy states that the total energy of an isolated system remains constant; it is said to be conserved over time.

Power

​Power is the work done per unit time.

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e.g. the power of an engine would be the work it does on the car per second.

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Or the power of an elevator system would be the work it does lifting people per second.

Multiple Choice

A student needs 10 Newtons of force to lift a box to the top of a 2 meter shelf. How much work do they do?

• 12 J

• 8 J

• 20 J

• 5 J

Multiple Choice

If the books is carried by a force which acts at right

angles to the motion, what is the work done?

• work done is maximum

• no work is done on the

  books.

• work done is 90J

• work done is minimum

Multiple Choice

The SI Unit for Energy is

• Joule

• Newton

• Calorie

• Watt

Multiple Choice

A battery converts what type of energy to another?

• sound to kinetic
• light to heat
• mechanical to electrical
• chemical to electrical

Multiple Choice

___________________________is the energy stored in a body or

system because of its position, shape and state.

• kinetic energy

• mechanical energy

• potential energy

• energy due to spring

Multiple Choice

Which of these items is not part of the law of conservation of energy?

• energy cannot be created

• energy cannot be destroyed

• energy can be transferred between forms

• energy is destroyed by friction

Multiple Choice

What is the difference between calculating power and work?

• Power takes into account the time it takes to complete a task
• Work takes into account the time it takes to complete a task
• Power is force/area
• Work is force/area

Multiple Choice

Bob (80 kg) uses 180 N of force to get him up the slope. The vertical distance is 2 m and the diagonal is 10 m. Calculate the work he does.

• 1600 J

• 160 J

• 3600 J

• 1800 J

Multiple Choice

A weight-lifter raises a 2000N weight through a vertical height of 2.0m in 0.80s. What useful power does he develop in doing this?

• 800 W

• 700W

• 4000W

• 5000W

Multiple Choice

Energy Efficiency is calculated by useful energy (Energy Out) divided by energy consumed (Energy In) , expressed as a percentage

• False

• True

Multiple Choice

Where does nuclear energy come from?

• Electric generators

• radioactive elements

• fossil fuels

• geothermal energy

Multiple Choice

Why is burning fossil fuels harmful? 

• then we won't have any fossil fuels if we burn them all
• They can get extremely hot
• Increases the amount of greenhouse gases in the air
• this would destroy fossils

Multiple Choice

Some power plants pump water down deep holes into hot rock. The hot rock heats the water, which turns the water into steam. The steam rushes back to Earth’s surface and is used to make electricity. This plant is generating which alternative energy resource?

• biomass
• wind
• solar energy
• geothermal energy

Multiple Choice

Why are fossil fuels like coal, oil, and natural gas called nonrenewable resources?

• a.  They can never be replaced.
• a.  They cannot be easily replaced.
• a.  They can be recycled.
• a.  They can be made in factories.