P1aL7 Energy, Work, and Power

Key Words

Energy - ability to do work

Joules - unit of energy or work.

Power - how much energy is used per second, or how much work is done every second.

Heat energy - energy flowing from a hot object to a cold object.

Watt - unit of power.  1 watt = 1 joule per second

Weight - force on a mass due to gravity.

Work - force × distance moved.

Grade E

If you lift a bag of sand onto a lorry , you do a job of work and use energy.  You have lifted the bag against the force of gravity which is pulling it to the ground.  The energy you have used comes from your food.

Both work and energy have the same units, joules (J).

If you use a hoist to lift twenty 25 kg bags of sand, you get the job done quicker than if you lifted each one in turn.  You could not lift 20 bags of sand at once; you are not strong enough.  A machine does the same amount of work as you would do, but in a shorter time.  It is more powerful.

Power is how much energy is used (or work done) every second.  The units are watts (W).  1 W = 1 J/s

Grade C

The amount of work you do in lifting bags of sand onto a lorry is the same as the energy gained.  Work done (J) = energy gained (J).

Work done is calculated using the equation:

work done (J) = force (N) × distance moved in the direction of the force (m)

In Physics code:

W = Fd

In triangle form:

When you run upstairs you are doing work by moving your weight vertically.  Remember:

Weight is a force measured in Newtons.

Your mass, measured in kilograms, is the amount of material in your body.  Weight is due to the pull of gravity.

Weight (N) = mass (kg) × acceleration due to gravity (m/s²)

The acceleration due to gravity = 10 m/s².  Therefore 1 kg weighs 10 N.

The energy gained when you run upstairs is:

Energy gained (J)  = weight (N) × vertical height (m)

Power is the amount of work done every second.  The equation is:

power (W) = energy (J) ÷ time (s)

In Physics code:

P = E/t

In triangle form:

  If you see the time given as minutes, you must convert it into seconds.

Grade A

If, instead of lifting our 25 kg bags of sand onto the lorry, we used a sack trolley and a shallow slope to move them, we would find it easier.  This is because we are moving the bags against the component of the weight that is parallel to the slope, not the weight itself.  However we would do the same amount of work, but require less power.