Work Done & Energy Transfer Flashcards

All 17 cards in this deck

When is work done on an object?

When a force causes the object to move through a distance (a displacement) along the line of action of the force.

What is the equation linking work done ($W$), force ($F$) and distance ($s$)?

work done = force $\times$ distance, or $W = F\,s$ (distance moved along the line of action of the force)

In $W = F\,s$, what unit is each quantity measured in?

$W$ in joules (J), $F$ in newtons (N), $s$ in metres (m).

True or false? If a force acts on an object but the object does not move, no work is done on the object.

True. Work needs a displacement: with $s = 0$, $W = F \times 0 = 0$ J.

What are the steps to calculate work done from a force and a distance?

e.g. a car travels 7.5 km against an air resistance of 4000 N

1. Convert the distance to metres: $7.5\text{ km} = 7500$ m
2. Substitute into $W = F\,s$: $W = 4000 \times 7500$
3. Work out the answer in joules: $W = 30\,000\,000$ J

What are the steps to find the force when you know the work done and the distance?

e.g. 35 J of work is done pushing a walker 2.8 m

1. Write the equation: $W = F\,s$, so $35 = F \times 2.8$
2. Rearrange: $F = \dfrac{W}{s} = \dfrac{35}{2.8}$
3. Answer in newtons: $F = 12.5$ N

Complete the definition: one joule of work is done when...

...a force of one newton causes a displacement of one metre.

What is 1 joule equal to in newton-metres?

$1\text{ J} = 1\text{ N m}$

True or false? 1 joule is the same as 1 N/m.

False. 1 joule is the same as 1 N m (newton-metre).

How much energy is transferred when 1 J of work is done?

1 J of energy — work done is equal to the energy transferred.

Which energy transfer takes place when a person lifts a box at a steady speed?

Energy is transferred mechanically from the chemical energy store of the person to the gravitational potential energy store of the box.

What are the steps to find the increase in an object's kinetic energy store from the work done on it?

e.g. a 200 N force pushes a trolley 3 m along a smooth level floor

1. Calculate the work done: $W = F\,s = 200 \times 3 = 600$ J
2. Work done = energy transferred, so the kinetic energy store increases by 600 J
3. If the speed is needed, put this into $E_k = \frac{1}{2}mv^2$

What happens to an object when work is done against the frictional forces acting on it?

Its temperature rises.

Which energy store is filled when work is done against friction?

The thermal energy store of the object (and its surroundings).

True or false? Work done against air resistance causes a rise in temperature, just as work done against friction does.

True. Air resistance is a frictional force, so energy is transferred to thermal energy stores.

True or false? A car driving at a constant speed does no work, because its kinetic energy store does not change.

False. It still does work against air resistance and friction, transferring energy to thermal energy stores of the surroundings.

What effect does lubricating the moving parts of a machine have on work done against friction?

It reduces friction, so less work is done against friction and less energy is transferred to thermal energy stores.