Resistance
Objectives

Identify the effects of air resistance, water resistance and friction
Plenary
Drop a feather and a hammer. Of course, the hammer will reach the floor first. Then show
pupils the footage of David Scott’s feather and hammer drop on the moon (Apollo 15). Pupils
should discuss what they see and suggest why the drops may be different.
Pupils are unlikely to be able to state that is no air resistance on the moon because there is a
vacuum/no atmosphere. This discussion may be returned to at the end of the lesson.
Resistance
Give two volunteers sheets of paper (one flat, one scrunched into a ball) ask them to throw
their paper as far as possible. The pupil with the flat sheet of paper will struggle.
Explain that this is due to air resistance. Because the surface of the flat sheet is larger, there is
more air pushing against it as it moves; this causes it to slow down. When an object falls due
to gravity, it will be resisted by air resistance, which acts in the opposite direction.
Pupils must note:
Air resistance is a force, caused by air, which makes objects slow down.
Pupils may test their understanding of air resistance by filling in the worksheet. The objects
with large surface areas in the direction of motion (lorry, leaf, feather, sheet of paper) will
experience greater air resistance.
Now ask pupils to imagine moving some of these objects through water instead of air. Would
the race car still be faster than the lorry? Yes; this is due to water resistance. Pupils will
recognise it is easier to swim horizontally through water (minimising the surface area in the
direction of motion) than to walk through water and that it easier to push a sheet of plastic
underwater when it is vertical than when it is flat against the surface.
Pupils must note:
Water resistance is a force, caused by water, which makes objects slow down.
We have talked about resistance due to a gases and liquids, so how about resistance due to
solids? Study photographs of racing bicycle wheels. Ask how they are designed for speed.
Pupils may notice that they are thin and smooth; this is to minimise the area in contact with
the ground. The resistive force which acts between an object and a surface is called friction.
© Education Umbrella 2015
Pupils must note:
Friction is a force, caused by contact, which makes objects slow down.
Explain that it is harder for objects to move on rough surfaces because there is more friction.
Smooth surfaces, such as an ice rink, provide little friction.
EXPERIMENT: Provide a selection of materials, such as plastic bags, cardboard, string, tape
and paper clips. Explain that a bar of chocolate will be thrown from a window and their task
is to design the best parachute (in groups) to slow its fall. Ask the pupils how they may judge
which parachute is the best. They may suggest measuring the time it takes for the chocolate
to fall. Establish that they should maximise air resistance to slow the fall.
After construction is complete, drop the chocolate-loaded parachutes from a window (the
higher the better) while pupils record the fall time with stop watches. The group with the
longest fall time wins the chocolate.
OR
Provide groups of pupils with tin foil and water tanks. They should design boats which float
in the tanks and hold as many marbles as possible. Pupils should test their designs by floating
them in the tank and counting how many marbles may be contained. Pupils should record
their progress with diagrams of their boats and notes of how many marbles they held. Pupils
will discover that the best boats are large and nearly flat. This is because contact between the
boat and the water is greater, resulting in high water resistance.
OR
EXPERIMENT: Split pupils into pairs. One pupil will use a stop watch while the other will
be the runner. The pairs measure how long it takes for their runners to sprint 50m. The
runners should then attempt to sprint 50m holding out large sheets of card or plastic in front
of them. The pupils will find that it takes longer to run holding the sheet because they
experience greater air resistance.
Possible extra-curricular questions
What causes air/water resistance?
Air/water resistance is caused by an object pushing air/water out of its path as it moves.
Objects with larger surface areas in the direction of motion push more air/water so
experience more resistance.
Why is there no air on the moon?
The moon is much smaller than the Earth and therefore has a much weaker gravitational
field. It does not have enough gravity to hold down an atmosphere, so instead of having air,
there is just a vacuum. Some of the larger moons, such as Titan, have enough gravity to hold
down an atmosphere.
© Education Umbrella 2015