# Friction and Surface Area

In this worksheet, students will consider how surface area affects friction between solid surfaces and in air and water.

Key stage:  KS 2

Curriculum topic:   Forces

Curriculum subtopic:   Resistance and Friction

Difficulty level:

### QUESTION 1 of 10

Friction is created between surfaces which rub together. Smooth surfaces produce less friction.

Snowboards and skies are designed with very smooth surfaces underneath them, to allow them to travel as quickly as possible over the snow and ice.

Rough surfaces produce large forces of friction.

Although not solid materials, air and water create rubbing surfaces when objects travel through them. More friction is created when there is a large surface area 'rubbing' against the air or water. We call this friction resistance.

Fish have a smooth shape and surface to reduce the amount of water resistance as they swim, allowing them to swim quickly.

Rockets also have a smooth shape and surface to reduce air resistance.

Use this worksheet to see if there are any rough edges to knock off friction and resistance.

When the area of surfaces rubbing together is small, the friction is.....

increased

reduced

eliminated

Air produces which kind of friction?

air force

aeroplanes

air resistance

To increase air resistance, we must...

increase the surface area

reduce the surface area

keep the surfaces the same

Zeta and Vanya are investigating how surface area affects air resistance. They drop pieces of paper and measure the time taken to reach the ground.

Which THREE variables (factors) must they keep the same?

the type of paper

the height of the drop

who drops the paper

the surface area

the size of the sheets of paper

Which ONE variable must they change to investigate their question: "How does surface area affect air resistance?"

Vanya scrunches-up some of the sheets of paper into balls to change the...

surface area

weight

forces

They each make a prediction:

Zeta says: "I think that the paper will travel faster when the surface area is reduced."

Vanya says: "I don't think it will affect the speed because the weight is the same."

Who do you think is correct?

Zeta

Vanya

Zeta researches objects which use air resistance as a useful force.

Can you identify TWO of them?

parachute

kite

bicycle helmet

racing car

Water resistance is a force which.....

makes objects sink

makes objects wet

slows down objects

TWO of these will REDUCE water resistance.

Which are they?

a light weight

flippers

a smooth surface

a streamlined shape

Vanya takes three pieces of modelling clay which all weigh the same. She models them into different shapes: a ball, a cube and a torpedo (with a sharp pointed end).

She drops the clay shapes from the same height into a tank of water. She discovers that the torpedo shape sinks the quickest of all the shapes, pointed end first.

Which is the best explanation for this?

It has a smooth, streamlined shape.

It has a greater surface area.

It has more mass.

• Question 1

When the area of surfaces rubbing together is small, the friction is.....

reduced
EDDIE SAYS
If it's only a small area that is in contact, the amount of friction will be smaller too - did you know that when you are cycling on your bike the ACTUAL area of the tyre in contact with the ground is much smaller than you think. Like, when a snooker ball is rolling across the table, only a tiny bit of it is touching the surface at any one time.
• Question 2

Air produces which kind of friction?

air resistance
EDDIE SAYS
We can feel air resistance, for example the wind in our face, or cycling against the wind.
• Question 3

To increase air resistance, we must...

increase the surface area
EDDIE SAYS
What difference does a big parachute and a small parachute make? Or a big sail on a sailing boat compared with a tissue-sized sail? What about a tiny kite compared with a large kite? If there's more area to "catch the wind", there's going to be more air resistance. If you're relying on your parachute to get you safely to the ground, that's a big deal!
• Question 4

Zeta and Vanya are investigating how surface area affects air resistance. They drop pieces of paper and measure the time taken to reach the ground.

Which THREE variables (factors) must they keep the same?

the type of paper
the height of the drop
the size of the sheets of paper
EDDIE SAYS
Zeta and Vanya know that different types of paper have different weights, so they must use only one kind of paper. They must keep the size of the sheets the same so that they all weigh the same. The height of the drop must always be the same to make it a fair test.
• Question 5

Which ONE variable must they change to investigate their question: "How does surface area affect air resistance?"

surface area
EDDIE SAYS
If they are trying to find out the effect of surface area on the speed that paper falls through the air, that is the variable they are going to be changing to see what effect it has. What do you think they are predicting will happen?
• Question 6

Vanya scrunches-up some of the sheets of paper into balls to change the...

surface area
EDDIE SAYS
Folding and scrunching change the surface area of the paper sheets. Try it yourself: take two identical pieces of paper, scrunch one up and what do you observe has happened to its surface area?
• Question 7

They each make a prediction:

Zeta says: "I think that the paper will travel faster when the surface area is reduced."

Vanya says: "I don't think it will affect the speed because the weight is the same."

Who do you think is correct?

Zeta
EDDIE SAYS
Zeta tests her prediction and finds that the balls of paper travel to the ground quickest because the air resistance is reduced when the surface area is reduced. The surfaces 'rubbing' together are smaller!
• Question 8

Zeta researches objects which use air resistance as a useful force.

Can you identify TWO of them?

parachute
kite
EDDIE SAYS
Parachutes and kites have a large surface area to increase the air resistance. Vitally important when you're trying sky-diving! It also makes a big difference to the controllability of a kite and how long it stays in the air.
• Question 9

Water resistance is a force which.....

slows down objects
EDDIE SAYS
One example: the world record for SWIMMING 100m is 47 seconds. For RUNNING 100m it is 9.5 seconds. Why do you think it is so much slower in water? Could it be anything to do with how much denser water is than air (in other words, how much harder it is to push it out of the way so you can move through it)?
• Question 10

TWO of these will REDUCE water resistance.

Which are they?

a smooth surface
a streamlined shape
EDDIE SAYS
When you are trying to move through any medium, like water and air, you have to push it out of the way. The easier your shape makes that, the better. That's why submarines and aeroplanes are shaped the way they are: smooth and streamlined. Can you think of other examples?
• Question 11

Vanya takes three pieces of modelling clay which all weigh the same. She models them into different shapes: a ball, a cube and a torpedo (with a sharp pointed end).

She drops the clay shapes from the same height into a tank of water. She discovers that the torpedo shape sinks the quickest of all the shapes, pointed end first.

Which is the best explanation for this?

It has a smooth, streamlined shape.
EDDIE SAYS
Pointed, streamlined shapes are used in the design of boats, ships and submarines to reduce water resistance, but animals which live in water used this technology long before we did!
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