KS3

: P

2.3

Mo

tio

n &

Pre

ssu

reK

no

wle

dge

Org

anis

er

Ad

dit

ion

al In

form

atio

n

Ke

y p

oin

ts t

o le

arn

Key

po

ints

to

lear

n

Fan

tast

ic f

act!

Co

nte

nt

To p

rod

uce

th

e sa

me

pre

ssu

re o

n t

he

flo

or

that

yo

u e

xert

wh

en y

ou

pu

sh in

a d

raw

ing

pin

, yo

u w

ou

ld n

eed

ove

r 5

00

0 p

eop

le

stan

din

g o

n y

ou

r sh

ou

lder

s.

Spee

dSp

eed

= d

ista

nce

/tim

e m

easu

red

in m

etre

s p

er s

eco

nd

(m/s

).

Ave

rage

sp

eed

is t

he

tota

l

dis

tan

ce t

rave

lled

/to

tal t

ime

take

n.

Dis

tan

ce-t

ime

grap

hs

You

can

sh

ow

wh

at is

hap

pen

ing

to t

he

po

siti

on

of

an o

bje

ct o

n a

dis

tan

ce-t

ime

grap

h.

The

slo

pe

of

the

dis

tan

ce-t

ime

grap

h is

th

e

spee

d.

Gas

pre

ssu

reG

as p

ress

ure

is d

ue

to t

he

colli

sio

ns

of

gas

mo

lecu

les

wit

h

the

sid

es o

f th

e co

nta

iner

or

ob

ject

. If

th

e ga

s is

ho

tter

, or

com

pre

ssed

, th

ere

will

be

mo

re

colli

sio

ns

and

th

e p

ress

ure

will

be

grea

ter.

Atm

osp

her

ic

pre

ssu

re

Atm

osp

her

ic p

ress

ure

is d

ue

to

the

colli

sio

ns

of

air

mo

lecu

les

wit

h o

bje

cts.

Atm

osp

her

ic

pre

ssu

re d

ecre

ase

wit

h h

eigh

t

bec

ause

th

ere

are

few

er

air

mo

lecu

les

hig

her

up

.

Liq

uid

pre

ssu

reTh

e p

ress

ure

at

a p

arti

cula

r

dep

th o

f a

liqu

id d

epen

ds

up

on

the

we

igh

t o

f w

ater

ab

ove

it.

Pre

ssu

re in

crea

se w

ith

dep

th.

Liq

uid

s ar

e in

com

pre

ssib

le.

Cal

cula

tin

g

pre

ssu

re

Pre

ssu

re =

fo

rce/

area

mea

sure

d

in N

/m2

or

N/c

m2.

The

pre

ssu

re

tells

yo

u h

ow

th

e fo

rce

is s

pre

ad

ou

t o

ver

an a

rea.

Mo

men

tsTh

e tu

rnin

g ef

fect

of

a fo

rce

is

calle

d a

mo

men

t. Y

ou

cal

cula

te a

mo

men

t b

y m

ult

iply

ing

the

forc

e

by

the

dis

tan

ce f

rom

a p

ivo

t.

Equ

ilib

riu

mIf

th

e cl

ock

wis

e m

om

ents

act

ing

on

an

ob

ject

eq

ual

th

e

anti

clo

ckw

ise

mo

men

ts t

he

ob

ject

will

be

in e

qu

ilib

riu

m.

This

is h

ow

see

-saw

s b

alan

ce.

Cen

tre

of

grav

ity

The

cen

tre

of

grav

ity

is t

he

po

int

at w

hic

h a

ll th

e w

eig

ht

of

the

ob

ject

ap

pea

rs t

o a

ct

Turn

ing

forc

eTh

e w

eig

ht

of

an o

bje

ct a

ctin

g

thro

ugh

th

e ce

ntr

e o

f m

ass

can

pro

du

ce a

tu

rnin

g ef

fect

Ph

ysic

s1

.1 F

orc

es

1.2

So

un

d

1.4

Sp

ace

1.3

Lig

ht

2.1

Ele

ctri

city

an

d m

agn

etis

m

2.3

Mo

tio

n a

nd

pre

ssu

re

2.2

En

ergy

P2

Ch

eckl

ist

Cha

pter

3

Less

on

Dev

elop

ing

S

ecu

re

Exte

nd

ing

P2 3

.1 S

peed

I ca

n st

ate

the

equa

tion

for

spee

d.

I

can

calc

ulat

e sp

eed

usin

g th

e sp

eed

equa

tion.

I

can

use

the

spee

d eq

uatio

n to

ex

plai

n un

fam

iliar

situ

atio

ns.

I ca

n de

fine

rela

tive

mot

ion.

I ca

n de

scribe

rel

ativ

e m

otio

n.

I

can

expl

ain

wha

t is

mea

nt b

y re

lativ

e m

otio

n an

d ho

w it

can

be

cal

cula

ted.

P2 3

.2 M

otio

n gr

aphs

I ca

n de

scribe

sim

ply

wha

t a

dist

ance

–tim

e gr

aph

show

s.

I

can

inte

rpre

t di

stan

ce–t

ime

grap

hs.

I

can

draw

dis

tanc

e–tim

e gr

aphs

fo

r a

rang

e of

jou

rney

.

I ca

n us

e a

dist

ance

–tim

e

grap

h to

des

crib

e a

jour

ney

qual

itativ

ely.

I

can

calc

ulat

e sp

eed

from

a

dist

ance

–tim

e gr

aph.

I

can

anal

yse

jour

neys

usi

ng

dist

ance

–tim

e gr

aphs

.

P2 3

.3 P

ress

ure

in g

ases

I ca

n st

ate

two

thin

gs t

hat

affe

ct g

as p

ress

ure.

I ca

n de

scribe

the

fac

tors

th

at a

ffec

t ga

s pr

essu

re.

I

can

expl

ain

gas

pres

sure

in

differ

ent

situ

atio

ns.

I ca

n st

ate

the

caus

e of

at

mos

pher

ic p

ress

ure.

I

can

desc

ribe

how

at

mos

pher

ic p

ress

ure

chan

ges

with

hei

ght.

I

can

com

pare

som

e ef

fect

s of

at

mos

pher

ic p

ress

ure.

P2 3

.4 P

ress

ure

in li

quid

s

I ca

n st

ate

sim

ply

wha

t ha

ppen

s to

pre

ssur

e w

ith d

epth

.

I ca

n de

scribe

how

liqu

id

pres

sure

cha

nges

with

dep

th.

I

can

expl

ain

why

liqu

id p

ress

ure

chan

ges

with

dep

th.

I ca

n de

scribe

cha

ract

eris

tics

of

som

e ob

ject

s th

at flo

at a

nd

som

e th

at s

ink.

I

can

expl

ain

why

som

e th

ings

flo

at a

nd s

ome

thin

gs

sink

, us

ing

forc

e di

agra

ms.

I

can

expl

ain

why

an

obje

ct w

ill

float

or

sink

in t

erm

s of

for

ce o

r de

nsity

.

P2 3

.5 P

ress

ure

on s

olid

s

I ca

n st

ate

the

equa

tion

of

pres

sure

.

I ca

n ca

lcul

ate

pres

sure

.

I ca

n ca

lcul

ate

pres

sure

in

mul

tiste

p pr

oble

ms.

I ca

n us

e id

eas

of p

ress

ure

to

desc

ribe

fam

iliar

situ

atio

ns

qual

itativ

ely.

I

can

appl

y id

eas

of p

ress

ure

to d

iffer

ent

situ

atio

ns.

I

can

com

pare

pre

ssur

e in

di

ffer

ent

situ

atio

ns,

expl

aini

ng

the

diff

eren

ces

in p

ress

ure

usin

g sc

ient

ific

know

ledg

e.

P2

Ch

eckl

ist

Cha

pter

3

Less

on

Dev

elop

ing

S

ecu

re

Exte

nd

ing

P2 3

.6 T

urni

ng

forc

es

I ca

n st

ate

the

law

of

mom

ents

.

I ca

n de

scribe

wha

t is

mea

nt

by a

mom

ent.

I ca

n ap

ply

the

conc

ept

of

mom

ents

to

ever

yday

situ

atio

ns.

I ca

n st

ate

the

equa

tion

to

calc

ulat

e a

turn

ing

forc

e.

I

can

calc

ulat

e th

e m

omen

t of

a for

ce.

I

can

use

calc

ulat

ions

to

expl

ain

situ

atio

ns in

volv

ing

mom

ents

.

Chapter 3 Glossary

© Oxford University Press 2015 www.oxfordsecondary.co.uk/acknowledgements This resource sheet may have been changed from the original.

Key word Definition

acceleration The amount by which speed increases in one second.

atmospheric pressure Pressure caused by the collisions of air molecules that produce a force on an area.

average speed The total distance travelled in the total time taken for a complete journey.

centre of gravity The point in an object where the force of gravity seems to act.

centre of mass The point in an object where the mass of an object seems to act.

compressed Squashed into a smaller space.

density The mass of a material in a certain volume.

distance-time graph A graph that shows how far an object moves each second.

gas pressure The force exerted by air particles when they collide with a surface.

incompressible Cannot be compressed (squashed)

instantaneous speed The speed at a particular moment.

law of moments An object is in equilibrium if the clockwise moments equal the anticlockwise moments.

liquid pressure The pressure produced by collisions of particles in a liquid.

metres per second A unit of speed.

moment A measure of the ability of a force to rotate an object about a pivot.

newton metres The unit of moment.

newtons per metre squared A unit of pressure.

pivot The point about which a lever or see-saw balances.

pressure A force exerted on a certain area.

relative motion The difference between the speeds of two moving objects, or of a moving and a stationary object.

speed A measure of how far something travels in a given time.

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This resource sheet may have been changed from the original.

Study guide information sheet P

Balanced and unbalanced

What are balanced forces?

When the forces acting on an object are the same size but in opposite

directions we say that they are balanced. You can think of balanced forces

like two teams in a tug of war. If each team pulls with the same force the

rope doesn’t move. The forces cancel out. The object is in equilibrium. All

stationary objects are in equilibrium. There must be a support force acting

on them to balance out their weight.

What are unbalanced forces?

The forces acting on this rocket-powered car are unbalanced. They are not

the same size so they do not cancel out. The driving force from the engine

is much, much bigger than the resistive forces from air resistance and

friction.

How do unbalanced forces change speed and

direction?

When the car's rocket-powered engine starts up the driving force will

become very big very quickly. When the driver wants to stop he will fire a

parachute to slow the car down. In both cases the forces on the car are

unbalanced.

The driving force is bigger than the

resistive forces acting on the car. The

speed of the car increases.

The only forces acting on the car are

resistive forces. The speed of the car

decreases.

Every time you go around a corner in a car the friction between the tyres

and the road changes the direction of the car.

State what equilibrium means.

Quick question

State the difference between balanced forces and unbalanced

forces.

Quick question

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This resource sheet may have been changed from the original.

Study guide information sheet P

Introduction to forces A force can be a push or a pull.

Forces explain why objects move in the way that they do, or why they don’t

move at all. Forces can change the direction that objects are moving in, and

change their shape.

Force diagrams

You can’t see forces but you can see the effect of them.

When you draw a diagram you add arrows to the show forces that are

acting. Force arrows show the direction and the size of the force.

Forces act on objects so the arrow must touch the object in the diagram.

Interaction pairs

Forces always come in pairs. The pairs are called interaction pairs.

In the diagram of the tennis ball sitting on the table:

Gravity pulls the tennis ball down. This is the force of the Earth on

the tennis ball.

The tennis ball pulls the Earth up. This is the force of the tennis ball

on the Earth.

How do you measure forces?

You can measure force with a newtonmeter). All forces are measured in

newtons (N).

Draw a force diagram to show the forces acting on an object sitting

on a table.

Quick question

Give the unit of force.

Quick question

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This resource sheet may have been changed from the original.

Study guide information sheet P

Non-contact forces

Acting at a distance A gravitational force acts on a diver jumping off a diving board. It is a non-

contact force. There are other types of non-contact force. Magnets exert a

magnetic force on magnetic materials or other magnets without touching

them. If you rub a balloon you can pick up bits of paper with it. This is an

electric or electrostatic force. Magnetic and electrostatic forces are non-

contact forces.

Force fields

In physics a field is a special region where something experiences a force.

There is a magnetic field around a magnet where magnetic materials

experience a force. There are gravitational fields where things with mass

experience a force. The further away from the mass, magnet, or charge, the

field gets weaker. Contact forces only act when the objects are touching

each other. Non-contact forces act at any distance, even if the objects are

not touching.

Weight and mass

Weight is a force so it is measured in newtons (N). Mass is the amount of

'stuff' something is made up of and it is measure in kilograms (kg) You can

calculate weight using an equation:

weight (N) = mass (kg) x gravitational field strength, g (N/kg)

On Earth gravitational field strength is about 10 N/kg. This means that, if

your mass is 50 kg, for example, then your weight on Earth is:

weight = 50 kg × 10 N/kg = 500 N

Gravitational field strength is different on other planets and stars. Your

weight would be different on different planets because g would be different

but your mass would remain the same.

Identify three forces that act at a distance.

Quick question

State the unit of mass and the unit of weight.

Quick question

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This resource sheet may have been changed from the original.

Study guide information sheet P

Types of forces

Changing shape

When a ball hits the floor the ball deforms. Forces can compress (squash)

or stretch objects. When you exert a force you can deform an object. You

can compress it or you can stretch it.

How can the floor push you up?

The floor is a solid; solids are made up of particles arranged in a regular

pattern. The particles are joined strongly together by bonds. When you

stand on the floor your weight pushes the particles together. The bonds are

then compressed and push back. This ‘push back’ supports you.

Stretching

Bungee cords, springs, and even lift cables all stretch when you exert a

force on them. The amount that they stretch is called the extension.

Springs are special. If you double the force on the spring the extension will

double. If the extension doubles when you double the force then the object

obeys Hooke’s Law. The graph of force against extension is a straight line,

or linear. Hooke’s Law is a special case. Not everything behaves like a

spring when you stretch it. If you double the force on an elastic band the

extension may not double.

What is friction?

When a book is resting on the table you can push on it but it may not move.

Friction grips objects. As you increase the force by pushing harder the book

will start to move. If you remove the force the book slows down and stops.

This is because the rough surfaces can no longer move past each other.

Describe what happens to a tennis ball when it hits the ground

the ground.

Quick question

State Hooke’s Law.

Quick question

State two things that friction does.

Quick question

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Study guide information sheet P

What are drag forces?

A dolphin swimming through the water and a surfer paddling through water

will both experience water resistance. As a snowboarder jumps through

the air he will experience air resistance. Water resistance and air

resistance are drag forces. To understand drag forces you need to think

about the particles in the air and the water.

As a dolphin moves through the water it pushes the water particles out of

the way. This produces a drag force, which slows it down.

Name the drag force acting on an aeroplane in flight.

Quick question