KS3

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KS3 Forces and Motion
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Contents Motion Weight, mass and
gravity Balanced and unbalanced
forces Friction Moments Pressure and Hydraulics
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Distance, Time and Speed

• To work out the speed of an object you need to
  know
• the distance traveled
• how long it took to travel that distance

distance traveled
time taken
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Formula triangle
Use this equation SPEED
d
distance
time
s
x
t
Speed is measured in many different units, e.g.
m/s, km/h, km/s, miles per hour. The units of
distance and time used will give the units to be
used for speed.
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Examples
A boy travels from his home to the cinema, a
distance of 10 km in 1 hour. Calculate his speed
in km/h.
d (distance in km)
Speed in km/h
t (time in h)
d
10 km

1 h
s
x
t
10 km/h
Cover the quantity you want to calculate - s
(speed)
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You sometimes have to change the units in this
type of problem - here is the same problem again
A boy travels from his home to the cinema, a
distance of 10 km in 1 hour. Calculate his speed
in m/s. (1km 1000m)
d (distance in m)
Speed in m/s
t (time in s)
d
10,000 m

3600 s
s
x
t
2.8 m/s
Cover the quantity you want to calculate - s
(speed)
Click for solution
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Questions 1. A group set off from home and walk
at an average speed of 3.6 km/h. How far would
they travel in 2 hours? Give your answer in km.
Click for solution
Distance (km) Speed (km/h) x time (h)
3.6 km/h x 2 h 7.2 km
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2. How long would it take a woman to walk 10 km
if her average speed is 5.4 km/h ?
Click for solution
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Experiments - Speed
1. Time how long it takes you to run 100m. 2.
Then calculate your speed for the run. SPEED
(m/s) 3. Repeat the experiment for each
member of your group. What was the fastest speed
for your group ?
Distance (m)
Time (s)
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Results
Name distance (m) time (s) speed
(m/s) 100 100
100 100 100
Conclusion The fastest member of the group with
a speed of ______ was _______.
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Weight, mass and gravity
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Weight and Mass
Weight and mass are not the same.
Mass is the amount of matter in an object. This
will have the same value anywhere in the Universe
including space.
Weight is a force and it is caused by the pull
of gravity.
In fact, weight is the pull of gravity acting on
a mass.
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Weight and Mass
Weight is a force so is measured in Newtons.
Like other forces it has both magnitude and
direction.
Mass is not a force, it is measured in kilograms.
A 1 kg mass will weigh less on the moon than it
does on Earth. This is because the force of
gravity is less on the moon because the moon is
smaller than the Earth. An astronaut could jump
20 feet into the air on the moon because gravity
is less. However, he still has the same body, and
the same mass, it just weighs less, because he is
on the moon and gravity is weaker.
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Weight and Mass
So, a scientist should never say He weighs
50kgs but, should say He has a mass of
50kgs, or the scientist could say the
gravitational force acting on his mass is about
500 Newtons. This is the same as saying his
weight is about 500 Newtons.
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Gravity
Gravity is an attractive force that acts between
all masses. The force depends on the mass of the
object. All objects produce a gravitational
force but it is only significant when the mass is
about the size of a moon or planet.
Think about it When you jump the gravitation
force of the Earth pulls you down. Your
gravitation force pulls the Earth up!
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Space shuttle - smaller mass
Force of gravity
The force of gravity depends on the mass of the
planet and how close you are. Objects will have
higher weight on Jupiter because it has a larger
mass than Earth.
Earth - large mass
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Balanced and unbalanced forces
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Forces
Balanced Forces
10 N
10 N
If you link two newton meters and pull equally
hard from both ends, the forces recorded on both
will be the same. We say that the forces acting
on the central hooks cancel each other out - they
are equal in magnitude and opposite in direction.
Because the forces are balanced the hooks do not
move.
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Forces
unbalanced Forces
11 N
10 N
movement
What happens if the pull on one end is harder
than on the other? The forces acting on the hooks
are no longer balanced. Both hooks will start to
move to the left, that is, their speed will
change. This is called acceleration.
Unbalanced forces lead to a change in speed or
direction.
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More Balanced and Unbalanced Forces
Think of a car traveling at a constant 50 mph.
The engine provides sufficient force to just
overcome all the frictional forces that are
acting to decrease the speed.
50 mph
500 N
500 N
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Now a cross-wind acting on the car produces a
sideways force.
50 mph
Cross wind
This causes the direction of the car to change.
This happens because the sideways forces on the
car are not balanced. If the car turns left so
that the wind is now BEHIND the car, what will
happen to the speed?
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The air resistance will decrease because the car
has a tail wind (it is being blown from
behind). This means the forces acting on the car
are no longer balanced. The car will increase in
speed (accelerate) until the forces are balanced
again.
gt 50 mph
400 N
500 N
60 mph
500 N
500 N
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• Summary
• If the forces on an object are balanced
• If it is stopped it will remain stopped.
• If it is moving then it will continue to move
  at the same speed.
• In other words, it will continue to do what it is
  already doing without any change.
• If the forces are unbalanced two things can
  happen
• The speed will change.
• The direction of motion will change.
• This is called acceleration.

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Resultant Forces The sum effect of more than one
force is called the resultant force. You can
find out the resultant force by calculating the
difference between opposing forces.
100 N
400 N
500 N
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Find the resultant force
1.
5N
5N
Click for solution
10N
20N
Resultant force 20N -10N 10N down The block
will accelerate down.
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2.
5N
Click for solution
5N
5N
Resultant force 5N - 0N 5N right. The
vertical forces are equal in size and opposite in
direction so there is no resultant force in the
vertical direction. The block will accelerate to
the right.
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7N
3.
3N
10N
20N
13N
10N
Click for solution
Resultant force 30 - 13 17N right. The
vertical forces are equal in size and opposite in
direction so there is no resultant force in the
vertical direction. The block will accelerate to
the right.
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Friction
Friction always tries to slow moving object down.
We say it opposes motion. Friction is created
whenever two touching objects or surfaces move
past each other. Friction also occurs when
things move through air. This is called air
resistance or drag.
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Friction
On the diagram label all sources of friction.
Click for answers
One more? Probably the most important
Air resistance, or Drag
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Air Resistance or drag

• Air resistance is a type of friction caused when
  objects move through the air.
• Cars are designed so that they are streamlined.
  The flow of air around the body is made as smooth
  as possible so that air resistance is minimized.
• Air resistance depends on
• the size of the car
• the shape of the car
• the speed of the car.

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Other Sources of Friction in Cars
One of the most important sources of friction in
cars in that between the tyre and the road. When
the car brakes, the
maximum possible amount of friction is desirable
so that the car does not skid.

• The friction between the tyre and the road is
  affected by
• inflation pressure of the tyre
• the road surface
• whether the surface is wet.

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Force and Rotation
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Force and Rotation
pivot
5N
A force acting on an object can cause it to turn
about a pivot. What would happen to the see-saw
above ? Would it turn? If so, clockwise or
anti-clockwise?
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Force and Rotation
pivot
The left goes down - an anticlockwise turn.
A turning force is called a moment.
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Moments
Pivot
Distance from force to pivot
Force
Suppose you were trying to unscrew a nut using a
spanner. The spanner exerts a moment or turning
force on the nut. If the moment is big enough it
will unscrew the nut. If not there are 2 ways of
increasing the moment.
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Increasing the moment
1. Increase the distance from the force to the
pivot - apply the force at the end or use a
longer spanner.
Pivot
Distance from force to pivot
Force
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2. Increase the force applied - push/pull harder
or get someone stronger to do it!
pivot
Distance from force to pivot
Force
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Moment
The moment of a force is given by the
relationship
Moment Force (N) x Distance (cm or m).

Moments are measured in Newton centimeter (Ncm)
or Newton metre (Nm).
moment
F
x
d
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A 500 N woman stands on one end of a see-saw. She
is 0.5m from the pivot. What moment does she
exert?
Click for solution
0.5m
pivot
500N
Moment 500 x 0.5 250 Nm.
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Principle of Moments
pivot
The green girl exerts an anti-clockwise moment
equal to ... her weight x distance
from pivot.
The yellow girl exerts a clockwise moment equal
to... her weight x distance from
pivot.
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Principle of Moments
pivot
If the two moments are equal then the seesaw is
balanced. This is known as the principle of
moments.
When balanced Total clockwise moment total
anti-clockwise moment c.m.
a-c.m.
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The principle of moments can be investigated
using the balance shown below with 10g masses
Moment (right) (3 x 10)
(4x10) 70gcm
Moment (left) 7 x 10 70gcm
Both moments are equal therefore the seesaw is
balanced.
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Using Moments in Calculations
1. Two girls are on a seesaw. One weighs 200N and
is 1.5m from the pivot. Where must her 150N
friend sit if the seesaw is to balance ?.
Click for solution
c.m. clockwise moment a-c.m.
anti-clockwise moment
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Pressure and Hydraulics
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Pressure
Pressure is exerted whenever a force is applied
over an area.
2.
1.
Which one exerts the biggest pressure, 1 or 2?
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1.
Case 1. The arm applies a force onto a board via
a finger tip. The force applied produces a high
pressure because the force acts over a small area.
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2.
Case 2. The arm applies the same force onto the
board. The force is now acting over a larger area
- the area of the palm is greater than the finger
tip. Thus, a lower pressure is produced.
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Pressure
Pressure is the force per unit area so is
calculated using the expression shown below
Pressure is measured in Newtons per metre
squared (N/m2) which is called a PASCAL
(Pa) Pressure can also be measured in Newtons
per millimetre squared (N/mm2) Newtons per
centimeter squared (N/cm2).
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The same force spread over a big area means low
pressure. Which shoes would you choose for
walking over a muddy field?
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The boots on the right spread the weight over a
larger area. Therefore, the pressure exerted on
the ground is low. In contrast, fashion shoes
have a smaller area and exert a higher pressure.
These shoes are likely to sink into soft ground.
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Application of Pressure
A force spread over a large area means low
pressure, e.g. skis and snowboards.
The large surface area of the board means the boy
exerts very little pressure on the snow. He
therefore slides over the top and does not sink
in.
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Application of Pressure
A force concentrated on a small area means high
pressure, e.g. razor blades, needles, high heeled
shoes, spurs, ice skates, sharp knives.
The high pressure on the cutting edge of an
ice-skate melts the ice and helps the skater
slide across the surface.
On the cutting edge of a knife a very high
pressure is exerted - this makes it easier to cut.
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Pressure in Liquids
In a liquid Pressure acts in all directions
and pressure increases with depth.
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The relationship between pressure and depth is
shown by a water bottle with holes along its
length.
low pressure
High pressure
Pressure (N/m2) 10 N/Kg x depth (m) x density
(Kg/m3)
The pull of gravity
The deeper you go, the higher the pressure
The denser the liquid, the heavier it is!
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Hydraulics
Hydraulic systems use the principle that pressure
is transmitted throughout a liquid. They are used
to transfer movement from one part of a machine
to another without linking them mechanically.
All hydraulic systems use two pistons linked
via a pipe carrying a special oil called
hydraulic fluid.

Force applied here
Force transferred here
Pressure inside all parts of the hydraulic system
is the same
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Hydraulics Brakes
All hydraulic brake systems (eg in a car) use a
small master piston and a bigger slave piston.
The master piston is used to apply a force. This
puts the liquid under pressure.
The pressure is transmitted to the pistons on all
four wheels.
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The slave piston always has a much larger area
than the master piston. The force exerted can be
calculated from the same equation
Much larger than master piston
Force exerted Pressure x Area slave piston
So, a greater force is exerted by the brakes than
the driver exerted on the pedal.
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The Hydraulic Brake
Friction shoes
Hydraulic fluid
Foot pedal
Slave pistons
drum
Master piston
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The Hydraulic Brake - example
The car master piston has an area of 5cm2. If a
force of 10N is applied to it, calculate the
pressure created in the brake pipes. If the
slave piston has an area of 50 cm2, calculate the
force exerted on the brake disc.
Click for solution
At the master piston, PF/A 10/5 2 N/cm2 At
the slave piston, F PxA 2x50 100 N (10
times the original force applied to the master
piston).