Forces

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Forces
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Menu

• What is a force?
• Drawing forces
• What can a force do?
• Terminal Velocity
• The Laws of Motion
• Gravity
• Weight, Mass Gravity

• Stopping Distances
• Work Done
• Power

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What is a Force?
A force is a PUSH
Or a PULL
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There are 6 different forces..

1. Gravity (always straight downwards)
2. Reaction force (from a surface)
3. Thrust/ Push
4. Drag (air resistance/ friction)
5. Lift (aeroplane wing)
6. Tension (rope / cable)

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How do we draw a force?
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We use arrows to show the size direction of a
Force
2N
This box has a force of 2N acting on it to the
right. It is an unbalanced force.
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In these examples, more than one Force is acting.
What is the RESULTANT FORCE?
Resultant 2N to left
4N
2N
Zero Resultant
2N
2N
Resultant 2N to right
3N
1N
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Are the forces Balanced or Unbalanced? State
whether the cars are at a steady speed or will
accelerate
Unbalanced Accelerate
A
Balanced steady Speed
B
Unbalanced Accelerate (slow down / decelerate)
C
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A Force can.
..Start something moving
..Stop something moving
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.Change an objects shape
..Change an objects direction
..and change an objects speed
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Look how this golf ball changes shape the split
second it is hit!
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The Terminal Velocity of a falling object depends
on their shape and area
The accelerating force acting on all falling
objects is gravity and it would make them all
fall at the same rate if it wasnt for air
resistance
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This means that 2 objects of different mass but
the same shape and area would fall at the SAME
rate
However, a feather a hammer would fall at
different rates because of air resistance
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What would happen on the Moon?
A hammer and feather would fall at exactly the
same rate because there is NO air and therefore
NO AIR RESISTANCE!
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The sky diver explains AIR RESISTANCE and
TERMINAL VELOCITY perfectly!
AIR RESISTANCE
.so the weight arrow will be the same size!
Note their weight doesnt change
WEIGHT
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Just Jumped
AIR RESISTANCE
Initially there is little air resistance and they
are unbalanced
The result is that they accelerate
WEIGHT
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As the speed increases the air resistance also
increases until it BALANCES the WEIGHT
AIR RESISTANCE
And there is NO acceleration
TERMINAL VELOCITY has been reached
WEIGHT
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The Parachutes have opened!
AIR RESISTANCE
The air resistance has now increased. The forces
are unbalanced
They decelerate (slow down)
WEIGHT
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The Laws of Motion
This is Sir Isaac Newton He is responsible for
the 3 Laws of Motion
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Newtons 1st law of Motion
BALANCED FORCES mean NO CHANGE IN VELOCITY
This car is travelling at a steady speed it
will not change direction or speed!
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Newtons 2nd law of Motion
If there is an UNBALANCED FORCE then the object
will ACCELERATE IN THAT DIRECTION
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Newtons 3rd law of Motion
If object A EXERTS A FORCE on object B.. Then
object B exerts the EXACT OPPOSITE FORCE on
object A
OR for every action there is an equal
opposite reaction
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Questions on- Force, mass and acceleration

1. A force of 3000N acts on a motor bike to make it
   accelerate by 2m/s2. What is the mass of the
   motor bike?
2. A car accelerates at a rate of 7m/s2. If it has
   a mass of 700kg how much driving force is the
   engine applying?
3. A force of 10N is applied by a boy while lifting
   a 15kg mass. How much does it accelerate by?

F
m x a
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Gravity is the force of attraction between all
masses

• It makes all objects accelerate to the ground

• It gives everything its weight

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Weight Mass are NOT the same!
MASS is the amount of matter in an object (for
any given object this will be the same ANYWHERE
in the Universe)
WEIGHT is caused by the pull of gravity
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Mass Weight Gravity are all related with this
formula-
W m x g
Weight mass x g Mass is in kg Weight is in
Newtons
This represents the strength of the gravity (on
Earth it is 10N/kg)
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Weight vs. Mass
On Earth the Gravitational Field Strength is
10N/kg This means- a 1kg mass is pulled
downwards by a force of 10N.

1. What is the weight on Earth of a book with mass
   1.5kg?
2. What is the weight on Earth of an apple with mass
   120g?
3. Debbie weighs 700N. What is her mass?

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What is the weight, in newtons of a 5kg mass,
both on the Earth on the Moon? (On Earth g 10
N/kg. On the Moon g 1.6 N/kg)
On Earth- W m x g 5 x 10 50N
On Moon- W m x g 5 x 1.6 8N
5kg
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Stopping distance of vehicles is affected by.
The Thinking Distance
And the
Braking Distance
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What affects the Thinking Distance?
Too much Alcohol
Tiredness
Thinking Distance
Poor Visibility
Drugs
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What affects the Braking Distance?
Icy Roads
Wet Roads
Braking Distance
Driving Too Fast
Brakes/ Tyres Worn
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Identify whether the Thinking distance or
Braking distance is affected in each case
C
A
B
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Stopping Distances Increase alarmingly with extra
speed (because velocity is squared!)
Kinetic Energy Transferred Work Done by
Brakes ½ mv2
f x d
v speed of car
f maximum braking force
d braking distance
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Work Done whenever a force moves an object
(whenever energy is transferred)
Work Done Force x Distance (Nm or J)
(N) (m)
1 joule of work is done whenever a force of 1
newton moves a distance of 1 metre
Wd
F x d
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Question Calculate the work done by a pupil
who pulls a car tyre 5 metres with a force of 60
N. Answer
Work Done FORCE x DISTANCE
60 N x 5 m
300 Nm
We could also give our answer as 300 J ( As 1Nm
1J)
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1) Calculate the work done when the broom moves a
3N force 6.5 metres
2) Calculate the distance moved by the broom if
it does 24J of work moving a 10N force
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What is POWER?
Power is the rate of doing work. A powerful
machine is one that can transfer a lot of energy
in a short space of time.
Work Done
J
Power
Time Taken
s
W
WD
The unit of power is the watt (W)
P x t
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To measure your power output

1. Measure the vertical height of the stairs
   (metres)
2. Measure your force (Newtons)
3. Measure the time it takes you to run or walk up
   the steps (seconds)
4. Calculate the WORK DONE (Nm)
5. Calculate the POWER (W)

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Results Table for Power
Name Force (N) Distance (m) Work done (J) Time (s) Power (W)





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