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C
Force
Forces
Types of forces
Balanced and unbalanced
forces
Investigating the
relationship between
force and acceleration
Force, acceleration and
mass
Example: Pushing cars
Example: Force
calculations
Example: Mass and
weight
Gravity and falling
Forces
A force is a push or a pull that one object exerts on
another.
It makes an object change its shape, speed or
direction of movement.
Force is measured in newtons (N) .
Forces can be added together, but to do so you must
know the size and direction of the forces. The answer
you get when you add the forces together is called the
total, net or resultant force.
Eg if a car has a thrust force of 50 N, but also has a
friction force of 20 N backwards then there is a net
force of 50 N – 20 N = 30 N acting forwards.
Types of forces
Forces can be contact forces, that need to touch when
they are acting. For example:
 A club hitting a golf-ball.
 The wind blowing the leaves on a tree.
 A ball rolling along the ground and slowing down.
Or forces can be field forces (non-contact) where
there is no touching. For example:
• Magnetic forces
• Electrostatic forces
• Gravitational forces.
Forces
A force is a push or a pull. It makes an object change
its shape, speed or direction of movement.
Force is measured in newtons (N).
If more than one force is acting on an object, the
forces can be added together but you must take into
account the direction. The sum of the forces is called
the net, total or resultant force.
Forces can be contact forces where the force touches
the object. Eg a bat hitting a ball, the wind blowing the
leaves. Or forces can be field forces (non-contact)
where there is no touching. Eg magnetic, electrostatic,
gravitational forces.
Balanced and unbalanced forces
If the net force is zero then the forces are said to be
balanced.
When the forces are balanced, the object will remain
stationary or, if it is moving, it will continue to move
with its speed and direction unchanged.
Examples of balanced forces:
300 N
500 N
If the net (resultant or total) force is not zero then
the forces are unbalanced. This results in the object
changing its speed and/or direction – accelerating in
the direction of the net force.
Balanced and unbalanced forces
If there is no net force (the net force is zero), then
the forces on an object are balanced. The object will
remain stationary or continue moving with its speed and
direction unchanged.
Examples of objects with balanced forces on them are
a cup on a table or a balloon flying at constant speed in
a straight line.
If there is an unbalanced force acting on an object it
will speed up, slow down, change its shape or change its
direction.
1C 1 Balanced and unbalanced forces
Mass and weight
Mass is a measure of how much matter something
contains. Mass is measured in kg. Your mass is the
same wherever you are.
Weight is the force of gravity on you. It changes as
gravity changes in different places (eg on the Moon).
On Earth, gravity (g) pulls
with an average strength of
9.81 N kg–1, which we often
approximate to 10 N kg–1 to
make the maths easier.
On the Moon, g = 1.63 N kg–1.
An astronaut has a mass of 80 kg.
On Earth her weight is given by:
Weight  m  g
 80 kg  10 N kg–1
 800 N
On the Moon g = 1.7 N kg–1 and her
weight is 136 N.
Weight  m

g
 80 kg  1.7 N kg–1
 136 N
Her mass on the Moon is the same
as it is on Earth: 80 kg.
Mass and weight
Mass is the amount of matter in an object in kg.
Weight is the force of gravity on an object in newtons
(N).
Weightweight
 m  mg × g
On Earth, g is about 10 N kg–1. On the Moon g is about
1.7 N kg–1.
Someone with a mass of 80 kg has a weight of
80 kg × 10 N kg–1 = 800 N on Earth,
80 kg × 1.7 N kg–1 = 136 N on the Moon.
Their mass remains 80 kg no matter where they are.
1C 2 A stretchy problem
Investigating the relationship between
force and acceleration
This experiment is in your Workbook.
We shall use a falling mass to apply a steady force on a
trolley, and use a ticker-timer to measure its
acceleration.
Use the following slides to help you to set up the
equipment below.
Set up the tickertimer with about 2 m
of tape.
Remember to connect
the timer to the AC
terminals of the power
supply.
Each of these masses is 50 g, while
the mass-carrier is also 50 g.
Two masses plus the carrier have a
combined mass of 150 g.
Attach one end of a piece of string about 2 m long to
the front end of a trolley.
Attach a pulley to a
clamp stand.
Tie the other end of
the string to the mass
carrier and hook it over
the pulley.
Put the clamp stand
with pulley on a stool,
and adjust the height
of the pulley so that
the string is horizontal.
Push the clamp stand
out from the stool until
the mass is able to fall
freely to the floor
without hitting the
stool.
Use a G-clamp to hold
the stand to the stool in
the correct position.
(If you don’t have a Gclamp, use a pile of
books.)
Pull the string until the
mass-carrier is right up
to the pulley.
Use tape or a drawing pin to attach the ticker-tape to
the back of the trolley.
Bring the trolley as close to the ticker-timer as
possible.
Move the stool away from the bench until the string is
tight.
Hold the trolley in place.
Switch on the power supply. You will hear the hammer
hitting the tape. THEN release the trolley.
As the mass falls, it pulls the trolley, which pulls the
tape behind it.
Now do the experiment for yourself.
1C 3 Force and acceleration
Results
The dots on the tape get further apart, indicating that
the trolley was accelerating.
Mark off every 5th
dot...
... and join the strips
together to make a
speed-time graph.
... cut up the tape...
Label your finished graph with the mass used.
Repeat the
experiment using a
mass of 300 g.
5 x 50 g weights
plus
1 x 50 g carrier
The greater mass has
produced a greater
acceleration – the dots
are further apart and
gradient of the graph is
steeper.
The gradient of the
second graph is exactly
double that of the first.
When the force pulling
the trolley doubles, its
acceleration doubles.
Force, acceleration and mass
An unbalanced force acting on an object will cause it to
accelerate.
We know now that the acceleration produced is
proportional to the force.
We could carry on making ticker-tape graphs to show
the relationship between acceleration and mass for a
constant force.
Another way to think of the relationship between
force, mass and acceleration is to imagine pushing
cars.
Pushing cars
The greater the force on the car, the faster it will
accelerate.
If the forces are equal but the mass is doubled, then
the acceleration halves.
Two people pushing two cars has the same acceleration
as two separate people each pushing one car.
We have seen that acceleration goes up as force goes
up, and acceleration goes up as mass goes down.
Thus:
or
force
acceleration =
mass
force = mass × acceleration
Fnet
Where
And
= m ×a
F, force is measured in newtons (N)
m, mass, is measured in kilograms (kg)
a, acceleration, is measured in metres per
second squared (m s–2)
Force, acceleration and mass
The greater the force applied to an object, the
greater its acceleration. If the net force is zero then
the acceleration is zero.
A moderate force will cause a small mass to accelerate
more than a large one.
These two relationships are combined in the formula
force = mass × acceleration
F
 ma
net
Where
And
F, force is measured in newtons (N)
m, mass, is measured in kilograms (kg)
a, acceleration, is measured in metres per
second squared (m s–2)
Example: Force calculations
A car with a mass of 1800 kg
can accelerate at
1.85 m s–2. What is the net
force on the car?
m = 1800 kg a = 1.85 m s–2
F=?
F
 ma
 1800 kg × 1.85 m s 1
 3330 N
1C 4 Force, mass and acceleration
The car is loaded up with 5
people plus luggage. It now
has a total mass of 2210 kg.
What acceleration can they
expect to reach with this
force?
m = 2210 kg F = 3330 N
a=?
a
F

m
3330 N

2210 kg
 1.51 m s –2
Example: Mass and weight
A 70 kg person stands on the floor. How much force
pushes on the floor? What is the support force
supplied by the floor? The acceleration due to gravity
is 10 m s–2.
m = 70 kg,
a = 10 m s−2,
F
F=?
 ma
 70 kg  10 m s
 700 N
[Therefore, weight = mass × gravity]
Hence, the support force is 700 N.
-2
Gravity and falling
Gravity is the force that makes objects fall.
Gravity is always pulling objects down, whether they
are moving downwards, stationary, or going up.
All falling objects in the same gravitational field will
accelerate at the same rate (about 10 m s–2 on Earth).
Here an astronaut on the Moon
dropped a hammer and a feather.
Without air resistance, the
hammer and the feather fall with
the same acceleration and land
together.
See the drop online.
feather
On Earth, air resistance acts as a friction force
opposing the motion (it pushes in the opposite
direction to your movement).
Air resistance or friction only happens when an object
is moving.
Stopped at the top
Going up
Gravity Friction
force
force
Gravity
force only
Friction
force
Gravity Falling
force down
As the speed of a falling object increases, its air
resistance increases.
Eventually the friction force (air resistance) is equal
to the force of gravity.
When the forces are balanced, the object stops
accelerating. Physicists say that the object has
reached terminal velocity.
A parachute has a large surface
area, giving it a high air
resistance. With the parachute
open, the sky-diver slows down
until the forces again balance at
a much slower terminal velocity.
Gravity and falling
The force of gravity always acts downwards, even if an
object is moving upwards.
Gravity makes all objects fall with the same
gravitational acceleration, 10 m s-2 approx. (assuming no
air resistance) on Earth.
Air resistance and any other friction opposes the
motion and only acts when things are moving. Air
resistance increases with speed.
If there is air resistance then the net force between
gravity and air resistance gives the acceleration of the
object.
F
 ma
net
1C 5 More force calculations
1C 6 Living in gravitational fields
1C 7 Burnt out satellite
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