# Forces and gravity

```1 of 54
2 of 54
What are forces?
3 of 54
Push and pull
4 of 54
Types of forces
All forces involve interactions
between objects. There are
several different types.
Gravity and magnetism
are forces that can act over
distances.
Friction and upthrust are
forces that involve direct
contact between objects.
All types of forces can occur whether objects are still or moving.
What forces are acting during this rocket launch?
5 of 54
Forces affecting objects
6 of 54
Forces changing an object’s shape
7 of 54
Balanced forces
There are two wind machines applying forces to the ice skater.
The forces acting on the skater are equal in magnitude and
opposite in direction.
The forces are balanced, so they cancel each other out.
The skater does not move.
8 of 54
Unbalanced forces
What if only one wind machine is blowing on the skater?
The forces acting on her are no longer balanced so she
will start to move to the left. Her speed will change – this
is called acceleration.
Unbalanced forces lead to a change in speed or direction.
9 of 54
Measuring forces
10 of 54
11 of 54
Force pairs
12 of 54
Balanced and unbalanced forces
Imagine a car travelling at a constant speed of 50 km/h.
The engine provides sufficient force to balance all the
frictional forces that are acting to decrease the speed.
500 N
13 of 54
500 N
Balanced and unbalanced forces
A crosswind acting on the car produces a sideways force.
500 N
500 N
100 N
cross wind
The crosswind causes the direction of the car to change
– this happens because the sideways forces on the car
are not balanced. The car will veer sideways.
If the car turns right so that the wind is now behind the
car, what will happen to the speed?
14 of 54
Resultant force
The sum effect of more than one force is called the
resultant force.
The resultant force is calculated by working out the
difference between opposing forces in each direction.
What is the resultant force on this truck?
A resultant force of 100 N is accelerating the truck.
400 N
15 of 54
500 N
Balanced and unbalanced forces – summary
If the forces on an object are balanced:
 and the object is stationary, it will remain stationary
 and the object is moving, it will continue to move at
the same speed and in a straight line.
In other words, the object will continue to do what it is
If the forces are unbalanced, two things can happen:
 The speed can change. This is called acceleration.
 The direction of motion can change.
16 of 54
Resultant forces – question 1
1. What is the resultant force on the satellite?
5N 5N
Resultant force = 20 N – 10 N
= 10 N down
The satellite will accelerate
downwards.
20 N
17 of 54
Resultant forces – question 2
2. What is the resultant force on the bird?
The forces acting in each
direction horizontally are equal
in size, so there is no resultant
force in this direction.
Resultant force = 5 N – 5 N = 0 N
5 N The vertical forces are not
balanced, the bird will accelerate
in a downwards direction.
5N
5N
18 of 54
Resultant force
= 5N – 0N
= 5 N down
Resultant forces – question 3
3. What is the resultant force on the yacht?
10 N
10 N
13 N
20 N
10 N
The vertical forces are equal in size and opposite in direction
so there is no resultant force in the vertical direction.
The horizontal forces are not balanced, so the yacht will
accelerate to the right.
Resultant force = (20 N +10 N) – 13 N
= 17 N right
19 of 54
20 of 54
What is gravity?
When the netball is thrown, why does it fall back down?
There is a gravitational force
pulling it towards the Earth.
Gravity is a force that occurs between all objects.
Gravity always acts to pull objects
towards each other.
The Earth and the ball are
pulling each other together.
However, the ball moves much
more than the Earth because
it has a much smaller mass.
21 of 54
Sir Isaac Newton
Sir Isaac Newton is a very
famous physicist who lived in
England 1643–1727.
Newton wrote down his ideas
in the Philosophiae Naturalis
Principia Mathematica; a very
and gravity.
Some accounts suggest that one
of Newton’s greatest discoveries
occurred when an apple fell on
22 of 54
Gravity and Newton
Newton realized that the motion
of falling objects and objects
orbiting in space must be caused
by the same force – Gravity!
He wrote in the Philosophiae
Naturalis, “It is an attractive
force that makes apples fall
from trees and the planets orbit
the Sun.”
noted the effects of gravity but
Newton was the first to calculate
the force of gravity on objects.
23 of 54
What is gravity?
Gravity is an attractive
force that acts between
all objects that have
mass. The size of the
force depends on the
masses of the objects.
Satellite’s
gravity
Earth’s gravity
All objects produce a
gravitational force. This
is very large for huge
masses such as planets.
When you jump, the gravitational force of the Earth pulls you
down. Your gravitational force also pulls the Earth towards
you, but you don’t notice it because the Earth is too heavy to
be visibly affected by your gravity.
24 of 54
Gravity and distance
The force of gravity between two
objects depends on their masses
and the distance between them.
Spacecraft produce a very
large force, called thrust, to
overcome the force of gravity.
As a spacecraft gets further
away from Earth, the force
of gravity gets smaller.
Why do spacecraft lose their large fuel tanks and booster
rockets once they have left the Earth’s surface?
25 of 54
Gravity during a rocket launch
26 of 54
What are mass and weight?
Mass is the amount of
matter in an object and is
measured in kilograms.
Mass is not a force.
An object, such as this
satellite, has the same
mass at any point in the
Universe.
Weight is a force and is caused by the pull of gravity acting
on a mass.
Weight is measured in newtons and has both magnitude
and direction. An object’s weight changes depending on
where it is in the Universe.
27 of 54
Investigating mass and weight
28 of 54
Mass and weight on the Moon
The force of gravity on the Moon
is only one-sixth of that on Earth
because the Moon has a much
smaller mass.
Any object on the Moon weighs
one-sixth of the amount it would
weigh on Earth.
Astronauts can jump up 20 feet
on the Moon due to there being
such a low gravitational force.
However, the astronaut still has
the same mass – they just weigh
less because gravity is weaker.
29 of 54
Mass and weight on different planets
30 of 54