1. Adding forces Consider a book lying on a table: Force of book on Earth
Weight Fg
Since the book is not moving, the forces are equal and thus balanced.
The resultant of the 2 forces on the book is thus 0 N.
The other pair of forces is the book on the table and the table on the book.
In both cases – no resultant force.

2. Adding forces
Consider the horizontal forces acting on the box in the sketches below. (Remember the weight of the box is cancelled by the upward normal force of the floor on the box.)
Forces on object:
Resultant force on object
2 N
2 N
R = 0 N A A
3 N
2 N
R = 1 N B B
3 N
R = 5 N C C
2 N
3 N
3 N
R = 2 N D D
2 N

3. Inertia The property of matter which maintains an
object’s state of rest, or its motion in a straight
line, is called inertia.
Law of inertia

4. Inertia
The inertia of an object is the tendency of the object to resist a change in it’s state of rest or of constant motion in a straight line.
Inertia is measured in kg.
The beaker remains at rest since the force due to friction between the paper and the beaker does not overcome the inertia of the beaker.
7 Inertia Demos

5. Inertia A force is required to change the motion of an object.
This means that objects resist a change to their motion.
This ‘resistance to the change in motion’ of an object is known as its inertia.
The bigger the mass of an object, the bigger its inertia.
The greater the inertia, the more difficult it is to accelerate the object and the harder it is to stop it moving or change its direction of motion.
If the baby and the baby seat are not properly strapped in, the baby will continue to move forward – as a result of its inertia – when the brakes are applied sharply by the driver.

6. Inertia & mass
Mass is the amount of matter contained by an object and depends upon the number of atoms and their size.
Inertia is the tendency to resist change in an object’s motion.
Both mass and inertia are scalar quantities and are measured in kg. They are equal to one another.
A box having a mass of 5kg also has an inertia of 5kg.
Inertia & mass
This equal arm balance would be used to measure the mass of an object.
However, the inertia would be measured by establishing the acceleration an object would have when a certain force is applied to the object.

7. Newton’s first law.
Definition: An object continues in a state of rest or uniform (moving with constant) velocity unless it is acted upon by an unbalanced (net or resultant) force.
This means:
1. If object has no forces acting on it, or if forces acting on it are balanced (zero resultant) the object will remain at rest forever.
2. If object is moving and no resultant force acts on it, it will continue to move in a straight line forever.
3. If object moves at constant speed, the resultant forces on the object are zero.
However, if a resultant force acts on the object, it will accelerate in the direction of the resultant force.
Newton's 1st law

8. Newton’s first law. Stationary: 2 important forces – name them.
Moving with constant velocity:
Which forces are balanced?

9. Newton’s first law. For Newton’s first law there should be:
One, two or more pairs of balanced forces acting on the object.
The balanced forces only act on the one object
Balanced forces on one object will cause the object to remain at rest or move with a constant velocity in a straight line.
Balanced forces
Balanced & unbalanced forces

10. Balanced & resultant forces
Force of table on box
Stationary:
Fg weight
Force of table on box
2N friction
Stationary:
2 N applied
Fg weight
Force of table on box
Constant velocity:
Already moving
3 N friction
3 N applied
Fg weight
Resultant force – acceleration:
Force of table on box
3 N friction
4 N applied
Fg weight

11. Newton’s first law. The shuttle ‘Atlantis’ is orbiting the Earth.
Discuss and explain the following questions:
Are the engines running?
What forces are there on the shuttle?
What happens when the shuttle re-enters the atmosphere?

12. Newton’s second law of motion.
The Second law of motion is derived from the fact that a resultant force produces an acceleration of an object.
N.B. Def:
Newton’s Second law of Motion:
When a net force, Fnet, is applied to an object of mass , m, it accelerates in the direction of the net force. The acceleration, a, is directly proportional to the net force and inversely proportional to the mass.
Fnet∝ a and a ∝ 1/m
i.e. Net or resultant force: Fnet = ma
Newton 2 experiment
Newton's second law experiment Click here

13. Newton’s second law of motion.
This 2000 kg hot rod moves from 20 m.s-1 to 30 m.s-1 in 5s.
Find the resultant force applied by the car engine.
Fnet = ma
m(v f – vi) t = =
2000 x (30 – 20) kg.m.s s
= 4000 N in direction of motion.

14. Alternative for Newton 2
Fnet =
∆p ∆t
mv – mu t =
Since
v – u t
= a
and
∆p = change in momentum
then
Fnet = ma
Newton 2
When a resultant force acts on an object, the object accelerates in the direction of the resultant force. The acceleration is directly proportional to the resultant force and inversely proportional to the mass.

15. Force & free body diagrams
A box being pulled across the floor can be shown as:
A force diagram – with arrows representing the force vectors: OR
2. A free body diagram. Object represented as a dot with forces acting from the dot:
applied
friction
weight
Normal
normal
This is the free body diagram for the same situation as above.
applied
friction
weight

16. Newton’s Third Law If object A exerts a 3 N force on object B, then
Simultaneously object B exerts an equal and opposite 3 N force on object A.
This is known as an action – reaction pair of forces. Although they are equal and opposite, they are not balanced forces – since they act on different objects and not the same object.
Equal & opposite forces

17. Newton’s Third Law Consider this couple on ice skates.
If the man pushes on the lady with a force of 10 N, she pushes back on him with a force of 10 N.
Newton stated: To every action there is an equal but opposite reaction.
It is easier to explain and understand as follows:
N.B. Definition Newton’s Third Law:
When object A exerts a force on object B, then object B simultaneously exerts an oppositely directed force of equal magnitude on object A.
Newton's third law

18. Newton’s Third Law Consider a book on a table:
If the book exerts a 2 N action force on the table then:
The table exerts and equal and opposite reaction force of 2 N on the book.
Newto 2 & 3

19. Newton’s Third Law A man pushes his vintage car with a force of 400 N.
If this is the ‘action’ force, give the size and direction of the ‘reaction’ force.
On what object would the reaction force act?
If the maximum friction of the car is 300 N, will it accelerate? Explain.
What prevents the man from being pushed backwards? Explain.
Action-reaction forces

20. Newton’s Third Law The parachutist is in ‘free fall’.
His weight is 1000 N (mass = 100 kg) and the earth attracts him with a force of 1000 N.
The parachutist also attracts the earth with a force of 1000 N, but since the earth is huge compared to the mass of the man, the man is seen to fall towards the earth. w
Ignoring air friction, the parachutist falls towards the earth at a rate of 9,8 m.s-2.
Here the Earth attracts the parachutist and simultaneously the parachutist attracts the Earth with an equal and opposite reaction force.
Terminal velocity

21. Newton’s Third Law Earth pulls orbiting shuttle down. w
Space station orbiting w
Earth pulls orbiting shuttle down.
Shuttle pulls earth upwards.
If the weight of the shuttle is say N, this means that the earth pulls the shuttle downwards with a force of N.
At the same time the shuttle also exerts a force of N upwards on the earth.
Whether the objects move relative to one another, depends upon the relative masses of the two objects. Here the mass of the earth is huge relative to the shuttle – so the earth is not seen to move up to the shuttle – rather the shuttle moves down to the earth and that is why it stays in orbit.

22. Comparing Newton 1, 2 & 3 Newton 1 Newton 2 Newton 3
Man on car – N
Car on man – 200 N
Engine – 300 N
Friction – 300 N
Engine – 400 N
Friction – 300 N
One object – forces balanced, equal and opposite
One object – resultant force – object must accelerate Fnet = ma
Newton 1,2 & 3
2 objects – man on car and equal & opposite force of car on man
Remain at rest or move with constant velocity
Car must accelerate
Motion depends on other forces