The document discusses forces, balanced and unbalanced forces, and calculating resultant forces. It defines a force as a push or pull and notes they have both magnitude and direction. Balanced forces are equal in size but opposite in direction and do not cause a change in motion, while unbalanced forces cause a change in motion. To calculate a resultant force, you take the difference between two forces acting in opposite directions. The document provides examples of calculating resultant forces and determining the direction of motion based on whether the forces are balanced or unbalanced.

1. REVIEW:
• Force
• Motion
• Effects of forces on the object’s motion
• Forces on objects at rest
• Normal force
• Friction
• Gravitational force
• Scalar quantity
• Vector quantity

2. True or False
If an object is at rest,
no forces are acting upon it

3. Answer: False
There are many forces acting on it
(gravity, friction, air pressure, etc)
but they are all BALANCED

4. True or False
When there is force, there is motion.

5. Answer: False
Force may be present but it does not
necessarily result to motion. If forces are
balanced, no motion will be produced.

6. True or False
In the absence of force, object will continue
to move constantly.

7. Answer: True.
Because force will slow down or stop
motion.

8. TODAY’S LESSON
Learning Objective
Aim high - Be able to represent forces using a force diagram
Aim higher - Describe the effect that balanced and unbalanced forces
have on an object
Aim even higher - Be able to calculate the size of the resultant force
Key Words:
Balanced force
Unbalancedforce
Resultant or Net Force

9. What is a force?
• A force is a push or pull that causes an object to
move, stop, or change direction
• In physics a force is anything that makes an object
accelerate

10. Force
• Forces come in pairs
• Forces have a magnitude and a direction
5N, north (up)
Magnitude: 5N
Direction: north (up)

11. Balanced and Unbalanced Forces
• Forces occur in pairs and they can be
either balanced or unbalanced

12. Balanced Forces
• Balanced forces do not cause change in
motion
• They are equal in size and opposite in
direction

13. Balanced Forces

14. We call this NORMAL FORCE. The book is
said to be at equilibrium since there are no
unbalanced forces acting upon the book
and it maintains its state of motion (it still
sits on the table).

15. Lets say you are
standing on the
ground. Since
the floor pushes
upward on you
and gravity is
pulling down on
you, you are at
equilibrium.

16. Unbalanced Forces
• An unbalanced force always causes a change in
motion
• When unbalanced forces act in opposite
directions you can find the net force
– Net force
• Magnitude
– The difference between the two forces
• Direction
– Direction of the largest force

17. Unbalanced Forces
3 N, right – 6 N, left = 3N, left

18. Unbalanced Forces
4 N, left – 10 N, right = 6N, right

19. Unbalanced Forces
5 N, right + 10 N, right = 15N, right

20. Balanced Forces
Forces are balanced if they are the same
strength but in opposing directions:
40N 40N

21. Balanced forces

22. Balanced forces

23. Unbalanced Forces
When forces working in opposite directions
are unequal they are unbalanced:
100N 20N
Which direction will the box move?

24. Unbalanced forces

25. Normal
The forces are balanced/unbalanced
The man is moving/not moving
The forces are balanced/unbalanced
The woman is moving/not moving
Thrust
The forces are balanced/unbalanced The car is moving/not moving

26. Application Questions
1. A diver wears weights and dives into the
water, which force is bigger, his weight or
upthrust? Draw a diagram to show the
forces acting on him
2. The diver then releases the weight. Now,
which force is bigger, his weight or
upthrust? Draw a diagram to show the
forces acting on him.

27. Resultant Force
If two forces are unbalanced there is a
resultant force.
It could replace all the others to give the
same effect.
The difference between two forces is the
resultant force.

28. What is the resultant force?
100N 20N
Hint: work out the difference between
the two forces.
The resultant force is 100N – 20N = 80N

29. Resultant forces – question 1
5 N 5 N
1.A. What is the
resultant force on
the satellite?
B. To which
direction will the
satellite move? 20 N

30. Resultant forces – question 2
2.
a. What is the
horizontal resultant
force on the bird?
5N 5N
b. What is the
vertical resultant
force?
5N
c. To which direction
will the bird move?

31. Resultant forces – question 3
10N
13 N 10 N 20 N
10N
3.
a. What is the horizontal resultant force on the
yacht?
b. What is the vertical resultant force?
c. To which direction will the yacht move?

32. Resultant forces – question 1
1. What is the resultant force on the satellite?
5 N 5 N
Resultant force = 20 N – 10 N
= 10 N down
The satellite will accelerate
downwards.
20N

33. 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
5N 5N The vertical forces are not
balanced, the bird will accelerate
in a downwards direction.
5N Resultant force = 5 N – 0 N
= 5 N down

34. Resultant forces – question 3
3. What is the resultant force on the yacht?
10N
13 N 10 N 20 N
10N
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