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

True or False

If an object is at rest, no forces are acting upon it

Answer: False

There are many forces acting on it (gravity, friction, air pressure, etc)

but they are all BALANCED

True or False

When there is force, there is motion.

Answer: False

Force may be present but it does not necessarily result to motion. If forces are

balanced, no motion will be produced.

True or False

In the absence of force, object will continue to move constantly.

Answer: True.

Because force will slow down or stop motion.

TODAY’S LESSON

Learning ObjectiveAim 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

Unbalanced forceResultant or Net Force

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

Force

• Forces come in pairs• Forces have a magnitude and a direction

5N, north (up) Magnitude: 5NDirection: north (up)

Balanced and Unbalanced Forces

• Forces occur in pairs and they can be either balanced or unbalanced

Balanced Forces

• Balanced forces do not cause change in motion

• They are equal in size and opposite in direction

Balanced Forces

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).

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.

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

Unbalanced Forces

3 N, right – 6 N, left = 3N, left

Unbalanced Forces

4 N, left – 10 N, right = 6N, right

Unbalanced Forces

5 N, right + 10 N, right = 15N, right

Balanced Forces

Forces are balanced if they are the same strength but in opposing directions:

40N 40N

Balanced forces

Balanced forces

Unbalanced Forces

When forces working in opposite directions are unequal they are unbalanced:

Which direction will the box move?

100N 20N

Unbalanced forces

Normal

The forces are balanced/unbalancedThe man is moving/not moving

The forces are balanced/unbalancedThe woman is moving/not moving

Thrust

The forces are balanced/unbalanced The car is moving/not moving

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.

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.

What is the resultant force?

Hint: work out the difference between the two forces.

The resultant force is 100N – 20N = 80N

100N 20N

20 N

1.A. What is the resultant force on the satellite?

B. To which direction will the satellite move?

Resultant forces – question 1

5 N 5 N

5 N

2. a. What is the horizontal resultant force on the bird?b. What is the vertical resultant force?c. To which direction will the bird move?

5 N 5 N

Resultant forces – question 2

10 N

13 N

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?

10 N

10 N

Resultant forces – question 3

20 N

20 N

1. What is the resultant force on the satellite?Resultant force = 20 N – 10 N

= 10 N down

The satellite will accelerate downwards.

Resultant forces – question 1

5 N 5 N

5 N

2. What is the resultant force on the bird?

5 N 5 N

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

Resultant forces – question 2

The vertical forces are not balanced, the bird will accelerate in a downwards direction.

Resultant force = 5 N – 0 N = 5 N down

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.

10 N

13 N

3. What is the resultant force on the yacht?

10 N

10 N

Resultant force = (20 N +10 N) – 13 N

= 17 N right

Resultant forces – question 3

20 N