Forces & motion Gr.7 - 2018 Chapter 1 physics

2. MASS
Mass is the amount of matter in an
object.
Mass is measured in grams (g).
We use a balance/scale to measure mass.

3. A force is a push or pull.
What can a force do?
1. Change the position of an object.
2. Change the speed of an object.
3. Change the direction of a moving
object
4. Change shape of an object.
FORCES

4. LETS LOOK AT EXAMPLES OF PULLING OR
PUSHING FORCES.

5. WEIGHT
Weight is the pull of gravity on an object.
It is measured in Newtons. (N)
We use a spring balance to measure it.

7. ACTIVITY
• Find the mass and weight of different
objects and write them down in the
table on your worksheet.
• What do you notice?

9. CONTACT FORCE

10. NON-CONTACT FORCE
A force experienced
by bodies even if
they are not in
physical contact
with each other.

11. FORCES
Friction
Contact Non-contact
Thrust
Upthrust
Tensio
n/sprin
g
Air /water
resistanc
e
Applied
force
Gravitationa
l
Electrostatic
Magneti
c

12. 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.
BALANCED FORCES
The forces are balanced, so they cancel each other out.
The skater does not move.

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Unbalanced forces
Unbalanced forces lead to a change in speed or direction.
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.

14. CALCULATING FORCE

15. CALCULATING FORCES

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Force pairs

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500N 500N
Imagine a car travelling at a constant speed of 50km/h.
Balanced and unbalanced forces
The engine provides sufficient force to balance all the
frictional forces that are acting to decrease the speed.

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The net force is calculated by working out the difference
between opposing forces in each direction.
What is the resultant force on this truck?
The sum effect of more than one force is called the Net
force.
A net force of 100N is accelerating the truck.
Net /resultant force
500N400N

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Balanced and unbalanced forces – summary
 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.
If the forces are unbalanced, two things can happen:
If the forces on an object are balanced:
In other words, the object will continue to do what it is
already doing without any change.
 The speed can change. This is called acceleration.
 The direction of motion can change.

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

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5N
2. What is the net force on the bird?
5N 5N
The forces acting in each
direction horizontally are equal
in size, so there is no net force
in this direction.
Net force = 5N – 5N = 0N
Resultant forces – question 2
The vertical forces are not
balanced, the bird will accelerate
in a downwards direction.
Net force = 5N – 0N
= 5N down

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The vertical forces are equal in size and opposite in direction
so there is no net force in the vertical direction.
The horizontal forces are not balanced, so the yacht will
accelerate to the right.
10N
13N
3. What is the net force on the yacht?
10N
10N
Net force = (20N +10N) – 13N
= 17N right
Net forces – question 3
20N