Grade 8 Physics

2.  ______ is simply “How fast?” whereas
 ____________ is “How fast, and in which
direction?”
 _____________ is the distance traveled
divided by the time interval for the motion.
 _____________________ defined as the
displacement divided by the time interval
during which the displacement occurred
RECALL:
SPEED
VELOCITY
AVERAGE SPEED
AVERAGE VELOCITY

3. __________________is the quantity that
describes the rate of change of velocity in
a given time interval
ACCELERATION

6. ACCELERATION is caused by a
NET FORCE.
Change in motion is
ACCELERATION.

7. What makes objects move the
way they do?
Why do objects move in
different ways? Why are some
objects faster than the others?
What makes objects stay in
place?

8. Our common sense idea of a force is
that it is a push or a pull

9. A Force can either be contact or nonconta

11. When a string or
rope or wire pulls
on an object, it
exerts a contact
force that we call
the tension
force.
The tension within a rope and the
tension force experienced by an
object at the end of the rope are really
the net spring force being exerted by
billions and billions of microscopic

12. The force exerted by a
surface (the agent)
against an object that is
pressing against the
surface is called as the
Normal Force.
In essence, the normal
force is just a spring
force, but one exerted
by a vast number of
microscopic springs
acting at once.

13. The pull of a
planet on an
object on or near
the surface is
called the
gravitational
force. The agent
for the
gravitational force
is the entire
planet. Gravity

14. Friction, like
the normal
force, is
exerted by a
surface. But
whereas the
normal force is
perpendicular
to the surface,
the friction
force is always
tangent to the

16. Magnitude refers to the size or strength of the force. It is
commonly expressed in Newton (N). The direction of the arrow
indicates the direction of the force while the length of the arrow
represents the relative magnitude of the force. If the force
applied on the ball is doubled, the length of the arrow is
increased two times. The line of action is the straight line
passing through the point of application and is parallel to the
direction of the force.
Consider the diagram in Figure 2 showing a force,
represented by the arrow, acting on a ball.

17. ACTIVITY 1

20. Gravitational force is the attraction between
any two bodies with mass.
- Gravitational force increases with mass. If
the mass of either object increases, the
gravitational force between them also
increases.
- As the Earth attracts objects around it, these
objects also attract the Earth. But the Earth is
much more massive than them that is why
their attraction is not as great as the
gravitational pull of the Earth.
- All things on Earth fall (or are attracted)
towards the center of the Earth.

22. BALANCED AND UNBALANCED FORCES

23. 𝑭𝒈
𝐹𝑛
Are the forces balanced or Unbalanced?

24. Are the forces balanced or Unbalanced?
Knight, Randall. 2013. Physics for scientists and engineers : a strategic approach. Illinois: Pearson Education, Inc.
𝑛
𝑭𝒈
𝑭𝑷
𝑭𝒇
Department of Education
National Capital Region

25. Are the forces balanced or Unbalanced?
Knight, Randall. 2013. Physics for scientists and engineers : a strategic
approach. Illinois: Pearson Education, Inc.
𝑻
𝑭𝒈
https://goo.gl/images/C6zuwE
Department of Education
National Capital Region

26. Combining Forces
If the sum of the forces equate to zero,
they are considered balanced. If the
algebraic sum is not equal to zero, the
forces are not balanced. The non-zero
sum is the net or unbalanced force.
This unbalanced or net force would
cause a change in a body’s state of
motion.

27. 1. A boy and a girl are pulling a
heavy crate at the same time with 10
units of force each. What is the net
force acting on the object?

28. 2. What if the boy and the girl pull the
heavy crate at the same time in
opposite directions with 10 units and 5
units of force respectively, what will be
the net force on the object? Will the
object move? To what direction will it
move?

29. 3. Suppose another girl pulls the heavy
crate in with 5 units of force in the same
direction as the girl, what will be the net
force that will act on the object? Will the
object move?

30. Balanced Forces
•The Net Force acting on an object
sums up to ZERO
•It does not produce acceleration
•There is no change in the motion
of an object
•A body at rest remains at rest
•A body moving with constant
velocity will maintain it’s constant
velocity
Department of Education
National Capital Region

31. Unbalanced Forces
•The Net Force acting on an
object is not equal to zero
•Produces acceleration
•Causes a change in motion
– Speeding up or slowing
down
Department of Education
National Capital Region

33. 1. If the forces acting on an object are
balanced, what will happen to the motion
of the object?
A. its motion will not change
B. will change direction
C. its motion will change
D. none of these

34. 2. As you sit on your chair and study your
science (presuming that you do), the force of
gravity acts downward upon your body.
Where is the reaction force to the force of the
Earth pulling you downward directed?
A. the force of the chair pushing you
upward
B. the force of the floor pushing your chair
upward
C. the force of the Earth pushing you
upward
D. the force of your body pulling the Earth
upwards

35. 3. An object experiences no
acceleration. Which of the following
cannot be true for the object?
A. A single force acts on the object.
B. Forces act on the object, but the
forces cancel.
C. No forces act on the object.
D. All of the above.

36. 4. Are the forces balanced or
Unbalanced?
Knight, Randall. 2013. Physics for scientists and engineers : a strategic
approach. Illinois: Pearson Education, Inc.
https://goo.gl/images/C6zuwE
𝑻
𝑭𝒈
Department of Education
National Capital Region

37. 5. Are the forces balanced or
Unbalanced?
http://sugarloaf.nbed.nb.ca/sites/sugarloaf.nbed.nb.ca/files/doc
//y2016/Oct/sec2.1.3-solutions.pdf
𝑛
𝑭𝒈
𝑭𝑷
𝑭𝒇
https://goo.gl/images/d3Gf
Department of Education
National Capital Region

38. 1. A physics book glides to the right
across a smooth table at a constant
speed. Ignore air resistance.
2. A crate sliding across a rough floor
while being pulled using a rope to
the right.

40. Magnitude length increased
Direction line of action application
Complete the sentence. Select your answer inside the box.
_____________ refers to the size or
strength of the force. It is commonly expressed in
Newton (N). The _______ of the arrow indicates the
direction of the force while the _________ of the
arrow represents the relative magnitude of the force.
If the force applied on the ball is doubled, the length
of the arrow is _____ two times. The _________ is
the straight line passing through the point of
__________ and is parallel to the direction of the
force.