1. The document discusses different types of forces including gravitational, normal, tension, and friction. 2. It provides equations for calculating resultant, centripetal, and frictional forces. 3. Examples are given to demonstrate calculating the force of friction for a skidding car and the forces on an object on an inclined plane.

1. Different Types of Forces
Bodies Subject to an Number
of Forces
Chapter 12 & 13

2. Resultant Force
• The resultant force (FR ) is the vector sum
of all forces exerted on a body.
• Forces are vector quantities having
magnitude and direction.
• FR = F1 + F2
• Force vector addition follows the same
rules as velocity vector addition.

3. Different Types of Forces
• The different types of forces are:
– Gravitational (FG) – the force
of attraction between two bodies
depending on their masses
and distance
- Normal (FN) – the resistant
reaction of a body’s surface
to a force by another body
in contact, usually due to gravity

4. Exam Question
The following statements concern the mass and the weight of an object. Which one(s) is
(are) true?
1. The weight of an object depends on the planet on which it is situated.
2. The mass of an object depends on the planet on which it is situated.
3. The weight of an object is directly proportional to its mass.
A) 1 only
B) 2 only
C) 1 and 3 only
D) 2 and 3 only

5. Exam Question
An object is in free fall near the surface of the earth.
Which of the following factors affect the acceleration of the object?
1. The location on the Earth where the object is in free fall
2. The mass of the object in free fall
3. The shape of the object in free fall
4. The altitude of the object in free fall
A) 1 and 2
B) 1 and 4
C) 2 and 3
D) 3 and 4

6. Different Types of Forces
– Tension (FT ) - the force
when an object is pulled from
another object
– Friction (FF ) – the force
that opposes motion

7. Force of Friction
• Friction is a contact force due to
electromagnetic attraction at the atomic
level.
• Standing or Static friction is a force that
prevents sliding from starting.
• Sliding or Kinetic friction is a force that
opposes motion once sliding has started.
• Kinetic friction is less than static friction.

8. Friction Force Equation

9. Example
• What is the force of friction exerted when
a 1500 kg car skids on a wet asphalt road
(μ = 0.5)?
• FF = μ FN, but FN = mg
• = 0.5 x 1500 x 9.81 m/s2
• = 7350 N
• Page 276, Q. 1-2

10. Activity
• Page 289, Q. 1,2
• Page 292, Q. 1,2
• Coefficient of Friction – Running Shoe Lab

11. Centripetal Force
• Centripetal Force (FC) maintainsa body in
a circular motion.
• The force points towards the centre of the
circle.
• At release, the object
moves on a perpendicular
tangent at right angles
to the radius.

12. Equation

13. Example

14. Activity
• Page 281, Q. 1-2
• Page 286, Q. 1-4

15. Equilibrium
• A body is in a state of static equilibrium
when the resultant force exerted on the
object is zero and the object is stationary.
• A body is in a state of dynamic equilibrium
when the resultant force is zero and the
object is moving at constant velocity.
• A resultant force of zero means that the
total of its components (x & y) are zero.

16. Equilibrant Force
• The equilibrant force (FEQ) is the force that
cancels out the effect of an unbalanced
force and results in a net force of zero.
• FEQ = -FR
• The resultant force and equilibrant forces
are opposite vectors, meaning that they
have the same magnitude and are parallel
but in opposite directions.

17. Unbalanced Net Force

18. Inclined Plane
• An inclined plane is a plane (flat) surface
set an angle to the horizontal.
• An object on a inclined plane is only
subject to the force due to gravity.
• The x,y axes are rotated to be parallel to
the slope of the inclined plane.
• Thus, the normal force (FN) rises at 90°
from the slope of the inclined plane.

19. Inclined Plane Forces
FP & FF are parallel to the
slope.
FG has 2
components –
FGX = mgsinθ
FGY = mgsinθ
The net Fy = 0,
So FN = -FGY

20. In & Out of Equilibrium
• In a situation of static or dynamic
equilibrium, the FF and the x component of
FG are opposite and equal.
• FF = -FGx

21. Activity
• Page 298, Q. 1-4
• Page 302, Q. 1- 4

22. Summary
• 1 N is the force required to accelerate 1 kg
by 1m/s2.
• Mass is the amount of matter in an object,
measured in kg. This does not change.
• The Gravitational Field Strength, g, at a
given point is the Force of Gravity on each kg
of mass at that point.
• At the surface of the earth, g = 9.8N/kg.
• The acceleration due to gravity, ag = 9.8 m/s2.

23. Summary
• The Force of Gravity on an object with mass m,
or the Weight is: Fg, = mg.
• The Force of Static Friction is the force required
to set an object in motion on a horizontal
surface.
• The Force of Kinetic Motion is the force required
to keep an object in motion on a horizontal
surface.
• It depends on the roughness of the surface and
the force applied downwards.

24. Summary
• The Coefficient of Friction, μ = Ff
FN
Where FF is the Force of Friction
And FN is the Normal Force or weight.