Different Types of ForcesBodies Subject to an Number

of ForcesChapter 12 & 13

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.

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

Exam QuestionThe 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

Exam QuestionAn 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

Different Types of Forces

– Tension (FT ) - the force

when an object is pulled from

another object

– Friction (FF ) – the force

that opposes motion

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.

Friction Force Equation

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

Activity

• Page 289, Q. 1,2

• Page 292, Q. 1,2

• Coefficient of Friction – Running Shoe Lab

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.

Equation

Example

Activity

• Page 281, Q. 1-2

• Page 286, Q. 1-4

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.

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.

Unbalanced Net Force

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.

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

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

Activity

• Page 298, Q. 1-4

• Page 302, Q. 1- 4

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.

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.

Summary

• The Coefficient of Friction, μ = Ff

FN

Where FF is the Force of Friction

And FN is the Normal Force or weight.