Page 1 of 9

EDEXCEL IGCSE Physics

Mechanics C

Types of Forces

1. Gravitational Force Gravitational force is the force which exists between two

masses. Simply, it is the attractive force between two objects with a mass.

E.g.: Weight is the gravitational force that a planet applies to a body.

Dynamics the study of motion of an object with reference to its mass and the

forces acting on it.

A force is a push or a pull exerted by one object on another. Force is a vector quantity.

A force can:

I. Stop a moving object

II. Move a stationary object (stationary not moving) III. Increase speed of an object (Accelerate)

IV. Decrease speed of an object (Decelerate)

V. Change the direction of a moving object

VI. Change the shape of an object

The concept of forces was introduced by Sir Isaac Newton. Therefore, the unit of

force is named after him A force is measured in Newtons. The SI unit of force is

Kgms-2. The device used to measure a force is called the Newton meter.

Page 2 of 9

2. Solid-Solid Contact Forces:

(a) Normal Reaction or Normal Contact Force When two solids are in contact,

they will push each other. This force exerted by the one solid to the other solid is

called the normal reaction force or the normal contact force. The word normal is

used because the force is perpendicular (forms 90) to the plane of contact.

(b) Frictional Force This is the force that opposes motion.

Why does friction occur?

When we magnify the surface of contact between two objects, they never

appear to be smooth lines. It may look like :

Therefore, when these rough surfaces are pulled (one on another) a force

which opposes motion arises and this force is known as Friction.

What are the uses of friction?

Friction helps us to:

Walk

Write

Drive a car

Light a match etc

Weight

Weight

Normal Reaction Force Normal Reaction Force

Page 3 of 9

What are the problems about friction?

Makes movement difficult in parts of machines

Wastes Energy

Heats parts of machines

Wears things out (tyres etc)

How can we reduce friction?

Use lubricants like oil

3. Solid Fluid contact forces:

(a) Solid Liquid contact forces Drag force

When an object is moving through a liquid, it experiences an opposing force

called drag. Drag depends on the shape of the object. Streamlined objects feel

less drag force.

(b) Solid gas contact forces Air Resistance

When an object is moving through a gas, the opposing force it experiences is

called Air Resistance. Air resistance also depends on the shape of the object.

Streamlined objects feel less Air resistance.

Thrust Drag

Air Resistance

Page 4 of 9

4. Thrust This is the force that an engine produces to push an object.

5. Tension Force When we pull a string down, an equal and opposite force called

tension acts.

6. Electrostatic force This is the force between two charges. Like charges repel

each other while Unlike charges attract each other.

7. Magnetic Forces Between two magnets or between a magnet and another

piece of magnetic material there are magnetic forces. Like poles of two

magnets repel. Unlike poles of two magnets attract. A magnet and magnetic

material attract.

Weight

Thrust

Tension

Pull Down

Like Poles

Repel

Unlike Poles

Attract

Page 5 of 9

8. Upthrust This is the upward force a liquid or a gas exerts on an object.

9. Nuclear Force This is the force which holds the protons and neutrons

together inside the nucleus of an atom.

More than one force

Forces are vectors and they always have a direction. When there is more than one

force, the sum of the force vectors is called the Resultant force. If the resultant

force is not zero, the object will move in the direction of the resultant force.

Q Calculate the resultant force:

Upthrust of water

Weight of boat

Weight

Upthrust

Page 6 of 9

Some important terms to know

Extension - increase in length

Compression - decrease in length

Brittle material - material that cannot be stretched noticeably and

breaks easily

Resilient/Elastic material - material that can be stretched noticeably and

returns to original length when the force applied is

removed

Plastic material - material that can change shape permanently

even when a small force is applied to them

- Acceleration due to gravity

When an object is dropped through a small distance close to the earths

surface, it accelerates. When calculated, it is observed that the value we get

as the acceleration is a constant value for all objects which are dropped

through a small distance close to the earths surface. This value is referred to

as the acceleration of free fall and has a value of 9.8ms-2. For IGCSE

purposes, we assume that the acceleration of free fall on earth is 10 ms-2.

- Equation to find Weight

W = mg

W Weight

m Mass

g Acceleration due to gravity

Mass - Mass is the amount of matter in an object. Units are Kilograms Kg.

Acceleration due to gravity The value of g depends on how strong a planets

gravity is. The units are Meters per second squared ms-2. It is sometimes

called Gravitational Field Strength. The value of g varies. For example the

value of g in:

The moon = 1.6ms-2

The earth = 9.8ms-2 (but for IGCSE, we assume it is 10ms-2) Jupiter = 25.9ms-2

Page 7 of 9

Q Calculate the weight of an object of mass 450.53Kg when the object is in

(a) The moon

(b) The earth

(c) Jupiter

Free body force diagrams

A free body force is a diagram which shows the forces which act on a particular

object. E.g.:

Q Draw free-body force diagrams for the following:

(a) The Music Department wants to move a piano up a flight of stairs. The movers

tie a rope to the piano, and pull it up a ramp placed over the stairs. The ramp is

very smooth (no friction). What forces are acting on the piano?

Weight

Reaction

force

Weight

Friction Reaction force

Page 8 of 9

(b) Assuming the pendulum in the figure consists of a massive bob hung on a light

rope, what forces are acting on the bob?

(c) You take the elevator to the fourth floor. What forces are acting on you while

in the elevator?

(d) Box B accelerates constantly to the right while box A sits on top of it. What

forces are acting on box A?

(e) Two blocks are joined together by a rope as in the figure.

i. What forces are acting on the block 1 if it is sliding down the ramp?

A

B

Page 9 of 9

ii. What forces are acting on the block 1 if block 2 is sliding down the ramp?

(g) The drawing below shows a car pulling a caravan.

i. Label the forces that are acting on the caravan

ii. Label the forces that are acting on the car