TOPIC THREE - FORCE

Force is a push or a pull. Force is therefore that which changes a body’s state of motion or

shape. The SI unit for force is Newton (N). It is a vector quantity. It is represented by an arrow.

Types of forces

1.   Gravitational force –this is the force of attraction between two bodies of given masses.

-    Earth’s gravitational force is the force which pulls a body towards

its center. This pull of gravity is called weight.

2.   Force of friction – this is a force which opposes the relative motion of two surfaces in contact with each other. Friction in fluids is known as viscosity.

3.   Tension force – this is the pull or compression of a string or spring at both its ends.

4.   Upthrust force – this is the upward force acting on an object immersed in a fluid.

5.   Cohesive and adhesive forces – cohesive is the force of attraction of molecules of the

same kind while adhesive is the force of attraction of molecules of different kinds .

6.   Magnetic force – this is a force which causes attraction or repulsion in a magnet.

7.   Electrostatic force – this is the force of attraction or repulsion of static charges.

8.   Centripetal force – this is a force which constrains a body to move in a circular orbit

or path.

9.   Surface tension – this is the force which causes the surface of a liquid to behave like a

stretched skin. This force is cohesive.

Factors affecting surface tension

a)   Impurities – they reduce the surface tension of a liquid i.e. addition of detergent

b)   Temperature – rise in temperature reduces tension by weakening inter-molecular

forces.

Mass and weight.

Mass is the amount of matter contained in a substance while weight is the pull of gravity on an object. The SI unit for mass is the Kg while weight is the newton (N). Mass is constant regardless of place while weight changes with place. The relationship between ma ss and weight is given by the following formula, W = mg where g = gravitational force.

Differences between mass and weight

Mass	Weight
It is the quantity of matter in a body	It is the pull of gravity on a body
It is measured in kilograms	It is measured in newton’s
It is the same everywhere	It changes from place to place
It is measured using a beam balance	Measured using a spring balance
Has magnitude only	Has both magnitude and direction

Example

An astronaut weighs 900 N on earth. On the moon he weighs 150 N. Calculate the moons’

gravitational strength. (Take g = 10 N/kg). Solution

Moons’ gravitational strength = weight of astronaut on the moon / mass of astronaut.

= 150 / 90 = 1.67 Nkg-1.

Measuring force

We use a spring balance to measure force. A spring balance is an instrument that uses the extension of a spring to measure forces.

Example

The length of a spring is 16.0 cm. its length becomes 20.0 cm when supporting a weight of

5.0 N. calculate the length of the spring when supporting a weight of:

a)   2.5 N     b) 6.0 N         c) 200 N

Solution

5N causes an extension of 4.0 cm, therefore 1.0 cm causes an extension of 4 /5 = 0.8 cm. a)   2.5 N => 2.5 × 0.8 = 2.0 cm therefore length becomes = 16.0 + 2.0 = 18.0 cm.

b)   6.0 N => 6.0 × 0.8 = 4.8 cm therefore length becomes = 16.0 + 4.8 = 20.8 cm.

c)   200 N => 200 × 0.8 = 160.0 cm therefore length becomes = 16.0 + 160.0 = 176.0 cm.

Vector and scalar quantities

A scalar quantity is a quantity which has magnitude (size) only . Examples are distance, mass, speed

A vector quantity is a quantity which has both magnitude and direction. Examples are

displacement, weight, velocity.