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 the following symbol.

**Types of forces**

**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**.

**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.**Tension force**– this is the pull or compression of a string or spring at both its ends.**Upthrust force**– this is the upward force acting on an object immersed in a fluid.**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 .**Magnetic force**– this is a force which causes attraction or repulsion in a magnet.**Electrostatic force**– this is the force of attraction or repulsion of static charges.**Centripetal force**– this is a force which constrains a body to move in a circular orbit or path.**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**

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

See also:

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