## FORM 1 PHYSICS NOTES-FORCES

FORM 1 PHYSICS NOTES-FORCES

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*

– *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 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* - 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* - Electrostatic force –
*this**is**the**force**of**attraction**or**repulsion**of static* - Centripetal force –
*this is a force**which constrains**a body to move in a circular orbit or* - Surface tension –
*this**is**the**force**which**causes**the**surface**of**a**liquid**to**behave**like**a*

*stretched* *skin.* *This force is cohesive.*

FORM 1 PHYSICS NOTES-FORCES

**Factors affecting surface tension**

- Impurities – they reduce the surface tension of a liquid e. addition of detergent
- Temperature – rise in temperature reduces tension by weakening inter-molecular

**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 mass 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 |

FORM 1 PHYSICS NOTES-FORCES

*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.

FORM 1 PHYSICS NOTES-FORCES

##### Example

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

*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) 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 = 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.