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CHAPTER THREE 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 masses.
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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
- Impurities – they reduce the surface tension of a liquid i.e. addition of detergent
- 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 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 |
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.
- 2.5 N => 2.5 × 0.8 = 2.0 cm therefore length becomes = 16.0 + 2.0 = 18.0 cm.
- 6.0 N => 6.0 × 0.8 = 4.8 cm therefore length becomes = 16.0 + 4.8 = 20.8 cm.
- 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.
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