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
Factors affecting surface tension
they reduce the surface tension of a liquid i.e. addition of detergentb) 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.
An astronaut weighs 900 N on earth. On the moon he weighs 150 N. Calculate the moons’ gravitational strength. (Take g = 10 N/kg).
Moons’ gravitational strength = weight of astronaut on the moon / mass of astronaut.
= 150 / 90 = 1.67 Nkg-1.
Measuring forceWe use a spring balance to measure force. A spring balance is an instrument that uses the extension of a spring to measure forces.
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 NSolution
b) 6.0 N
c) 200 N
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.
Physics notes form 1 to 4