Causes of motion
If a book or pencil is placed on a table, it stays there without moving until a push or pull is applied or given to it.
When a body is moving with uniform velocity, it will continue that way until a push or pull is applied on it. The push or pull is called force.
Sir Isaac Newton experimented on the importance of the force to motion. Hence, Newton’s first law of motion states that a body will continue in its state of rest or uniform motion in a straight line unless it is acted upon by a force, so the cause of motion is force.
Types of force
There are two major types of force;
Contact force
Force field
Contact force: this is the type of force which exists between any surfaces in contact. This includes push, pull, friction, reaction forces, tension forces in strips and wires.
Above is a diagram of a block of wood resting on a table. The weight (W) of the wood acts downwards and a force of reaction (R) tries to counteract this by acting upwards when the block of wood is being pulled by force(f), which is being opposed by the frictional force acting between the table surface and block.
Force field: These are vector fields, e.g. Gravitational fields, Electrostatic field, Magnetic field.
*Magnetic field is created in the surface around a bar magnet or in the region surrounding a conductor carrying a current.
*Gravitational force is the force which the earth exerts on every object on its surface.
*An electrostatic field exists on the area surrounding some charged bodies.
Friction
This is the force that opposes one surface moving or trying to move over another. Static or stating friction occurs when the force causing motion is just equal to the frictional force of the surface in contact. The dynamic frictional force is less than the static frictional force.
Frictional force is directly proportional to the weight of the body (w) moving or the normal reaction (this is because, for a body to be at rest, its weight must be equal to the normal reaction).
F…w or R
F= µ R
m=… where µ is the coefficient of friction.
Notes
Starting or static friction occurs when the force causing motion is just equal to the frictional force of the surfaces in contact.
Dynamic frictional force is less than the static frictional force.
Frictional force is directly proportional to the weight of the body.
Moving or the normal reaction (this is because, for a body to be at rest, its weight must be equal to the normal reaction).
Frictional force is independent of the cross-sectional area of the two surfaces in contact.
Advantages of friction
The usefulness of friction is many and varied.
Walking: The friction between the soles of our feet and the ground makes it easy for us to walk. There is little or no friction on slippery surfaces and that is why it is difficult to walk on.
Brakes: When brakes are applied in a moving vehicle, the brake shoe are pushed apart, and this brings the brake lining in contact with the brake drum. Friction between the drum and lining brings the vehicle to a halt.
Tyres: Friction between the vehicle tyre and the road allows the tyre to have a grip on the road. The same friction prevents a car from skidding off the road.
Nuts and bolts: Nuts and bolts can stay tight because of the friction which is built up between them when they are tightened.
Ladders: The ladder is prevented from slipping because of the friction between its foot and the ground on which it stands.
