Explain contact and non-contact forces.


1. Contact forces

The forces which come in action, when two objects come in contact with each other are called contact forces.

e.g. Muscular force and Friction are contact forces.

2. Non-Contact forces

The forces which the body exerts on another, without any contact with one another are called non-contact forces.

e.g. Electrostatic and Magnetic forces are non-contact forces. 

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What is pressure? What happens to the pressure when area in which it is applied increases?


Pressure is the amount of force acting on a unit area of a surface.

Mathematically, pressure is given by

Pressure = Force / Unit Area

That is, pressure and area on which the force is applied has an inverse dependance.

Keeping force constant, if the area on which the force applied is increased, the pressure decreases. 

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Why is it difficult to hold a school bag having a strap of a thin and strong string?


We know that

Pressure = ForceArea

A school bag has a weight of the books and exerts a vertically downward force.

If the string is thin, it has less area of cross-section, and therefore, exerts greater pressure on the hand for the given weight of the school bag.

Therefore, it becomes difficult to hold the school bag with a thin string. 

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What is electrostatic force? Why is it called non-contact force?


The force exerted by a charged body on another charged or uncharged body is called electrostatic force.

This force is known as non-contact force because these forces comes into play even when the bodies are not in contact with each other. 

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Why are double tyres provided at the rear wheels of a truck?


A truck carries load whose weight is largely dependent on the rear axle.

Since,
Pressure = ForceArea

The double tyres increase area of cross-section without increasing friction, and hence decrease pressure on the axle. 

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