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Types of Forces- Physics Guide for Class 8

Information about Types of Force

Title

Types of Forces

Class

Class 8

Subject

Class 8 Physics

Topics Covered

  • Contact Forces
  • Non-contact forces
  • Gravitational force
  • Magnetic force
  • Electric Force


Type of Forces 

It is clear now that whenever there is an interaction between different bodies forces come into play. We can classify all forces in two broad categories:
  1. Contact forces 
  2. Non-Contact forces ('action at a distance' forces) 

1. Contact Forces 

We call those forces as contact forces which result when two interacting bodies are in direct physical contact with each other. Some of the examples of contact forces include muscular force, frictional force, air resistance force and so on.
  • Muscular force: In our daily life, we push, pull or lift many things. The effort (force) is caused by the action of muscles in our body. Animals, like bullocks, horses and camels, have been used for pulling carts. In arctic regions, reindeers are made to pull the sledges that are used as passenger vehicles. In these cases, the muscles of animals apply the force. This force is called muscular force. All animals, including human beings, use muscular force for most of their activities. 
  • Frictional force: Ball rolling along the ground, gradually slows down and finally comes to rest. In order to move a bicycle along a straight level road, we have to keep pedalling it all the time. This is because of the frictional force acting between the two surfaces in contact. The magnitude of the frictional force depends upon the nature of the two surfaces in contact. The direction of the frictional force is (usually) opposite to the direction of motion of the object.

2. Non-Contact Forces ('Action at a distance' forces)

We call those forces as non-contact forces which can cause their effects even when the two interacting bodies are not in direct physical contact with each other. Here they are able to exert a push, or pull, despite their separation. Some examples of non-contact forces are:
  1. gravitational force
  2. magnetic force
  3. electric force.
  • Gravitational force: We know that when we throw a ball upwards, the ball goes up in the air but then falls down again. Ripe fruits, that grow on trees, fall to the ground by themselves. This happens, due to a force, we call as the gravitational force.
  • Magnetic force: The magnetic property of lodestone was known to mankind since quite early times. Magnets have the well-known property of attracting objects made of iron. Likes poles of two magnets repel and unlike poles of two magnets attract each other. A magnet can exert a force on another magnet without being in contact with it. The force, exerted in this case, is known as a magnetic force.
  • Electric force: Two charged bodies exert a force on each other. When a glass rod is rubbed with a silk cloth, the rod becomes positively charged. Similarly, when an ebonite rod is rubbed with wool, the rod acquires a negative charge. Bring this charged ebonite rod near the suspended (and charged) glass rod. You will find that the suspended glass rod moves towards the charged ebonite rod. From these observations, we observe that like charges repel and unlike charges attract. This attraction, or repulsion, between charges is due to the (non-contact) electric force between them.

Some important Points

  • There is a popular story that one day, when Newton was sitting under an apple tree, an apple fell on his head, and this led him to think about the force of gravitation. As in all such legends, this story is almost certainly not true in its details, but the story contains elements of what actually happened. Probably the more correct version of the story is that Newton, upon observing an apple fall from a tree, began to think along the following lines :— The falling apple is getting accelerated; there must be a force acting on the apple. If the force can reach to the top of the highest level of tree might it not reach even further (all the way to moon!). By such reasonings, Newton came to the conclusion that any two objects, in the universe, exert gravitational attraction on each other. This force is directly proportional to the product of their masses and inversly proportional to square of the distance between them. The weight of an object is a measure of the gravitational force exerted on that object by the earth.
  • A single isolated force does not exist by itself. Forces are always pushes or pulls between two objects, so they always occur in pairs. When two objects interact, the force, exerted by one object on the other, is equal and opposite to the force exerted by the second object on the first. 
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