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Force and Motion

In ancient Greece the word motion means the change in quantity over a period of time and not just the movement of a body from one position to another. But today what we see has bit different meaning which tells even more about displacement, velocity, time variation etc. Force could be observed in all quantities and it could vary from barely noticeable to something which is huge in quantum. Force also causes motion and the study of these is the branch of physics better known as Mechanics. These two topics are very important in today’s contemporary world as almost everything around us involves these two factors in some manner or other. 

Almost everything around us is in motion and motion could take various forms. They move in straight line, a curvilinear pathway, spin around a particular axis or make to and fro movements. Force simply put means something that can help in push or pull and for every push, pull; twist or stretch the application of force is ultimate. Forces also keep tiny atoms together which makes up everything else in this Universe. Any particular object if moves would come to rest only because of force. An object can go faster or slower when force is applied.

In physics it is proven that different forces in this Universe belong to one of the four basic kinds. These are gravitational force, electromagnetic force and the two atomic forces. Gravitational force helps in keeping everything on Earth intact and keeps the Earth in orbit, while magnetic force and chemical forces is included under electromagnetic force. The two atomic forces hold the nucleus of an atom in place without which the atom would disintegrate.

To describe motion one has to know speed and direction. Speed is about how fast a body moves and in relative to either a stationary object or other moving objects. To describe motion we also need to clarify the direction in which the body is moving. Speed with direction gives us velocity. 

The change in motion of a body is due to unbalanced force in action while a constant motion is the resultant of a balanced force. In constant motion the friction force tends to be minimal or almost close to zero.

Motion of falling objects: Galileo long ago discovered something very important and it was about falling objects. He discovered that the force acting on massive objects is more than the force acting on less massive objects and hence both will reach the ground at the same time if dropped from a height at the same instant. He also concluded that if forces were same then the object with less mass would reach first.

In principle all objects will reach the ground at the same time provided there is no air resistance. Balanced and unbalanced forces are yet another way of checking Newton’s first law of motion. The first law states that a body will remain at rest or motion until an external force is applied to either set it in motion or bring it to rest. Unless there is an external unbalanced force is applied nothing could be completed. 

When a book is kept on a table it remains at rest because the force of gravity pushes the book down and an equal amount of force is pushing it up. As the forces working on book are balanced the book stays in one place. Similarly a body is in constant motion when the body has balanced force with respect to frictional force and the body keeps on moving until another unbalanced force stops it.



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Need force to move an object, but do we need more force to move heavier objects?

Experiment 1:

Let us fill two separate boxes with two different weights. When we push the first box with the lighter weight we use less force. We use the same force for the second box with larger weight but we cannot move it. To move the second box with larger weight we need larger force. 

Experiment 2:

A toy truck is on an inclined plane with a smaller weight and released from rest position. The truck travels some distance and comes to rest. The same truck with larger weight at the same inclined plane is released from rest. The truck moves with larger magnitude and covers a longer distance. 

The above experiment shows that a body with larger momentum covers longer distance than a body with smaller momentum.

Experiment 3:

Unbalanced force related experiment. Two hand wrestlers of same calibre exert same force on each other’s wrist. None of them move from their spot and are in deadlock. This is because balanced force is working and cancelling each-others force. 

One of the wrestlers applies more force and immediately there is a shift in position of the arms and the stronger arm pushes down the other arm. This implies that only unbalanced forces can result in motion.


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Activity related to force of gravity and motion 
As we know gravitational force is nothing but force of attraction between two objects and centre of gravity lies right exactly at the centre of mass of the object. Any two object would definitely attract each other with same gravitational force and hence the pull towards each other. The meteors that fall on Earth are attracted towards the centre of Earth.
The person who goes for a high jump needs to jump off against the force of gravity and then because of the gravitational force acting on him returns to ground. While lifting off the force applied by the person was greater than force of gravity and after reaching the apex the force of gravity takes over and the person returns safely.

Activity related to action reaction or newton’s third law of motion
For every action there is an equal and opposite reaction. When two bodies with different mass come to collision they tend to accelerate at different rates and since acceleration is change in velocity the smaller of the two objects will accelerate more while the bigger will be less accelerating. 
When bodies with equal mass and speed collide they will accelerate with same magnitude in opposite direction. This happens as both the bodies press against each other in opposite direction.
More topics in Force and Motion
Electrical Force Magnetic Force
Role of Gravity Gravitational Field
Related Topics
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