Balanced and unbalanced forces on a body

3(b) describe the effect of balanced and unbalanced forces on a body 
3(c) describe the ways in which a force may change the motion of a body.

Newton's First Law Of Motion

"Every object continues in its state of rest or of uniform motion in a straight line unless acted upon by an external unbalanced force."

Balanced forces

To better understand the concept of balanced forces, you will have to know the meaning of the term resultant force. The topic is covered in this post here.

Now that you know about the term resultant force, we will come to balanced forces. When the resultant force in a system is 0, we will say that the forces are balanced in the system. Look at the following picture.

The toy car is initially at rest. A man pushes it in both the forward and the backward direction.The

car does not move. This is so because the two forces acting on the car are opposite in direction and of the same magnitude, lets say of 10N. As a result, they cancel each other out, and the car continues its state of rest as stated in Newton's law.

Newton's first law also states that a body will continue its state of motion even when the resultant force is zero. This is only possible, when the body is already in motion, and the overall forces in the system become balanced. Look at the following picture

Even though the thrust force and the friction force cancel each other out, the car is moving at a constant speed. This is so because the car was already moving at some speed, and when the forces became balanced, the car continued on at that constant speed. Remember, whenever the forces are balanced, the object will not accelerate or decelerate.

Unbalanced forces

In a system where the forces are unbalanced, the resultant force is never zero. As Newton's first law states, if the body is at rest, and an unbalanced force is applied on it, it would start moving. Look at the following picture. A man applies a force on a car in only one direction. As there is no other force from the opposite direction to cancel the first force out, the car will be displaced. This is so because the resultant force in the system is more than zero.

Why does the car stop after traveling some distance?

According to Newton's first law, the car should have continued moving once it was pushed. But Newton's law also states there should be no outer forces acting on it. In the above example, some outer forces like friction, air resistance etc are acting on the car, which eventually cause the car to stop.

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