FORCE

Force Laws of Motion of Class 9

Consider a ball kept on a table, we can move it by pulling or pushing. We can increase its speed by pushing it in the direction of motion. If we push it opposite to the direction of motion its speed will decrease. If the ball is in motion towards east, we push it towards north, the direction of will change.

Take a soft rubber ball between your palms and push the ball from both sides, the shape of the ball is distorted. In all the above cases we have applied the force on the ball.

So force in a push or pull which can move the object. It can change the speed of the object, it can change the direction of motion, it can change the shape of the object. In all the above cases we have applied the force on the ball and the ball is accelerated so we can define force as follows:

“Force is the cause which can produce acceleration in the body on which is acts”.

(a) Effects of Force :

The force or a set of forces acting on a body, can do three things :

  • A force or a set of forces can change the speed of the body.
  • A force or a set of forces can change the direction of motion.
  • A force can change the shape of the body.

TYPES OF FORCES:

The forces that we come across in our daily life, are divided into two types :

  • Balanced Forces
  • Unbalanced Forces

(i) Balanced Forces : If the resultant of all the forces acting on a body is zero, the forces are called balanced forces. 

FORCE

When balanced forces act on a body (here a heavy box), they do not produce any motion in it.

e.g.

  • A heavy box lying on ground has four forces acting on it as shown in the  above figure. Since the box does not move at all, the resultant of all the forces acting on it is zero. The force of our push is balanced by the force of friction, and the force of gravity is balanced by the force of the reaction of the ground. So, the forces acting on this stationary box are an example of balanced forces.
  • Similarly, in a tug of war, i.e. in rope pulling between two teams, if the resultant of forces applied by the two teams is zero, the rope does not move in either direction. The forces exerted by two teams are balanced. From the above example we also conclude that balanced forces do not change the state of rest or of motion of an object i.e. these forces cannot produce motion in a stationary body or stop a moving body. However, they can change the shape of the body. For e.g. when a spherical balloon is pressed between our two hands, its shape changes from spherical to oblong. Here, the balloon does not move, but its shape changes. Friction is a force which opposes the motion of a body moving over the surface of another body.
  • Unbalanced Forces : If the resultant of all the forces acting on a body is not zero, the forces are called unbalanced forces.
  • (a) The heavy box shown in figure can be moved with a very strong force. This is because in that case the force of push will become greater than the opposing force of friction. An unbalanced force will then act on the heavy box and make it move.
  • (b) A ball rolling on the ground stops after some time because an unbalanced force of friction acts on it.
  • (c) When we stop pedaling, the bicycle begins to slow down. This is again because of the unbalanced friction force acting opposite to the direction of motion.From the above examples, we also conclude that unbalanced forces can change the state of rest or state of uniform motion or the direction of the body or the size and shape of the body. An unbalanced force is needed to move a body from state of rest but no such force is required to maintain uniform motion of the body.
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