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The motion of an object can be uniform or non-uniform in nature. Let us look at the cause of motion. When and why does the speed of an object change with time?
We all know that an object at rest does not start moving on its own. Some 'effort' is needed to make it move.
From our everyday experience, we know that we have to push, or pull, a table, a chair, or an almirah, if we wish to change its position in a room.
When we push or pull an object, we are exerting a force. A football, at rest, has to be kicked to send it over a distance. We again say that we are exerting a force on it. We, therefore, say that we exert a force when we push, pull, kick or lift a given object.
In all these situations, some kind of external agency (very often a muscular effort) is involved and its effect can be noticed or felt quite easily. It follows that we need an external force to move a body from its rest position, or to stop a moving body.
For example, we can stop a ball rolling down an inclined plane by applying a force against the direction of its motion.
We can thus say that force is a push or pull which comes into play when there is an interaction of one object with another object.
Thus an interaction of one object with another object can result in a force between the two objects.
A force is a push or pull upon an object resulting from its interaction with another object. Whenever there is an interaction between the two objects, some force acts between them; when the interaction ceases, the force between them no longer exists. Force exists only as a result of some interaction.
Change in the state of motion: A change, in either the speed of an object, or its direction of motion, or both, is described as a change in its state of motion.
(i) Take a rubber ball and place it on a smooth level surface (like a table top). Gently push the ball; it starts to move. Now, push it harder. What do you observe? Is there any change in its speed? Does it increase or decrease?
(ii) Roll the ball on the table top and now, push against the motion of the ball. What do you observe in this case? The speed of the ball decreases and it can come to rest.
(iii) Next push the ball at an angle to the direction of motion of ball. what do you observe?
From the above activity, we understand that a force, applied on an object, may change its speed, or direction of motion, or both.
We also realise that—
if the applied force acts on a body along its direction of motion, the speed of the body will increase.
if the direction of force, on the body, is opposite to its direction of motion, the speed will decrease. (In both the above two cases, the object is supposed to be moving in a straight line).
if the force acts at an angle to the direction of motion, it can change the speed as well as the direction of motion.
Our common experience also tells us that many a time the application of force does not result in a change in the state of rest or motion of the body. For example, we do not observe any motion, when we try to push a heavy stone.
A force may not, therefore, always succeed in bringing a change in the state of motion of an object. Sometimes, it only tends to do so.
Change in size/shape of an object
We know that to make a chapatti, we first take same dough and then roll it between the palms to make it spherical. We can also change the shape of an inflated balloon by gently pressing it between our palms. Some of us might have had a chance to observe the potter at work. A potter makes pots of different sizes and shapes from kneaded clay. In all these situations, the changes in size, or shape, or both take place due to the force applied on them.
We can now say that a force may—
make an object move from rest.
change the speed of a moving object.
change the direction of a moving object.
bring a change in the size, or shape, of an object.
cause two, or more, or all, of these effects.
Factors associated with the Magnitude of Force needed
We know that harder we kick a football, faster it moves. It means that: 'greater is the applied force, greater is the change in the speed of the object.'
Consider a lighter mass (car) and a heavier mass (loaded truck) parked on a horizontal road. We all know from our experience that a much greater push (force) is needed to move the truck than the car.
Now, suppose we apply the same force, for the same time, to both the car and the truck. The car picks up a greater speed than the truck in that time.
We thus realise the mass of an object, and the value of the change in its speed (in a given time) are both important parameters that determine the magnitude of the force needed.
Force has Both Magnitude and Direction
It is now easy to understand that we need to know both the magnitude of force, and the direction in which it acts, to completely specify it. When we change either the direction, or the magnitude, or both (magnitude and direction) of the applied force, its effect changes.
Balanced and Unbalanced Forces
From our experience we know that sometimes more than one force can act on an object. The effect of all these forces on the object, would be due to the net force acting on It.
Consider a large 'container' filled with a heavy material.
Try to push it, all by yourself, from one corner of the room to another corner. Can you move it?
Take the help of one of your friends. Ask him to push the container in the same direction in which you are pushing it.
Next ask him to push from the opposite direction. What do you observe? In which case does it become easier, or difficult, to move the container?
When your friend pushes the container in the same direction, his effort adds to yours, but when he pushes in the opposite direction, his effort tries to cancel your effort. In each case, an unbalanced force can act on the container.
It is the net unbalanced force, acting on an object, that changes its speed or direction of motion, or both. When the two forces, acting on an object, are different, the object undergoes a change in its state of rest or motion. The change caused depends on the net force acting on it. We call a pair of different forces as unbalanced forces.
1. Unbalanced forces, acting in the same direction, combine by addition.
2. Unbalanced forces, acting in mutually opposite directions, combine by subtraction. The net force is equal to the difference between the two forces and is exerted in the direction of the larger force.
The resultant (net) of unbalanced forces is always non-zero.
For example, in a game of tug of war, the weaker team always gets pulled towards the stronger team.
The figure shows a block of wood, lying on a table, that has been tied to two springs.
If we pull the block from both sides, with the same force, the block remains stationary. The forces are equal and opposite. The net force is zero. Similarly, in a game of tug of war, when both the teams pull the rope, with equal and opposite forces, the rope remains stationary. The net force again is zero. We call such a pair of forces as balanced forces.
Now, try to squeeze a rubber ball between your palms by applying nearly same force from both sides. As the forces applied are equal and opposite, net force is almost zero. It does not move the ball, but can deform it.
We thus conclude that—
equal and opposite forces (balanced forces) do not change the state of rest or motion of an object. They may, however, cause a change in the size and/ or shape of the object.
unequal (unbalanced) forces may lead to
(i) change in state of rest or motion as well as
(ii) change in size and/or shape of an object.
Spring balance is a simple device that can be used for measuring the force acting on an object. It consists of a coiled spring which gets stretched when a force is applied to it. Stretching of the spring is measured by a pointer moving on a graduated scale. The reading on the scale gives the magnitude of the force. The SI unit of force is 1 newton (1N). The force is said to be 1N if it produces an acceleration of lm/s2 in a body of mass 1kg.
The force need not always act in the direction of motion. Depending upon the situation, force may act at any angle to the direction of motion. A force, acting perpendicular to the direction of motion, does not cause any change in speed but can still cause a change in direction.