To understand the meaning of the term 'force', let us examine some effects when a force is applied to an object. When a force is applied to a body, it can essentially have two effects:
1. A force can bring about a change in the state of motion of a body, or
2. A force can deform a body, i.e., change its shape.
A force can produce two kinds of motion, translator and rotator.
The motion of a body is said to be translator if each constituent part of the body suffers the same displacement in the same time.
When a force is applied to an object, it can have the following effects on its translator motion:
1. A force can move an object originally at rest.
2. A force can increase or decrease the speed of a moving object.
3. A force can change the direction of motion of an object.
In other words, a force produces acceleration in the body to which it is applied.
The forces acting on the body may be of two types: Balanced or Unbalanced.
Balanced forces cannot change the speed or the direction of motion of a body. In order to change the speed or the direction of motion of a body, an unbalanced force must act on the body.
Inertia and mass:
Inertia is the property or tendency of a body to resist any change in its state of rest or of uniform motion in a straight line. The name 'inertia' was given by Galileo. A body is said to have a large inertia if it is hard to change its state of rest or of uniform motion. An object such as a heavy truck or a heavy stone has a large amount of inertia, whereas a pencil or a safety pin has very little inertia. Thus, the term mass is a measure of inertia of a body. The more inertia a body has, the greater is its mass.
Newton's laws of motion:
After Galileo had clarified the idea of motion, Isaac Newton (1642-1727) of England made a detailed and systematic study of the motion of objects. He was the first person to give a scientific meaning to the concepts of mass and force. He used these concepts to formulate the three fundamental laws governing all motion. These laws are called Newton's Laws of Motion.
These laws of motion are applicable not only to the motion of earthly bodies but also to the motion of heavenly bodies.
First Law of Motion:
A body continues in its state of rest or of uniform motion in a straight line unless it is compelled to change that state by an unbalanced force.
Interpretation of the First Law:
The first part of the law, everybody continues in its state of rest unless it is compelled to change that state by an external force' is a common experience and, hence, is easy to understand.
If you place a book on a table, it will keep on lying there unless someone removes it, i.e. applies a force on it. The second part of the law, i.e., 'everybody continues in its state of uniform motion in a straight line unless it is compelled to change that state by an unbalanced force' is difficult to understand because it is contrary to our common experience. A stone rolled along a road comes to rest after going only a short distance.
But replace the stone by a body which is smoother, e.g., a ball bearing and replace the road by a surface which is smoother. Now roll the ball bearing on the smooth surface with the same force as before.
It travels a much longer distance before coming to rest. If this experiment is done in a chamber from which air has been exhausted, the ball bearing will go still further because the friction due to air resistance has been removed. In other words, the speed of a body in uniform motion can be changed only if a force acts on it.
The phrase 'in a straight line', which occurs in the statement of the first law, should be carefully noted. The direction as well as the speed of a moving body will remain the same, i.e. its velocity remains the same unless a force acts on it.
Hence, from the first law, we may qualitatively define force as something which when acting on a body, changes or tends to coinage its state of rest or uniform motion in a straight line.
Second Law of Motion:
The acceleration produced by unbalanced force acting on a body is directly proportional to the magnitude of the applied force and inversely proportional to the mass of the body; i.e. the acceleration taking place in the direction in which the force acts.
Interpretation of the Second Law:
Newton's second law tells us what happens when a force is actually exerted on an object. It tells us how to measure the effect of the force exerted on a body. Suppose a force F acts on a body of mass m.
The second law tells us that the acceleration a produced in the body is directly proportional to F and inversely proportional to m, i.e. we, therefore, conclude that the effect of a force is to produce acceleration. Hence, the force is something that produces acceleration in a material body.
Unit of Force:
Newton's second law gives us a method by which force can be measured.
The Newton is defined as that force which, applied to mass of 1 kg, produces an acceleration of 1 ms-2.
Third Law of Motion:
Whenever a body exerts a force on the second body, the second body exerts an equal and opposite force on the first.
To every action, there is an equal and opposite reaction.
It may be noted that action and reaction which occur in pairs act on different bodies. If they acted on the same body, the resultant force would be zero and there could never be accelerated motion ever possible.
When a body exerts a force on a second body, the second body at the same time exerts a force on the first body. It is impossible to have a single isolated force. Furthermore, the above two forces are found to be equal in magnitude and opposite in direction.
For example, if we try to push a heavy door, the force we exert on it accelerates it as it opens. Simultaneously, we feel the force exerted by the door on us impeding our movement.
Sometimes, we can feel the reacting force. If we fire a rifle, the forward thrust of the projectile is matched by the backward thrust, which we call the recoil or 'kick' of the rifle. When two bodies thus interact with each other, one of the two forces is termed action and the other reaction. There is no cause-effect relation between action and reaction.
They occur simultaneously, and any one of the two forces may be called the action or the reaction. The relation between these two forces is stated in Newton's third law of motion.
We know that
The force required to stop a moving body depends on its mass; and
The force required to stop a moving object depends also on its velocity.
We conclude that the force required to stop a body depends on two factors: (1) its mass m and (2) its velocity v. Newton defined momentum as the product of mass and velocity of a body
Qualitatively, momentum may be regarded as the quantity of motion possessed by a body. Momentum is a vector quantity since it is equal to the product of a scalar quantity m and a vector quantity v.
Unit of Momentum:
The unit of momentum in the SI system is kg ms-2.
A girl standing at rest with her right foot on a skateboard thrusts backwards on the ground with her left foot. The push of the ground on her left foot will increase the forward momentum of the girl and skateboard. The velocity v acquired by the girl will depend on the force F which can be applied and the time for which it acts.
Suppose a force F acts on a body for a time interval t. The product Ft is called the impulse of the force, i.e.
Impulse = force x time interval According to Newton's second law, Force = mass x acceleration
Change in velocity
Force x time interval = mass x change in velocity or Impulse = mass x change in velocity
Impulse of a force is a useful concept when a force acts for a short duration of time. It tells us by how much the velocity of a body of a given mass will change if a given impulse is applied to it. When we hit a ball with a hockey stick, we give it an impulse.
The SI unit of impulse is Newton second (Ns). Impulse is a vector quantity.
Whenever a body slides over the surface of another body, each body exerts a force on the other which retards motion. This force is called the force of friction.
The magnitude of the force of friction depends on the roughness of the surface in contact, the area of contact and the weight of the sliding body.
Static and Kinetic Friction:
Suppose we place a block on a horizontal surface. When a horizontal force F is applied, and the block does not move, it is because F is opposed by an equal force due to friction. This is called static friction. If the applied force is increased, the block may still not move which means the frictional force has also increased.
However, if we gradually increase F, at a certain maximum value of F, the block just starts to slide. This is the maximum force of static friction. Once the block starts sliding, the frictional force developed decreases. This force is called kinetic friction. The laws of static friction are:
The maximum force of static friction is independent of the area of contact.
The maximum force of static friction is proportional to the normal force.
The constant of proportionality is called the coefficient of static friction. (The laws of kinetic friction are similar to those of static friction).
Rolling and Sliding Friction:
When a body rolls over a surface, the frictional force developed is known as rolling friction. And, when a body slides over the other body, the frictional force acting between the two is called sliding friction. Sliding friction is always more than rolling friction.
Importance of Friction:
Many everyday activities like walking or gripping objects are carried out through friction and most people have experienced the problems that arise when there is too little friction and conditions are slippery.
However, friction is a serious nuisance in devices that move continuously, like electric motors or rail road trains, since it constitutes a dissipation of energy, and a considerable proportion of all the energy generated by humans is wasted in this way.
Most of this energy loss appears as heat, while a small proportion induces loss of material from the sliding surfaces, and this eventually leads to further waste, namely, to the wearing out of the whole mechanism.
Modes of Reducing Friction:
Friction can never be completely eliminated. It can only be reduced. The various methods of reducing friction are enumerated below:
Polishing makes surfaces smooth and thus reduces friction.
In machines, the possibility of metal-to-metal contact is reduced by 'oiling'. Friction between liquid layers is much smaller than between solid surfaces. The oil used to give a slippery coating to solid surfaces is known as a lubricant. Lubrication minimizes the heat due to the friction and, hence, reduces wear and tear of the surfaces in contact.
3. Ball Bearings:
The fact that rolling friction is much smaller than sliding friction is made use of in reducing friction in the moving parts of machines.