Motion and Force

Everywhere around us we see that objects are in a state of motion. We see cars, trains move, we see a person walking on the street. We know that the earth is rotating around its axis because we see its effect as day and night. The pendulum in a grandfatherís clock repeats itís back and forth motion; this is also an example of motion.  In fact, it is a basic fact in Physics that nothing is completely stationary!!

What we will study in this chapter :

1. Idea of motion
2. Definition of force

1. Idea of motion

If an object moves from one point to another point in a straight line, we say that the object has undergone a linear motion. If on the other hand, the object moves in a zig-zag motion and reaches its final destination, we say that the object has undergone a non-linear motion. 

Change of position of an object is always relative. Hence motion is relative. Thus motion means a change of position of an object with respect to an observer. 

When the displacement of all the particles in the object takes place simultaneously and in the same direction, we say that the motion is translational. When the object is moving around a fixed axis, we say that the object is undergoing a rotational motion. (In a rotational motion, as in the case of the earth, or a spinning top, the constituent particles of the object are actually moving at different angular velocities). In case of repeated motion, as in the case of a pendulum, we say that the motion is oscillatory.

When the object does not change its position with respect to the observer, we say that the object is at rest.  But one observerís remarks may not match with another observerís. For example, if you see a boy standing still on the road, you will say that the boy is at rest or is stationary. But another observer on the moon will see the boy rotating around earthís axis. So both your observation and the observation of the person on the moon differs a lot. Thus we see that the concept of motion is relative. 

As soon as we say motion, a few  terms get attached to it. They are speed, velocity and acceleration.  
 
When we say that a car is moving at a speed of 30 km/hour, we mean that the displacement the car undergoes in 1 hour is 30 km.

Therefore, speed =  displacement per unit time. 

Velocity is defined as displacement in a particular direction. 

Thus speed and velocity are the same, but speed is a scalar (only magnitude but no direction) and velocity is a vector (has a magnitude and a definite direction). Although speed and velocity are used loosely as identical terminologies, in Physics they are different. When speed is increased or decreased, only the rate of displacement is changed. When there is a change in velocity, both the rate of displacement as well as the direction may have a change. 

When a car starts to move, first it is at rest, velocity is zero. Then it starts to move, picks up speed.  Then when the car reaches its destination, it stops and comes to rest. At rest again the velocity is zero. In between start and stop, the velocity of the car has continuously changed. Rate of change of velocity is called acceleration.  Acceleration is also a vector quantity. When the car picks up speed, the acceleration is positive. When the car reduces its speed before coming to a halt, the car is decelerating, which is nothing but negative acceleration (direction of deceleration is opposite to the direction of acceleration).           

2. Definition of force 

All motions take place under the influence of some force. For example, a car is moving because of the force delivered by the engine. A ball when dropped from a height falls on the ground under the influence of earthís gravitational force.  The earth is moving around the sun because of the gravitational force of the sun.  Electrons move in an atomic orbit due to electrical attractive force of the protons.  

Thus force can be defined as the influence that tends to change the speed and direction of a stationary or a moving body. 

Force directly affects acceleration of the body on which it is acting. Thus force is directly proportional to acceleration. 

F  a 

Force is inversely proportional to the mass of body on which it is acting; larger the mass more difficult it would be to move it.

F  1/m 

The relation can be written as 

    F = m. a

When a force acts on a body, it produces an acceleration in the same direction in which the force is acting. Force is a vector quantity and the above equation is correctly written as  

            
  F     =   m.   a 


Unit of force is that force which produces a unit acceleration on a unit mass. 1 kg is the unit mass and 1 m/s² is the unit of acceleration in the MKS system.  A unit of force in the MKS system is  a  Newton and it is defined as the  force that causes an acceleration of 1 m/s². 
A unit of force in the CGS system is called a dyne. (1 Newton = 10⁵ dyne) . 
Newton is a derived unit  (m.kg.s^-2 ) and has dimension [ M¹L¹T^-2].    

Next        Main        Previous