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CBSE Class 10 Answered

explain newtons 3 laws with examples
Asked by uslearnings | 19 Nov, 2019, 07:23: PM
answered-by-expert Expert Answer
First Law of motion:
Every body continues to be in its state of rest or of uniform motion in a straight
line unless compelled by some external force to act otherwise.
 
examples  :-
Consider a book at rest on a horizontal surface. It is subject to two external forces :
the force due to gravity (i.e. its weight W) acting downward and the upward force on the book by the table,
the normal force R . R is a self-adjusting force.  We observe the book to be at rest. Therefore, we conclude
from the first law that the magnitude of R equals that of W.
The conclusion for this observation is : “Since the book is observed to be at rest, the net external force on it must be zero,
according to the first law "
 
Suppose we are standing in a stationary bus and the driver starts the bus suddenly. We get thrown backward with a jerk.
The reason for this is explained below
Our feet are in touch with the floor. If there were no friction, we would remain where we were, while the floor of the bus
would simply slip forward under our feet and the back of the bus would hit us. However there is some friction between the
feet and the floor. If the start is not too sudden, i.e. if the acceleration is moderate, the frictional force would be enough
to accelerate our feet along with the bus. But our body is not strictly a rigid body. It is deformable, i.e. it allows some
relative displacement between different parts. While our feet go with the bus, the rest of the body remains where it is
due to inertia. Relative to the bus, therefore, we are thrown backward. As soon as that happens,
however, the muscular forces on the rest of the body (by the feet) come into play to move the body along with the bus.
A similar thing happens when the bus suddenly stops. Our feet stop due to the friction which does not allow relative
motion between the feet and the floor of the bus. But the rest of the body continues to move
forward due to inertia. We are thrown forward.The restoring muscular forces again come into play and bring the body to rest..
 
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Second Law of motion expressed by Newton as follows :

The rate of change of momentum of a body is directly proportional to the applied force and
takes place in the direction in which the force acts.
 
Hence force F = d(mv)/dt = m×(dv/dt) = m×a
 
examples for second law are obvious. If force is applied to a body of mass, the body is accelerated as per newton's second law.
If many forces are acting simultaneously, net resultant force gives the acceleration.
 
acceleration due to gravitational force is familiar example.
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Newton's third law of motion :-
To every action, there is always an equal and opposite reaction.
 
Action and reaction are terms expressed for forces
 
A simple and clear way of stating the third law is as follows :

Forces always occur in pairs. Force on a body A by B is equal and opposite to the
force on the body B by A.
 
There is no cause-effect relation implied in the third law. The force on A by B and the force on B by A act
at the same instant.
 
example :-
if a block of mass is kept on the table, There are two forces acting on the block .
One force is due to gravitational force i.e. weight of the block that is acting on the table surface.
Other force is reaction force provided by table surface to the block.
 
if a block of mass is hangd by rope, then there are two forces. One force is weight of the block, i.e.,
gravitational force that is pulling the rope downwards.
Other force is tension force in the rope that is pulling the block upwards.

Answered by Thiyagarajan K | 20 Nov, 2019, 10:49: AM
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