Newton's Law

 Law of newton

First law of motion

It is also called law of inertia, two form of inertia: inertia of rest, inertia of motion and Inertia of direction. According to this a body maintain its uniform motion unless any external force is applied to it.

● Inertia of rest:  inability of a body to change its state from rest.

→ When a branch of a fruit tree is shaken, the fruits fall down. This is because the branch comes in motion and the fruits tend to remain at rest. Hence, they get detached.

→ The dirt particles in a durree fall off if it is stricken by a stick. This is because the striking sets the durree in motion whereas the dirt-particles tend to remain at rest and hence fall.

→ When a train starts suddenly, the passenger sitting inside tends to fall backwards. This is so because the lower part of the passenger’s body starts moving with the train but the upper part tends to remain at rest.

→ If a smooth paper having a coin on it placed on a table is suddenly drawn, the coin remains at the same place on the table due to inertia of rest.

→ When a horse starts suddenly, the rider tends to fall backwards due to inertia of rest

● Inertia of motion: inability of a body to change it state form uniform motion.

→When a horse at full gallop stops suddenly, the rider on it falls forward because of inertia of motion of the upper part of the rider’s body

→ When an athlete takes a long jump, he runs first for a certain distance before the jump. This is because his feet come to rest on touching the ground and the remaining body continues to move owing to inertia of motion.

→ When train stops suddenly, a passenger sitting inside tends to fall forward. It happens because the lower part of the passenger’s body comes to rest with the train but the upper part tends to continue its motion due to inertia of motion.

→ A person jumping out of a speeding train may fall forward due to inertia of motion of his body. Hence, he should

run a few steps on the platform in the direction of motion of train.

● Inertia of direction: The inability of a body to change its direction itself.

→ Use of an umbrella to protect us from rain is based on the property of inertia of direction because the raindrops cannot change their direction of motion.

→ When a bus or a car rounds a curve suddenly, the person sitting inside is thrown outwards. It happens so because the person tries to maintain his direction of motion due to directional inertia while the vehicle turns.

→ When a knife is sharpened by pressing it against a grinding stone, the sparks fly off tangentially(goes from upside, a tangent is a line that touches a curve in one spot but doesn't intersect it anywhere else.) because of the inertia of direction.

→ When a stone tied to one end of a string is whirled and the string breaks suddenly, the stone spins off along the tangent of its circular path. It happens so because of the pull in the string was forcing the stone to move in a circle. As soon as the string breaks, the pull disappears. The stone becomes free and in a bid to move along the straight line flies off tangentially.

→ The wheels of any moving vehicle throw out mud tangentially due to the inertia of direction.

 Second law of motion

‘The rate of change of linear momentum(the quantity of motion of a moving body, measured as a product of its mass and velocity.ie momentum= mass*velocity) of a body is directly proportional to the external force applied on the body and this change takes place always in the direction of the applied force’.

When a force is applied on a body, its momentum and hence, velocity change. The change in velocity produces acceleration in the body. The rate of change of linear momentum with time is equal to the product of the mass of the body and its acceleration which measures the magnitude of the applied force

→ Force =change is linear momentum/ time       → →  first equation

→ Force = mass * acceleration   → →   second equation of force

For the first equation:

While playing the cricket, when we catch the ball, we lower or hand, this reduce the jerk feel by our hand. This is because, when we are lowering our hand , we actually increasing the time of impact, that’s why we feel less jerk. Let us suppose the momentum of ball is 15, and if the time is less let suppose 3 then the force feel by us is more i.e. 15/3=5.  But if we increase the time of impact let time 5 then the force will automatically decrease i.e. 15/5=3, now we are feeling less force. This is how the buffer of train boogies work and shocker of bike work to protect us from jerk.

For the second equation:

Suppose the force is applied is 15. So if the mass is 5 then the acceleration will be 3, but if the mass is less, let suppose mass is 3 then the acceleration will be more that is 5. Because the product of mass and acceleration is always equal to force. Ie. 3*5=15.

Second law gives us the measure of force.

When a body is moving with a uniform velocity along a straight line, it neither experience nor require an external force. This is because, the acceleration is due to change in the velocity of the body and the velocity remains constant for a body moving with a uniform velocity along a straight line

When a body changes its velocity or direction of its motion, its velocity changes too. It results in an acceleration which is possible only by the action of an external applied force. Hence, an accelerated motion is always due to an external force

Applications:-

→ Crockery items are wrapped in paper or straw pieces before packing because paper or straw acts as buffers. It

Changes the time of impact and hence, avoids the chances of damage during the jerks.

→ An athlete should stop slowly, after finishing a fast race, so that the time of impact of his run increases at stop and hence, force experienced by him decreases.

→ When two vehicle collide, the change of linear momentum is equal to the sum of two vehicle's momentum, and the time of impact is very low, so the large force is developed which result the maximum damage to vehicles.suppose the momentum of first vehicles is 10 and 20, so when they collide the change in momentum is 10+20=30, and the time of impact is vary less let time is 1, so the force produced is 30/1=30.

→ When a person falls from a height on a concrete floor, the floor does not yield. The total change in linear-momentum

is produced in a very small interval of time. Hene, the floor exerts a much larger force and the person receives more injury. But when a person falls on a heap of sand, the sand yields. The same change in linear momentum is produced in a much longer time. The average force exerted on the person by the heap of sand is, therefore, much smaller and hence the person is not hurt.

Third law of Newton

‘’To every action, there is always, an equal and opposite reaction.’’

It signifies that forces in nature are always in pairs. A single isolated(alone) force is not possible. Force of action and reaction act always on different bodies. They never cancel each other and each force produces its own effect. The forces of action and reaction may be due to actual physical contact of the two bodies or even from a distance. But they are always equal and opposite. This third law of motion is applicable whether the bodies are at rest or they are in motion. This law is applied to all types of forces e.g. gravitational, electric or magnetic forces, etc.

Examples:

→ A book placed on a table exerts a force as an action on the table. This action is equal to the weight of the book. The table exerts a force of reaction equal and opposite to the reaction to support the book.

→ When a gun fires a bullet, it moves forward due to a force exerted by the gun. The bullet exerts a reaction due to

which the gun recoils backward

→ We can walk on a ground easily if it is tough because the ground provides sufficient reaction against our push. But it

is difficult to walk on sand or ice. This is because on pushing, sand gets displaced and reaction from sandy ground

is very little. In case of ice, force of reaction is again small, because friction between our feet and ice is very little

→ When a rubber ball is struck against a wall or floor, it exerts a force as an action on the wall. The ball rebounds with an equal and opposite force as reaction exerted by the wall on the ball.

→ A swimmer pushes the water with a force of action in backward direction while water pushes the swimmer with a force of reaction in the forward direction. Consequently, the swimmer is able to swim

→ When a jet-plane or rocket moves in the sky, the gases produced due to combustion of fuel escape through the nozzle in the backward direction due to the force of action exerted by the engine. The escaping gases exert a force of reaction on the jet-plane or rocket in the forward direction. Consequently, the jet-plane or rocket moves.