Chapter- Force and Laws of Motion
APPLICATIONS OF LAWS OF MOTION
APPLICATIONS OF LAWS OF MOTION
Class-IX
First law of motion
Introduction: First law of motion states that- ‘A body at rest will remain at rest and a body in uniform motion will continue in uniform motion with same velocity unless it is compelled by an external force to change its state of rest or of uniform motion.’ The property of a body due to which it resists a change in its state of rest or of uniform motion is called Inertia. So, we can say that first law defines inertia.
Physical significance of inertia: Mass is the measure of inertia of a body. Heavier objects have more inertia than lighter objects.
Practical applications/ Related phenomenon: 1) When a hanging carpet is beaten with a stick, dust particles start coming out of it. This is because the force of stick makes the carpet move to and fro slightly but dust particles tend to remain at rest due to their inertia and hence seperate from the carpet.
Introduction: First law of motion states that- ‘A body at rest will remain at rest and a body in uniform motion will continue in uniform motion with same velocity unless it is compelled by an external force to change its state of rest or of uniform motion.’ The property of a body due to which it resists a change in its state of rest or of uniform motion is called Inertia. So, we can say that first law defines inertia.
Physical significance of inertia: Mass is the measure of inertia of a body. Heavier objects have more inertia than lighter objects.
Practical applications/ Related phenomenon: 1) When a hanging carpet is beaten with a stick, dust particles start coming out of it. This is because the force of stick makes the carpet move to and fro slightly but dust particles tend to remain at rest due to their inertia and hence seperate from the carpet.
2) When a tree (having flexible stem) is shaken vigorously, its fruits and leaves fall down. This is due to the fact that when branch of tree is shaken, it starts moving to and fro but the fruits and leaves attached to it tend to remain at rest due to inertia and hence detach from tree and fall down.
3) When a car or bus stops suddenly, passengers are jerked forward because due to force of brakes bus stops but passengers tend to remain in uniform motion. The seat belts are provided in cars so that if a fast running car stops suddenly due to some emergency, then the passengers are not thrown forward violently, and injury is prevented.
4) When a car or bus starts suddenly, passengers are thrown backward because the vehicle starts but passengers tend to remain at rest.
5) When a car or bus turns a corner sharply, we tend to fall sideways because of our inertia or tendency to continue moving in a straight line.
6) It is dangerous to jump out of a moving bus because the jumping person, who was moving with high speed of the bus, would tend to remain in motion due to inertia even on falling to the ground and get hurt due to resistance offered by ground.
Definition of FORCE: Force is something which changes or tends to change the state of rest or of uniform motion of a body.
Second Law of Motion
Introduction: Second law of motion states that-‘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.’
Force α Change in momentum / time taken
Formula: F=ma where, m=mass of body, a= acceleration produced on applying force F
NOTE: first law is special case of second law (explained in class)
Practical applications/ Related phenomenon: 1) A cricket player moves his hands backward on catching a fast cricket ball. This is because a fast moving cricket ball has a large momentum. In stooping such a ball its momentum has to be reduced to zero. Now, when a cricket player moves his hands back, the time taken to reduce momentum of ball to zero is increased which reduces the rate of change of momentum( rate of change of momentum α change in momentum/ time taken; if denominator is increased, whole value decreases, so force decreases). Hence, a small force is exerted on the hands of player and he does not get hurt.
2) During athletics meet, a high jumping athlete is provided either a cushion or a heap of sand on the ground to fall upon. When the high jumper falls on a soft landing site, then the jumper takes a longer time to come to a stop. Thus rate of change of momentum is less due to which small stopping force acts on athlete’s body and the athlete does not get hurt.
Third Law of Motion
Introduction: Whenever one body exerts a force on another body, the second body exerts equal and opposite force on the first body i.e. to every action there is an equal and opposite reaction.
NOTE: Both the forces are opposite, so if one is positive, other will be negative.
Physical significance: Forces always exists in pairs.
Practical applications/ Related phenomenon: 1) When we walk on the ground, then our foot pushes the ground backward and in return ground pushes our foot forward. The forward reaction exerted by the ground on our foot makes us walk forward.
2) When a bullet is fired from a gun, the force sending the bullet forward is equal to the force sending the gun backward. But due to the high mass of the gun, it moves only a little distance backward and gives a backward jerk or kick to the shoulder of the gunman. The gun is said to have recoiled.
3) In jet airplanes and rockets, the hot gases obtained by rapid burning of fuel rush out of a jet (nozzle) at the rear (back) end of aircraft at a great speed. The equal and opposite reaction of the backward going gases pushes the aircraft forward at a great speed.
4) Moving of a boat is also similar to those of aircrafts with the difference that backward force is provided by oars (chapu).
5) When a man jumps out of a boat to the bank of the river, the boat moves backwards, away from him. This is because while stepping out of the boat, man pushes the boat in backward direction with his leg (action). In return of this boat exerts an equal force on man in forward direction which makes the man move forward.
6) When firemen are directing a powerful stream of water on fire from a hose pipe, they have to hold the hose pipe strongly due to its tendency to go backward. The backward movement of hose pipe is due to the backward reaction of water rushing through it in the forward direction at a great speed.
7) To make the cart move, the horse bends forward and pushes the ground with its feet. In return to this ground exerts a forward push on horse. When the forward reaction to the backward push of the horse is greater than the opposing frictional forces of the wheels, the cart moves.
Law of Conservation of momentum
Introduction: The law of conservation of momentum states that ‘When two or more bodies act upon one another, their total momentum remains constant provided no external forces are acting.’
Total momentum before collision= total momentum after collision
Practical application/ Related phenomenon: The chemicals inside the rocket burn and produce high velocity blast of hot gases. These gases pass out through the tail nozzle of the rocket in the downward direction with tremendous seed or velocity, and the rocket moves up to balance the momentum of the gases. Although the mass of gases emitted is comparatively small, but they have a very high velocity and hence a very large momentum. An equal momentum is imparted to the rocket in the opposite direction, so that, inspite of its large mass, the rocket goes up with a high velocity.
Q Why more force is required to move a heavier body?
A A greater force is required to move a body with more mass. This can be explained as follows-
a) First law of motion- According to first law of motion, a body with more mass has more inertia and more the inertia of a body, more is its tendency to resist change in its state of rest. So, a greater force is required to overcome this high inertia.
b) Second law of motion- According to second law of motion, F=ma i.e. Fα m when ‘a’ is constant. So, greater the mass of a body, more force it requires to produce a particular amount of acceleration.
Q How does a rocket move?
A A rocket moves due to gases produced from its nozzle. This can be explained as-
a) Third law of motion- see explanation above
b) Law of conservation of momentum- see explanation above
Q Why are seat belts used in cars?
A Seat belts are used in cars to save the passengers from injuries. This is because
3) When a car or bus stops suddenly, passengers are jerked forward because due to force of brakes bus stops but passengers tend to remain in uniform motion. The seat belts are provided in cars so that if a fast running car stops suddenly due to some emergency, then the passengers are not thrown forward violently, and injury is prevented.
4) When a car or bus starts suddenly, passengers are thrown backward because the vehicle starts but passengers tend to remain at rest.
5) When a car or bus turns a corner sharply, we tend to fall sideways because of our inertia or tendency to continue moving in a straight line.
6) It is dangerous to jump out of a moving bus because the jumping person, who was moving with high speed of the bus, would tend to remain in motion due to inertia even on falling to the ground and get hurt due to resistance offered by ground.
Definition of FORCE: Force is something which changes or tends to change the state of rest or of uniform motion of a body.
Second Law of Motion
Introduction: Second law of motion states that-‘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.’
Force α Change in momentum / time taken
Formula: F=ma where, m=mass of body, a= acceleration produced on applying force F
NOTE: first law is special case of second law (explained in class)
Practical applications/ Related phenomenon: 1) A cricket player moves his hands backward on catching a fast cricket ball. This is because a fast moving cricket ball has a large momentum. In stooping such a ball its momentum has to be reduced to zero. Now, when a cricket player moves his hands back, the time taken to reduce momentum of ball to zero is increased which reduces the rate of change of momentum( rate of change of momentum α change in momentum/ time taken; if denominator is increased, whole value decreases, so force decreases). Hence, a small force is exerted on the hands of player and he does not get hurt.
2) During athletics meet, a high jumping athlete is provided either a cushion or a heap of sand on the ground to fall upon. When the high jumper falls on a soft landing site, then the jumper takes a longer time to come to a stop. Thus rate of change of momentum is less due to which small stopping force acts on athlete’s body and the athlete does not get hurt.
Third Law of Motion
Introduction: Whenever one body exerts a force on another body, the second body exerts equal and opposite force on the first body i.e. to every action there is an equal and opposite reaction.
NOTE: Both the forces are opposite, so if one is positive, other will be negative.
Physical significance: Forces always exists in pairs.
Practical applications/ Related phenomenon: 1) When we walk on the ground, then our foot pushes the ground backward and in return ground pushes our foot forward. The forward reaction exerted by the ground on our foot makes us walk forward.
2) When a bullet is fired from a gun, the force sending the bullet forward is equal to the force sending the gun backward. But due to the high mass of the gun, it moves only a little distance backward and gives a backward jerk or kick to the shoulder of the gunman. The gun is said to have recoiled.
3) In jet airplanes and rockets, the hot gases obtained by rapid burning of fuel rush out of a jet (nozzle) at the rear (back) end of aircraft at a great speed. The equal and opposite reaction of the backward going gases pushes the aircraft forward at a great speed.
4) Moving of a boat is also similar to those of aircrafts with the difference that backward force is provided by oars (chapu).
5) When a man jumps out of a boat to the bank of the river, the boat moves backwards, away from him. This is because while stepping out of the boat, man pushes the boat in backward direction with his leg (action). In return of this boat exerts an equal force on man in forward direction which makes the man move forward.
6) When firemen are directing a powerful stream of water on fire from a hose pipe, they have to hold the hose pipe strongly due to its tendency to go backward. The backward movement of hose pipe is due to the backward reaction of water rushing through it in the forward direction at a great speed.
7) To make the cart move, the horse bends forward and pushes the ground with its feet. In return to this ground exerts a forward push on horse. When the forward reaction to the backward push of the horse is greater than the opposing frictional forces of the wheels, the cart moves.
Law of Conservation of momentum
Introduction: The law of conservation of momentum states that ‘When two or more bodies act upon one another, their total momentum remains constant provided no external forces are acting.’
Total momentum before collision= total momentum after collision
Practical application/ Related phenomenon: The chemicals inside the rocket burn and produce high velocity blast of hot gases. These gases pass out through the tail nozzle of the rocket in the downward direction with tremendous seed or velocity, and the rocket moves up to balance the momentum of the gases. Although the mass of gases emitted is comparatively small, but they have a very high velocity and hence a very large momentum. An equal momentum is imparted to the rocket in the opposite direction, so that, inspite of its large mass, the rocket goes up with a high velocity.
Q Why more force is required to move a heavier body?
A A greater force is required to move a body with more mass. This can be explained as follows-
a) First law of motion- According to first law of motion, a body with more mass has more inertia and more the inertia of a body, more is its tendency to resist change in its state of rest. So, a greater force is required to overcome this high inertia.
b) Second law of motion- According to second law of motion, F=ma i.e. Fα m when ‘a’ is constant. So, greater the mass of a body, more force it requires to produce a particular amount of acceleration.
Q How does a rocket move?
A A rocket moves due to gases produced from its nozzle. This can be explained as-
a) Third law of motion- see explanation above
b) Law of conservation of momentum- see explanation above
Q Why are seat belts used in cars?
A Seat belts are used in cars to save the passengers from injuries. This is because
a) First law of motion- see explanation above
b) Second law of motion- In a fast moving car when brakes are applied, car stops suddenly. Due to this car’s large momentum is reduced to zero in a very short time. The stretchable seat belts worn by the passengers of the car increase the time taken by the passengers to fall forward. Due to longer time, the rate of change of momentum of passengers decreases and hence less stopping force is applied on passengers. This prevents the passengers from getting hurt seriously.
NOTE: Similarly, there are many other phenomena which cannot be explained simply by one law of motion.
b) Second law of motion- In a fast moving car when brakes are applied, car stops suddenly. Due to this car’s large momentum is reduced to zero in a very short time. The stretchable seat belts worn by the passengers of the car increase the time taken by the passengers to fall forward. Due to longer time, the rate of change of momentum of passengers decreases and hence less stopping force is applied on passengers. This prevents the passengers from getting hurt seriously.
NOTE: Similarly, there are many other phenomena which cannot be explained simply by one law of motion.
