force & laws of motion
TRANSCRIPT
IX PHYSICS
YESH CHOPRA
IX- B
11
SCIENCE FA
FORCE & LAWS OF MOTION
FORCE
• In physics, a force is any influence that causes a free body to undergo a change in speed, a change in direction, or a change in shape.
• Force is a vector quantity• It is the cause of acceleration and retardation
in a object
• F=ma• It’s unit is Newton
Generally pull and push is known as force.
Force can make a stationary body in motion
For example a football can be set to move
by kicking it
Force can change the direction of a
moving object
For example; By applying force, i.e.
by moving handle the direction of a
running bicycle can be changed.
Force can stop a moving body
For example by applying brakes,
a running cycle or a running vehicle
can be stopped.
Effect of Force
Force can change the speed of a moving body
A hockey player hits the the ball
coming towards him/her and
in this way the speed of the
ball can be changed.
Force can change the shape and size
of an object.
For example By hammering,
a block of metal can be
turned into a thin sheet.
Effect of Force
Types of Force
There are 2 main types of forces:
1. Balanced Force- Which is cause of rest.
2. Unbalanced Force Which is cause of Motion.
Inertia
• The property of an object by virtue of which it is unable to change its rest state or uniform motion state itself, is known as Inertia.
• Inertia is the natural tendency of an object to resist a change in its state of motion or of rest. The mass of an object is a measure of its inertia.
Mass and Inertia
• The property of an object because of which it resists to get disturbed its state is called Inertia. Inertia of an object is measured by its mass. Intertia is directly proportional to the mass. This means inertia increases with increase in mass and decreases with decrease in mass. A heavy object will have more inertia than lighter one.
Mass and InertiaInertia of an object is directly proportional to its mass
• Heavy object has large inertia.
• Light object has less inertia.
Laws of motion
Galileo’s observation•By observing the motion of objects on an inclined plane Galileo deduced that objects move with a constant speed when no force acts on them.• He observed that when a marble rolls down an inclined plane, its velocity increases.
VS
At the end of comparison, Aristotle’s laws were failed and hence ,Galileo proved his laws wrong and won.
Newton’s Laws of motion
1. Law of Inertia 2. Law of momentum3. Law of action and reaction
Sir Isaac Newton made a systematic study of motion and proposed three law of motion witch are known as Newton's laws of motion.
Law of InertiaLaw of inertia on rest
• If an object is in rest , it will remain in the rest untill and unless an external force is not applied on it.
Law of inertia on motion
• If an object is in uniform motion in a straight path , it will remain in uniform motion , untill and unless an external force is not applied on it.
1st law of motion from 2nd law
That is, when F = 0, v = u for whatever time, t is taken. This means that the object will continue moving with uniform velocity, u throughout thetime, t. If u is zero then v will also
be zero.That is, the object will remain at rest.
• Dust is removed from a hanging carpet by beating it with a stick.
• Coin falls into the tumbler when the card is given a sudden jerk.
• Only the carom coin at the bottom of a pile is removed when a fast moving carom striker hits it.
• An athlete runs for a certain distance before taking a long jump.
• A ball thrown upward in a moving train comes back into the thrower’s hands.
Law of Momentum
• The rate of change of linear momentum is equal to the force applied and the direction of force is always in the direction of change in momentum.
MATHEMATICAL FORMULATION OFSECOND LAW OF MOTION
Every action has its equal and opposite reaction.
Law of Action & Reaction
• While walking ,we press the ground (action) in backward direction ,it pushes us in forward direction with equal force but in the opposite direction.
• The Rocket is launching in the backward direction and flies in forward direction.
The recoiling of gun also due to action and reaction.
A skateboard surfer exerts his action and in reaction the skate board runs
Conservation of Momentum
According to II law of motion
F = 𝑑𝑃
𝑑𝑡
If external applied force F in zero (F=0)
Then F = 𝑑𝑃
𝑑𝑡=0
i.e. P = Constant
In the absence of ext. applied force net linear momentum of a system is always constant.
Collision of two ballsAccording to the third law of motion, the force F exerted by ball A on ball B (action) and the force F exerted by the ball B on ball A (reaction) must be equal and opposite to each other.
So, the total momentum of the two balls remains unchanged or conserved provided no other external force acts.