Chapter 09 ––>Force and Laws of Motion

• In our everyday life we observe that some effort is required to put a stationary object into motion or to stop a moving object.

• By pushing, hitting and pulling objects we are bringing objects in motion. They move because we make a force act on them.

• We know that force can be used to provide acceleration.
• We also know that a force can change the shape and size of objects.

• By blowing air into the balloon we are applying force , which causes change in its shape

Balanced and Unbalanced force

• If net addition of all the forces on an object is zero then the forces action on it are called Balanced Forces
• If net addition of all the forces is not zero then the forces are termed as unbalanced forces.
• If an unbalanced force is applied on object it will have acceleration. And the acceleration will be there as long as the force is there.
• After the force is removed it will continue to move with the velocity it has gained till then.

Laws of motion

First Law of Motion
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• An object remains in a state of rest or of uniform motion in a straight line unless compelled to change that state by an applied force.
• The tendency of an object to remain at its natural position, rest or uniform motion, is because of its property called Inertia.

• When we beat carpet dust particles comes out of it because initially both the carpet and dust particles are in a state of rest as soon as you beat it, you provide carpet a motion which is resisted by dust particles and they come out of it.

INERTIA & MASS

• 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.
• A force that is just enough to cause a small cart to pick up a large velocity will produce a negligible change in the motion of a train, because of train’s large inertia.

Second Law of Motion
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• Second law state that the rate of change of momentum of an object is proportional to the applied unbalanced force in the direction of force.

• The impact produced by the objects depends on both their mass and velocity , this can be observed by following observations :
• A moving truck, even at speeds as low as 5 m/s, may kill a person standing in its path.
• A small mass, such as a bullet may kill a person when fired from a gun

• Momentum is introduced by Newton which combines the object’s mass and its velocity.
• Momentum has both direction and magnitude. Its direction is same as of velocity.

• SI unit of momentum is kg m s^-1 .  

Mathematical Formulation of Second Law of Motion

Third Law of Motion
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• The third law of motion states that when one object exerts a force on another object, the second object instantaneously exerts a force back on the first object.
• These two forces are always equal in magnitude but opposite in direction.
• These forces act on different objects and never on the same object.
• The two opposing forces are also known as action and reaction forces.

Note : Third Law states that action and reaction forces will be same, but there acceleration will depend on their masses, in accordance with the Second Law.

Momentum Conservation

 Law of conservation of momentum: In absence of an external unbalanced force the sum of momenta (plural of momentum) of two objects before collision is equal to sum of momenta of those objects after collision.

• All conservation laws such as conservation of momentum, energy, angular momentum, charge etc. are considered to be fundamental laws in physics. These are based on observations and experiments and cannot be proved. They can be verified, or disproved, by experiments.

Where  u_a = initial velocity of mass A       u_b = initial velocity of mass B

             v_a = final velocity of mass A       v_a = initial velocity of mass B