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Chapter: 11th Physics : Laws of Motion

Physics Laws of Motion: Summary

Physics : Laws of Motion


Aristotle’s idea of motion: To maintain motion, a force is required

Galileo’s idea of motion: To maintain motion, a force is not required

Mass is a measure of inertia of the body

Newton’s first law states that under no external force, the object continues its state of motion

Newton’s second law states that to change the momentum of the body, external force is required

Mathematically it is defined as 

Both Newton’s first and second laws are valid only in inertial frames

Inertial frame is the one in which if there is no force on the object, the object moves at constant velocity.

Newton’s third law states that for every force there is an equivalent and opposite force and such a pair of forces is called action and reaction pair.

To draw a free body diagram for an object,

·           Isolate the object from other objects and identify the forces acting on it

·           The force exerted by that object should not be taken into account

·           Draw the direction of each force with relative magnitude

·           Apply Newton’s second law in each direction

If no net external force acts on a collection of particles (system), then the total momentum of the collection of particles (system) is a constant vector.

Internal forces acting in the system cannot change the total momentum of the system.

Lami’s theorem states that if an object is in equilibrium under the concurrent forces, then the ratio of each force with the sine of corresponding opposite angle is same.

An impulse acting on a body is equal to the change in momentum of the body. Whenever a force acts on the object for a very short time, it is difficult to calculate the force. But impulse can be calculated.

Static friction is the force which always opposes the movement of the object from rest. It can take values from zero to μsN. If an external force is greater than μsN then object begins to move.

If the object begins to move, kinetic friction comes into effect. To move an object with constant velocity, the external force must be applied to overcome the kinetic friction. The kinetic friction is μkN.

Rolling friction is much smaller than static and kinetic friction. This is the reason that to move an object roller coaster is fixed in the bottom of the object. Example: Rolling suitcase

The origin of friction is electromagnetic interaction between the atoms of two surfaces which are touching each other.

Whenever there is a motion along a curve, there must be a centripetal force that acts towards the center of the curve. In uniform circular motion the centripetal force acts at the center of the circle.

The centripetal force is not a separate natural force. Any natural force can behave as centripetal force. In planetary motion, Sun’s gravitational force acts as centripetal force. In the whirling motion of a stone attached to a string, the centripetal force is given by the string. When Moon orbits the Earth, it experiences Earth’s gravitational force as centripetal force.

Centrifugal force arises whenever the motion is analysed from rotating frame. It is a pseudo force. The inertial motion of the object appears as centrifugal force in the rotating frame.

The magnitude of centrifugal and centripetal force is mω2r. But centripetal force acts towards center of the circular motion and centrifugal force appears to acts in the opposite direction to centripetal force.


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