Summary
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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