Laws of
Motion
Newton's Laws of Motion
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When you try to
push a wall, it pushes your right back! The harder
you push the more pressure is felt by your hands.
This is what Newton's third law of motion deals with.
Third law
of motion: If a body exerts a force on
another, there is an equal and opposite force, called
a reaction on the first body by the second.
Do the force
and its reaction cancel each other? No, as they are
acting on different bodies. For equal and opposite
forces to cancel, they must act on the same body. In
the case of pushing of the wall, the wall feels the
action and the hands the reaction.
Also forces
always exist in pairs. If there is a force on the
wall, then there has to be a force on the hands. It
is impossible to have a single isolated force.
This law is
true for bodies at rest and for bodies in motion.
Bodies may or may not be in contact. A magnet repels
another magnet from a distance and also gets repelled
right back without being in contact.
Let us take the
case of a person standing in an elevator. Let this
person be standing on a weighing machine placed in
the elevator. It will be interesting to note what
reading the weighing machine will show under the
following conditions
 N =
mg
Elevator at
rest (answer: equal to the actual weight of
the person)
 
N = mg
Elevator moving
up, or down with uniform velocity (answer:
equal to the actual weight of the person)
 N > mg
Elevator moving
up with uniform acceleration (answer: greater
than the actual weight of the person)
 N < mg
Elevator moving
down with uniform acceleration (answer: less
than the actual weight of the person)
A
helicopter of mass 1000 kg rises with a vertical
acceleration of 15 m/s2. The crew and the
passengers weigh 300 kg. Give the magnitude and
direction of (a) force on the floor by the crew and
the passengers (b) force of the rotor of the
helicopter, on the surrounding air (c) force on the
helicopter due to the surrounding air. Acceleration
due to gravity = 10 m/s2.
(a) Force on
the floor by the crew and passengers = 300 (15 +10) =
7500 N
(b) Force of
the rotor on the surrounding air
=(1000 +
300)(15 + 10) = 32500N downward.
(c) From
Newton's third law of motion, the air will have an
equal and opposite reaction on the helicopter rotor
blades = 32500 N upward.
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