HELP.
A body of mass 70 kg rests on the floor of a lift. Calculate the force, R, exerted on the body by the floor when the lift:
a) accelerate at 2ms^-2
b) decelerate at 3ms^-2
c) moving with constant velocity.
Thanks in advance!

Question

HELP.
A body of mass 70 kg rests on the floor of a lift. Calculate the force, R, exerted on the body by the floor when the lift:
a) accelerate at 2ms^-2
b) decelerate at 3ms^-2
c) moving with constant velocity.
Thanks in advance!

michele_laino · Answer

@kym02 when the lift accelerates with the acceleration a, there is an apperent force acting on the body, whose modulus is equal to m*a, where m is the mass of the body. Such apparent force is directed vertically downward, and is balanced by the reaction force exerted by the floor on the same body. when the lift is moving with constant velocity, there is no any apparent force, so also there is no the reaction force exerted by the floor on the body.

michele_laino · Answer

for example in the first case the force exerted by floor on the body has a modulus of 70*2=14 Newton, please try to calculate in other cases

michele_laino · Answer

oops 70*2= 140 Newton

anonymous · Answer

oh okay the use of Newton's second law of motion. Got it! Thanks! but i don't know what to do for b) and c)

michele_laino · Answer

for b) you have to subtract the apparent force from the weight of the body, please see the drawing below:
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