Question

a person weighing 60 kg. He stands on a weighing machine in a lift, which is moving 1) upward with uniform speed 5 m/s
2) downward with uniform acceleration of 4 m/s
3) Upward with uniform acceleration pf 4 m/s
What would be reading on scale in each case ( g = 10m/s )

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Answer 1

In case of an apple kept on a table, apple doesn't fall even if the downward force (i.e gravitational force acting on the apple) it's because of the upward force acting on the apple which is equal to the gravitational force and the name of that force is normal force.

As we know gravitational force acting on a body(F)=mass*gravity

R=F

R=m*g

Refer the above diagram

Whenever the two forces acts on a body and it is equal force then the body doesn't move as in case of apple.Another example of real life is of tug of war where when on both the sides equal forces acts then the rope doesn't move to any side.

Whenever velocity/speed remains same(that is equal distance/displacement covered by the body in equal time that means a car takes 2 metres to cover in 5 min

and in next 5 min it covers that same 2 metres and continues to be the same) then acceleration is zero{As we know acceleration= change in velocity(final velocity -initial velocity) /time hence as per the above example of a car aceleration=2-2/5+5=0}

As we move upward in a lift with increasing speed/velocity then acceleration will not be zero as velocity is changing(final velocity is greater than initial velocity)

As the lift is moving upward that means normal force(Upward force) is increasing

R=mg+ma

As we move downward in a lift with increasing speed/velocity then acceleration will not be zero but here the gravitational force(downward force) is increasing

mg=R+ma

1. As the speed is constant hence acceleration is zero but moving upward then put the formula as mentioned above R=mg+ma. R=mg (as acceleration i.e a =0) . R=60*10=600N or 600/10=60kgf
2. As the acceleration is uniform and it is moving downward hence put the formula as mentioned above mg=R+ma. 60*10=R+60*4 R=600N-240N=360N or 360/10=36kgf
3. As the acceleration is uniform and moving upward hence put the formula as mentioned above. R=mg+ma R=60*10+60*4 R=600+240 R=840N or 840/10=84kgf

Answer 2

Answer:

720N and 420N

Explanation:

this is the correct answer