IIT JEE , AIEEE Forum - Physics , Mechanics

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lets discuss frames

once someone asked this ques.

a block of mass m is kept on the surface of a rough car moving with acceleration "a"
in +icap direction.
what all forces are acting on 'm' whan it is viewed in ground frame as well as car frame

in ground (inertial) frame....(no pseudo force)
the body is pushed back due to motion of car.
Forces are:-
1) mg ( -j^)
2)reaction(N) by floor (+j^)
and N = mg

3)frictional force(f) = umg(+i^)
So the frictional force is supposed to provide the necessary force ma(+i^) on the body which makes it move as seen from ground
i,e, f = ma

in car (non inertial frame)
the block is still pushed back and remain at rest for an observer in the car at the pushed position
1) mg ( -j^)
2)reaction(N) by floor (+j^)
and N = mg

3)frictional force(f) = umg(+i^)
4)pseudo force = ma(-i^)
Since the body is at rest as seen from the car,
so f = ma

In both cases , f = ma.
But from ground .... its a single force which actually is f but its magnitude is equal to ma
and from car.... they are two different forces in opposite direction making the body stay at rest.

figure

same concept is applied for a ball hung from the ceiling of a car...

in ground (inertial) frame....(no pseudo force)
the ball is pushed back due to motion of car.
Forces are:-
1) mg ( -j^)
2)tension component Tsin

(+i^) and tension component Tcos

(+j^)
Now the tension component Tcos

balances the weight of the body

Tcos

= mg

So,tension component Tsin

provides the necessary force whic makes the body move forward with accl^n a (as seen from ground)

i,e, Tsin

= ma

in car (non inertial frame)
the block is still pushed back and remain at rest for an observer in the car at the pushed position
1) mg ( -j^)
2)tension component Tsin

(+i^) and tension component Tcos

(+j^)
3)pseudo force = ma(-i^)
Since the ball is at rest as seen from the car,
so Tcos

= mg

and Tsin

= ma

In both cases , Tsin = ma
But from ground .... its a single force which actually is Tsin but its magnitude is equal to ma
and from car.... they are two different forces in opposite direction making the body stay at rest.

This proves ........ pseudo forces come only in non-inertial frames.

We may consider pseudo forces in inertial frame (here, ground frame) and arrive at the same result....... BUT THE CONCEPT REMAINS WRONG

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