IIT JEE , AIEEE Forum - Physics , Mechanics - physics doubt [Admin]: Internal Forces query

anubha

do internal forces have any effect on the motion of the body?

if yes,

why doesn't a car move when pushed from inside?

if no,

why does a tennis ball(in which an coackroach is locked) moves when the coackroach inside it tries to escape?

shan

it is NO.

The scenario you mentioned moves not due to internal force, but due to weight (mg force) due to change in the centre of mass of the tennis ball-insect system. gravity in this case is NOT an INTERNAL FORCE.

cvramana

Internal forces does not effect the motion of the body. When we mean motion of a body, we mean motion of its center of mass. And motion of center of mass occurs only when there is an external force to the system. Therefore in the case of tennis ball and coackroach there is no external force acting and the center of mass of the system does not displace, although there is relative motion between the ball and the coackroach.

Just like the Universe, there is motion of different bodies relative to each other but there is no motion of the Universe itself. If at all, then to where?

rosey

when we consider three cases of AT WOOD's machine,in the first case one end of the string have mass "M"and other end have a mass of"2M". in the second case one end have mass "M" and other end of the string is pulled by "2Mg"force.in the third case one end of the string have mass "M"and other end have mass"M"and this mass is pulled by a force of Mg.In all these cases what is the difference? why there is difference in accelaration,though other end of the string has 2Mg force in different forms.

anubha

why is motion of the insect considered to be an external force?

cvramana

First case:

2Mg - T = 2Ma ---------------(1)

T - Mg = Ma ----------------(2)

Solving we get a = g / 3 and T = 4 Mg / 3 ---------------(3)

Second Case:

In this case we are pulling the other end with a force 2Mg, which is nothing but the tension in the string.

Therefore

T = 2Mg ---------------(4)

T - Mg = Ma --------------(5)

Solving we get a = g and T = 2 Mg ----------------(6)

Third Case:

In this case we are pulling the mass M with a force Mg. Therefore the equations of motion are,

Mg + Mg - T = Ma ----------------(7)

T - Mg = Ma ------------------(8)

Solving we get a= g / 2 and T = 3Mg / 2 --------------------(9)

The motion of the insect is possible only when there is friction. This frictional force is considered as external force for the insect.

vb_rdb

As the man inside pushes the car by his hand at the same time he applies a backward force on the car by his legs .

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