pl. explain the situation when T-mg=ma and T=ma. where T=Tension in the string passess over a pulley.

When weight of a body plays a role in it’s motion, then you use T – mg = ma

 

Suppose a body is hanging from a string and is moving upwards with accelaration a, then net force = ma whihc will be upward force T minus downward force mg

 

and T= ma is done when weight has no role in motion. example, a body is on a horizontal floor and is being pulled by a string having tension T and the accelaration of the body is a,

 

since weight acts vertically downwards it plays no role in horizontal motion. so the equation will be T = ma.

 

Please feel free to ask any follow up question and approve the answer if you like it :) (option given just below the answer) 

 

:)

A block of mass 4kg is placed on a horizontal frictionless surface connected by a string which passes over a frictionless pulley and its other end is attached to a mass of 6kg. and a different situation where a block of mass 4kg and mass of 6kg is attached with the two ends of the string which passess over a pulley fixed with the roof. please explain the factors involving.

 

Yes that is a very good question i will be able to explain it very clearly now. :)

 

In the first case, let’s assume block A is on horizontal surface and B is hanging veritcally with the help of a string which passes over a pulley.

since  B is 6kg and A is 4kg, B will moves DOWNWARDS and A will move horizontally towards B.

For B, the force acrting DOWNWARDS is mg, the force acting UPWARDS is tension T and the net accelaration is downwards = a,

since net force is acting downwards, the equation should be downward force minus upward force = net force which is downwards.

therefore, mg – T = ma 

 

for A , the weight mg is acting downwards but it’s motion is in horizontal direction so downward force plays no role in this motion so you cannot include mg in the equation. Now the net force is ma towards the pulley and the tension T is also acting towards pulley (in the string attached to the body) so the equation becomes T = ma

 

For the second case when both are hanging from the pulley, 4kg goes up and 6kg goes down.

For 6kg, the equation remains same since net force is downwards so the equation is mg – T = ma .

 

for 4kg which is now moving upwards, the net force is upward so the equation should be upward force ( T ) minus downward force (mg) = net force (ma)

= T – mg = ma

 

Please feel free to ask more questions :) And if this is clear then please approve the answers which i provided :)