what r the tricks to problems in laws of motion? how to solve pulley problems without differentiation

what r the tricks to problems in laws of motion?


how to solve pulley problems without differentiation


3 Answers

vikas singh
31 Points
11 years ago

Some tricks in laws of mation-

1. Always draw FBD(making a perticular diagram of a body and showing its contact forces with thier directions) of indivisual bodies given in the system like(Blocks,Pulleys etc)

         *when you have to show the direction of Tensionic force in contact with FBD of a body then always use the concept- "Tension always acts away from body."

2. Make a right sense in your mind  that in what direction the bodies will move.

3. Now make eqns according to sense of motion of abody.

          * for example,in a system a body of mass m is moving in +ve X direction with an acceleration a(let), then ma is net force acting on that body. This net force is the resultent of all contact forces acting on the body. suppose the eqn becomes-

          F1-F2-F3=ma, this represents ,

                 F1 is parellel to ma,

                 F1> F2+F3 ,                                                    

                 forces F2 and F3 are opposing the sense of motion of the body i.e., opposite to ma as well asb F1.

Note-  F1= F2+F3, iff a=0, i.e., the body is moving with contant velocity.

Fact- for making an eqn for an FBD(free body diagram)always take those contact forces with +ve sign which are along the sense of motion and take those contact forces with -ve sign which are opposite to sense of motion.

Important points-

1. Spring balance reads the tension in the string.

2. weighing machin reads the value of Normal Reaction( the force which is always perpendicular to the contact surface of the body).

3(imp)-If string  is inextensible and all polleys are fixed, then magnitude of all masses connected through the string is same.

4.In case of movable polley , the accelerationof all masses is different. In order to get relation between their acceleration we use THE TENSION TRICK !

Tension trick is Very short way to solve these problems within few seconds instead of long approaches. It is the trick to hide from other competeters, because concept of movable polley is very important for all exams including IITJEE.


Keshev Dugar
24 Points
7 years ago
1.Choose the frame of reference according to the question2.Draw the free body diagram of the forces acting on the bodies and remember not to include force by the body.3.Write the equations.
15 Points
5 years ago
Newton's laws of motion are of central impedance in classical physics. A large number of principles and results may be derived from Newton's laws. This chapter must be kept at high importance. 
1. Make FBD. It's always mandatory in each question you attempt.
2. Keep your frame of reference clear. 
3. Mark your directions of motion in you diagram.
4. It is very important to make a clean and neat diagram..a diagram will always help you and guide you in your sum. All the best.

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