## Pages

`“Life is like riding a bicycle.  To keep your balance you must keep moving.”–Albert Einstein`

## Monday, February 16, 2009

### A Free Response Practice Question Involving Newtonian Mechanics and Electromagnetism for AP Physics B

I give below a free response practice question involving fundamental principles in Newtonian mechanics and electromagnetism. Even though this question is meant for AP Physics B aspirants, I would advise AP Physics C aspirants also to try it. Here is the question:

A cart carries a block of mass m1 which is connected to a soft iron cylinder of mass m2 using an inxtensible string of negligible mass passing over a light frictionless pulley as shown in the adjoining figure. The soft iron cylinder is suspended vertically with its lower end inside a current carrying coil fixed to the cart. The cylinder and the coil are coaxial and their vertical sides touch each other without any friction between them. The current carrying coil exerts a magnetic force of constant magnitude f2 on the soft iron cylinder and the entire system consisting of the cart of mass M (with the current carrying coil), and the connected masses m1 and m2 are moving with a constant acceleration ‘a’ in the positive x-direction under the action of a constant force F (fig.) in the same direction. There is no friction between the cart and the block m1 and the block and the cylinder are at rest relative to the cart during the accelerated motion of the cart. Assume that g = 10 ms–2.

(a) Write an expression for the acceleration ‘a’ of the cart in terms of the net force on the cart and the masses involved

(b) If m1 = 11 kg, m2 = 1 kg and a = g/10 calculate the magnitude of the magnetic force f2 on the cylinder.

(c) If the current in the coil were in the opposite direction, will the direction of the magnetic force change? Justify your answer.