Friday, May 16, 2008

Some clarification

ok, partly because I need a new entry, and partly because some of you asked what in the story of Niels Bohr reminded me of AP physics, the questions on that test are similarly ridiculous, asking questions that are utterly ridiculous or convoluted and serve no practical purpose. This is an actual question:

Here is a question involving circular motion, for your practice:

A simple pendulum of length ℓ is suspended from the point O. The bob of the pendulum is a sphere of mass m and is initially at rest at A. An identical sphere S (not shown in the figure), which is a projectile with its trajectory in the plane of the figure, has its highest point at A. The projectile therefore collides (elastically) with the bob of the pendulum at A and makes it move with horizontal speed V1 as shown. The acceleration due to gravity at the place is g.

(a) If the speed V1 of the bob of the pendulum is just sufficient to make it travel along the circular path of radius ℓ, derive an expression for the speed V2 of the bob at the highest point B of the circular path.

(b) Derive an expression for the kinetic energy of the bob in terms of m, ℓ and g just when it starts moving from A.

(c) Obtain an expression for the speed of the bob at C, when the string of the pendulum is horizontal.

(d) What was the kinetic energy of the projectile (sphere S) just before it collided with the bob? Give reason for your answer without writing theoretical steps.

(e) Briefly explain the nature of the motion of the projectile after hitting the bob.

A poignant counter-point, WHO THE HELL CARES?! Now I love Physics, really I do. I make physics jokes (Hey, what did the cow say as he slid down the inclined plane? Muuu!) but I just groan with things like that. All year I had to do questions like "I f I throw a jug of water that weighs 8 Kg at a kid who weighs 140Kg riding a skateboard moving with an initial velocity of 8m/s opposite the direction the jug is thrown, describe what happens" I laugh at the kid and run away. That should be the answer.

Off my micro-rant (thanks paul!), I am done with school and these last three weeks are a formality. Seriously, no finals, no tests, no homework. I do nothing. It's boring and nice at the same time.



guess who!

Michael said...

I think I may actually be able to do this. Does this mean I kinda learned physics!?