## Thursday, November 12, 2009

### Mechanics question

Hi everyone, (especially Harley).
I have had one or two questions about the exam. I'll just post the explanations here, and you can have another go.
2006 Paper, question 2 (e)
They want you to label the diagram with all the information that you know. This includes:
The acceleration - gravity = -10 m/s^2
The velocity at the top of the flight is 0 m/s
They tell us that the initial upward velocity of the ball is 19m/s
Also, because gravity acts at the same rate all the time, we can assume that the ball will reach 19m/s on it's way down as it passes the exact height of the helmet.

2 (f)
So now we have to calculate the time from the ball first hitting the helmet to hitting the ground. They want you to assume that the ball bounces up vertically; there is not horizontal motion.
We can separate the motion into three parts: The hit to the top, the top to the height of the helmet, and the height of the helmet to the ground.
Doing vi=, vf=, a=, d=, t=, for each of these three parts, and adding the times for each together, will result in the correct time.

Good luck!

Oh and also for Harley: Waves with a lower frequency are able to diffract more. This means that they can be heard over a longer range of distances, and around and over more obstacles. The actual physical reason has to do with the amount of energy that is required per meter of travel. If you think about it, a wave which has a high frequency means its moving a lot of particle per second, whereas a low frequency is moving fewer particles per second. This allows it to travel further.

#### 1 comment:

1. Interestingly, the answer for time in question 2 (f) would not change even if there was horizontal motion. This is because the vertical and horizontal components are independent.