# Year 11 physics help

1. Jul 21, 2004

### alzig

Hey all
im having trouble with some old physics revison. I just cant remember it.
if you can help out with some of the questions please do.
Oh try to include working out, formulas and diagrams

1.A bicyclist accelerates from 1ms-1 for 10s and covers 110. calculate the magnitude of the runners acceleration.

2.A storman pushes a 100kg crate with a force of 100N. If the firction opposing the man is 50N calculate

a)the magnitude of the net force on the crate.

b)the magnitude of the accelaration of the crate.

3.If a 5kg rock is dropped from a 50m high cliff calculate the.

a)time taken to hit the ground

b)speed of the rock on hitting the ground.

4.A billiard ball travelling at 1.5.ms-1 due east collides with the cushion of the billiard table and rebounds at the same speed but now heads due south.

a) If the collision and consequent change in direction took just 0.1s, calculate the balls acceleration.

b)Calculate the size of the force that the ball exerted on the cushion.

2. Jul 21, 2004

### jamesrc

Try posting in the homework forum to get more hits and present the work you have done to show what you know and what you're having trouble understanding. I'll try to get you started with some of these:

This is a constant acceleration problem. The kinematic equation that will get you to the answer the quickest is:

$$(x-x_o) = v_ot + \frac{1}{2}at^2$$

You should be familiar with that equation so I won't explain the terms unless you ask about them. All you have to do is plug in the appropriate given information and solve for a.

Draw a free body diagram for this one. Say the guy is pushing the block to the right; this means the friction force is pointing to the left. The net force will be the difference between the forces (100N-50N pointing to the right (remember force is a vector so you need to give a direction))

Use Newton's 2nd law to find the acceleration from the net force (Fnet = ma).

In this one, the mass of the rock is irrelevant since we're just considering the acceleration due to gravity. Use the same equation as the first problem to find the time taken. Remember that the initial velocity is equal to 0 here and the acceleration is the acceleration due to gravity. Also be careful to have a consistent sign convention.

The most straightforward way to find the second part is to use the time you just calculated in the formula:

$$v = v_o + at$$

To find these answers, use the definition of impulse which is equal to the change in momentum:

$$\vec I = \vec{F}_{avg}t = \vec p - \vec p_o$$

3. Jul 21, 2004

### HallsofIvy

Staff Emeritus
By the way, "rule 1" for studying is: read careful and copy carefully!

The problem "1.A bicyclist accelerates from 1ms-1 for 10s and covers 110. calculate the magnitude of the runners acceleration." makes no sense at all because you are given no information about a "runner"!

4. Jul 21, 2004

### alzig

Come on Halls
i was referring to the bicyclist. A simple mistake that was present on my sheet. Jamesrc managed to figure it out.
If your not going to post helpful information dont post at all.

btw jamesrc thanks for your help

5. Jul 23, 2004

### futb0l

He's just trying to tell you to do your work more carefully - that's all.

6. Jul 23, 2004

### HallsofIvy

Staff Emeritus
But, of course, that's not "helpful".

7. Jul 23, 2004

### KnowledgeIsPower

All of the experts here are a fantastic help. Let them vent their frustration a little occasionally ^^.

8. Jul 26, 2004

### dave_gilmour

I'm doing a similar question in my physics class. How would you explain why this situation demonstrates acceleration, even though there has been no change in speed?

9. Jul 26, 2004

### Staff: Mentor

Acceleration means a change in velocity (a vector), not necessarily a change in speed.