x Turn on thread page Beta
 You are Here: Home >< Physics

# Experiment watch

1. A ramp was set up in the lab so a trolley could run down it. At the base of the ramp was a light gate to which allowed the speed of the trolley to be measured due to the time taken for the interupt card to pass through it.

Speed = (length of interupt card)/time taken for car to pass

The purpose of the expt was to seee how much of the GPE was converted into KE.

Explain what graph should be drawn and how the graph could be used to find the relationship.

Thanks
2. In the experiment are you given the height of the ramp and/or the mass of the trolley+card?
3. Btw, just realised I answered your questions on guard cells earlier today. Thought I'd just mention it in case you haven't looked - hope it helped.
4. ye you know height and mass of trolley + card
5. Umm...what is the independent variable - are you changing the height of the ramp at regular intervals and then working out the velocities by using recordings of the time taken - from your formula Speed = (length of interupt card)/time taken for car to pass??

If so, you can plot height along the x-axis and velocity along the y-axis. Then this will enable you to read values off the graph in order to calculate the KE and GPE using the formulas:

KE=1/2mv^2
GPE=mgh
6. And you can measure the velocity at the bottom?

It may be difficult to tell at once what the relation between kinetic energy and initial potential energy is. You could, however, notice that
.

Therefore I would suggest drawing versus . If it is a straight line, the problem is solved. If not, we will have to investigate it further.
7. (Original post by jaroc)
And you can measure the velocity at the bottom?

It may be difficult to tell at once what the relation between kinetic energy and initial potential energy is. You could, however, notice that
.

Therefore I would suggest drawing versus . If it is a straight line, the problem is solved. If not, we will have to investigate it further.
I'd follow jaroc's advice - the graph he has suggested will show you the relationship immediately.
8. But the question wants us to find the relationship NO MATTER WHAT
so how would i do that???

thanks
9. (Original post by jsmith6131)
But the question wants us to find the relationship NO MATTER WHAT
so how would i do that???

thanks
Plot the graph first. I cannot help you until you tell me how the graph looks like.

Experiments is working with experimental data. Without the data, it can be extremely hard to work something out based on pure theory.

If you're supposed to give the answer to the question about what graph should be plotted without actually doing the experiment nor being given at least the data, it really doesn't make sense to me. Of course we can try to predict the answers, which is very useful in planning of the experiment. For example, if resistive force is independent of velocity, you would expect kinetic energy to be proportional to initial potential energy. However, in doing such predictions, you have to assume something you can't really be sure of - for example, that resistive force is independent of velocity - and you do the experiment to verify it and draw conclusions. If we surely knew that resistance was independent of velocity, why do the experiment at all?
10. I have no more information other than what I have given you
I have to decide what graph to plot!!!
11. (Original post by jsmith6131)
I have no more information other than what I have given you
I have to decide what graph to plot!!!
All right. If you plot the graph I mentioned previously ( versus ), you will see how final kinetic energy depends on initial potential energy. Then there are two possibilities:

(1) the graph is a straight line. Then KE is proportional to GPE, and by working out the slope of the line you can find the factor of proportionality.

(2) the graph is not a straight line. Then, depending on how it looks like, it may be very difficult to say what the actual relation is. Resistive forces can be very complicated; if you wanted to proceed further on with the solution in this case, it would really be useful to know how the graph looks like.

Nevertheless, my opinion is that there is no point in deciding what graph to plot before taking measurements. You can try to predict, but it may well be that the prediction is wrong.
12. ok thanks

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: April 15, 2011
Today on TSR

### Ask the House of Commons anything

The digital outreach team is online until 4pm!

### Can I go to freshers even if I'm not at uni?

Poll

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE