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By: - 1st May 2006 at 00:07 Permalink - Edited 1st January 1970 at 01:00
Just to add, that any help would really really be appreciated! I have been pondering this for about an hour. I only have about 2 more hours tomorrow to get this whole coursework complete before it's due in on Tuesday.
By: - 1st May 2006 at 00:15 Permalink - Edited 1st January 1970 at 01:00
Does it state the track is frictionless? That may be the purpose of the air track. Otherwise frictional forces might be playing a part in the difference?
I remember doing something similiar a year or two ago, or I probably did it twice in two years as I failed the course first time round, ;) unfortunately I can't remember some of the possible causes. ;)
By: - 1st May 2006 at 00:19 Permalink - Edited 1st January 1970 at 01:00
Does it state the track is frictionless? That may be the purpose of the air track. Otherwise frictional forces might be playing a part in the difference?
I remember doing something similiar a year or two ago, or I probably did it twice in two years as I failed the course first time round, ;) unfortunately I can't remember some of the possible causes. ;)
I have already mentioned frictional forces as one of the possible causes. But I think if that was the case, you'd acually expect the Kinetic Energy to be less than the GPE, as energy would be lost in heat etc.
The fact that the KE is greater than the loss in GPE, makes me think that the air track would be leaning so that the truck is travelling slightly down hill. But I need some proper explanation, and maybe another way of proving that this is the case....
By: - 1st May 2006 at 00:59 Permalink - Edited 1st January 1970 at 01:00
I have already mentioned frictional forces as one of the possible causes. But I think if that was the case, you'd acually expect the Kinetic Energy to be less than the GPE, as energy would be lost in heat etc.The fact that the KE is greater than the loss in GPE, makes me think that the air track would be leaning so that the truck is travelling slightly down hill. But I need some proper explanation, and maybe another way of proving that this is the case....
Hmm yes, annoying, isn't it? ;)
By: - 1st May 2006 at 11:27 Permalink - Edited 1st January 1970 at 01:00
The truck is on wheels, and there is still friction between the track and the truck. The air track is meant to eliminate the friction as much as possible, to a very small amount. I don't have specific values for the friction, no.
The truck is being accelerated by the falling mass. The loss in GPE of the falling mass can be calculated using:
the mass of the falling mass x g (9.81N/kg) x the distance beween the velocity measurement points.
The KE is calculated by:
0.5 x the mass of the truck x (the final velocity)^2.
The KE is always greater in my calculations, although in an ideal situation you'd expect the KE to be equal to the loss in GPE.
By: - 1st May 2006 at 12:15 Permalink - Edited 1st January 1970 at 01:00
A few things about your method are not ideal. It is probably too late to change this now, but at least you can make amends in your evaluation.I'm not sure why you are using a wheeled truck and an air track. You usually use a "v-shaped" unwheeled metal "vehicle" which is then pretty frictionless. Or use a wheeled truck on a long(wooden or metal) runway which you friction-compensate by tilting the runway until the vehicle when given a gentle momentary push then moves down the runway at a steady speed(Newton's first law).
The way you are measuring the KE gain and GPE lost is not ideal as you are only measuring the average KE gain for a given GPE lost. This should give you a lower figure for the KE rather than the higher one quoted.
It may have been better to have measured the GPE lost from the fallen height to striking the floor(75cm say) and then to have measured the speed gained over a distance of 10-20 cm electronically AFTER the falling mass has hit the floor.( electronic timing should mean that you can measure over a shorter distance and still retain accuracy) The truck is then moving at a steady speed(no forces on it) and will be the final KE reached.
Your KE gained should ALWAYS be less than the GPE lost in such experiments due to energy losses -- air resistance with moving objects, friction in the pulley and possibly some friction due to poor friction-compensation being the most obviouse areas.
I hope this long answer is of some use to you.
Phil
Posts: 3,539
By: wannabe pilot - 1st May 2006 at 00:02
If so, I'd greatly appreciate your help.
I am doing my first year A-level Physics, and have come to my final piece of coursework before we start our exams in a few weeks.
Basically, the coursework is about an investigation where a truck is pulled by a falling mass (like in a pulley) as the truck travels along an air track (a track thing where air is blown up to reduce friction). The velocity of the truck is measured at 2 points along the track, and the time between these points is also measured (done precisely by a computer programme). The mass of the truck is constant, and the falling mass is increased.
I worked out the Kinetic Energy of the truck at it's different velocities, and also the loss in Gravitational Potential Energy of the falling mass at different masses. I was using the change in height to calculate the GPE as the distance between the points where the velocity of the truck is being measured. In this case, in an ideal World, you would expect the loss in GPE to be the same as the KE?
However, the KE is greater than the loss in GPE in all the measurements. This is obviously due to some extraneous variables. I am trying to discuss what these are, but need a little bit of help. I'm trying to argue that the air track may have been un-level (not exactly horizontal). I think this is a correct assumption, but I need a bit moe of an explanation to back it up. Could there be any other causes of this result?