Physics Coursework: Trolley experiment Essay Sample

Physics Coursework: Trolley experiment Pages
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Aim:

The aim of this experiment is to find out whether or not the mass of a trolley effects its acceleration when traveling down a ramp kept at a fixed height.

Planning

Set up the experiment as shown above. Run the cart down the ramp once to make sure the cart runs in a straight line. Start the cart with its front wheels behind the starting line each time. The run finishes when the back wheels cross the finishing line. Three runs will be taken for each weight and three runs will be taken without a weight. We decided to take seven different observations, increasing the weight by 0.8kg each time

Apparatus needed: 0.8kg weights (8)to add on to the cart)

Ramp (1.5m in length)

Cart (same cart used all the time)

Books (to hold up the ramp at a fixed height)

Stop watch (for timing the experiment)

Sand bags (to stop the car at the end of each run)

Safety: To keep this experiment safe, different precautions have to be taken.

* Sand bags will be used to prevent the cart falling of the table

* The ramp will be kept at a sensible height (17.5cm)

* The ramp will be kept on a flat surface.

* To prevent the ramp slipping, the books will be securely fastened.

The ramp will be kept at a fixed height for the whole experiment as if the height is changed, gravity will act differently on the car changing the results for each run. The only two forces that should act on the cart are air resistance as the surface area for the front of the cart changes, and friction, because the car gets heavier which slows the cart down.

Each weight will weigh the same amount, so the weight of the cart changes in equal amounts. The amount of slabs on top of the cart will affect the speed, as the air resistance will be greater.

The cart will always start with its front wheels behind the finish line, and will finish with its back wheels have crossed the finishing line, which is 1.5m away from the starting line.

Our group picked this method as we thought it would be a good method because there would be interesting results. With this experiment the results would not be predicable, as weight is not the only force that acts against the speed of the cart. The forces that act on the acceleration of the car have to be considered.

If an object moves in a straight-line acceleration is how much it speeds up by each second. The movement of an object depends on the relative size of the forces acting on it. To speed up or slow down requires an unbalanced force. More then 300 years ago Isaac Newton carried out an experiments which enabled him to describe how acceleration depends on the size of the unbalanced force and the mass it acts on. Here was his theory:

“The rate of change, of momentum, of an object is directly proportional to the force acting, and takes place in the direction in which the force acts.”

This means that the way the cart accelerates depends on the forces acting on it. And whether or not they are balanced.

I predict hat the acceleration of the cart will be constant, as the forces are always unbalanced, but because of the different mass for each observation, the forces will always be different.

Obtaining evidence

The results shown below are the results for the 7 observations for the cart

Weight of cart (kg)

Time for 1st run (sec)

Time for 2nd run (sec)

Time for 3rd run (sec)

Average time (sec)

0kg added

2.52

2.53

2.53

2.52

0.8kgs added

2.44

2.44

2.44

2.44

1.6kgs added

2.4

2.42

2.4

2.40

2.4kgs added

2.4

2.44

2.41

2.41

3.2kgs added

2.41

2.46

2.45

2.44

4kgs added

2.47

2.49

2.47

2.47

4.8kgs added

2.53

2.51

2.53

2.52

Each run was 1.5m in distance; the same cart was used and started from the same place, and finished in the same place.

I worked out the average by adding the 1st, 2nd and 3rd run times, and then dividing the answer by 3.

Here is a table to show the average speed for each weight added onto the cart:

Weight on top of cart (kg)

Average speed (m/s)

0

0.59

0.8

0.61

1.6

0.62

2.4

0.62

3.2

0.61

4

0.60

4.8

0.59

Calculations

Here are the calculations I did to find out the average speed for each weight:

1) For 0kg: 1.5m/2.52s = 0.59m/s.

2) For 0.8kgs: 1.5m/2.44s = 0.61m/s.

3) For 1.6kgs: 1.5m/2.40s = 0.62m/s.

4) For 2.4kgs: 1.5m/2.41s = 0.62m/s.

5) For 3.2kgs: 1.5m/2.44s = 0.61m/s.

6) For 4kgs: 1.5m/2.47s = 0.60m/s.

7) For 4.8kgs: 1.5m/2.52s = 0.59m/s.

Here is the formula I used to work out the average speed:

Distance/average time taken for each weight = average speed

Please turn to the last page to see the speed vs. mass graph. The graph shows that the speed stays constant no matter what the weight of the cart.

Conclusion

Newton’s 2nd law of motion applies to this situation. The mass did change but the unbalanced forces kept the cart at a constant speed. This is because every time an extra weight was added, the surface area for the front of the cart increased, causing the air resistance to be greater. Also as the cart got heavier the friction acting on the cart increased. The two forces acting together managed to slow the cart down more, every time an extra weight was added

Here is a diagram to back-up my theory:

It is important to realise that if a force on an object there will be no acceleration. For there to be acceleration there must be a resultant force or the forces must be unbalanced. By the cart accelerating down a slight slope to counterbalance the effects of resistive forces. As the graph shows that the speed does stay constant, so mass can’t affect the speed of the cart in a big way, only because other forces act on the cart preventing it from accelerating, so that a definite difference can be seen.

Evaluation

The results obtained enables us to know that they match the prediction and that mass doesn’t have a noticeable impact on the speed of a car, but the observations only back up this theory to a certain extent. If the ramp were raised higher then the results would have been faster as there would be less friction acting on the cart. If the ramp were longer, the cart would have taken more time to complete one run.

The observations did back up my prediction, and did back-up my prediction to a certain extent, but at

The method we used gave accurate results. Even though the results were accurate, different factors could have still affected the speed, like whether or not the cart ran down the ramp in a straight line, or if the cart hit the sides

If I had to do the experiment again I would use wider ramp so that there is chance of the cart hitting the sides of the ramp. The factor that would have probably had the biggest effect on the speed and time would have to be the stopwatch. It was extremely hard to stop the watch at precisely the right time, as it was hard to see when the back wheels crossed the finishing line. That would mean that the experiment is not 100%accurate, but that was one of the things our group found hard to control.

To go on a step further in the experiment, I propose that we do exactly the same experiment, but also do 7 observations with the ramp at a different height, to see if the force that does slow the cart down can be determined.

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