Physics Investigation Essay Sample

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I am trying to find out about:

The factors affecting the deflection of a cantilever when weights are added onto the end.

What I think will happen:

I think that as the weights are added to the cantilever, the deflection will increase.

I think this will happen because:

I think the deflection will increase because gravity will push down on the cantilever and weights while the atoms in the cantilever will be resisting this force: –


This means that the moment of the weights is the distance from the bench (or pivot) multiplied by the number of Newtons attached to the hook. As I know that the moment increases as more weight is added, I can use this to back-up my prediction by stating that more weight (or moment) causes more deflection e.g. I predict that weight is directly proportional to deflection. This could also be likened to a bimetallic strip, which will bend more as it is increasingly heated or cooled.

The atoms in a solid are rather like a spring in that if they are stretched, they will try to return to their original position when released (unless stretched beyond their elastic limit). An example is a cube (see next page):

When one atom is pulled, any atoms connected to this atom will try to pull each other back into their original shape.

The atoms in the cantilever are being stretched so that the beam deflects downwards. The top of the cantilever gets stretched (called tension) and the bottom of the cantilever gets squashed (called compression). A cantilever that has a high cross-section will bend less than one with a low cross-section. This means that as more weight is added to the cantilever, the more tension and compression there will be. I think the atoms at the middle of the cantilever will look like this at first –

To this when weight is added ~~~~~~~~~~~~–>


To do this experiment, I will need:

1. A wooden meter rule

2. A G-Clamp + small block of wood

3. A Hook

4. 7, 1 Newton weights

5. A small coil of string

First fit this equipment up like so:

The string is used to attach the hook to the ruler. String is useful for this, as it is thin and can be placed at a measured position with fairly good accuracy.

The block of wood goes in-between the G-Clamp and the ruler to stop the pressure damaging the wood. If I were to set-up the ruler without the G-Clamp I could not call it a cantilever, as a cantilever is defined as a projecting structure, fixed in position and direction at one end, and free at the other.

In my preliminary experiment I found that there was no way to hook the weights onto the ruler and that using too many weight would snap or split the ruler.

I used this information in my “primary” experiment by using string a certain distance from the bench and also adding no more than 7 Newtons onto my ruler. I have also decided to measure the deflection from the top corner of the ruler, as the bottom corner is 1/2cm away from 0 deflection.

I have decided to keep it fair and accurate by changing only one variable. This is mainly due to the time constraint on the practical. I will always use the same ruler, weights and hook.

I will take at least 2 readings and use the average in my results. After each reading I will verify it to see if it is near my prediction. If not, I will test it again to check if it was correct.


As my variable is length, for this experiment I will position my hook at the end to the meter rule:

I am leaving 20mm at the end of the ruler so that I can avoid the wear on the ruler that may cause the string to slip.


Instead of attaching my hook to the end, this time I will position my string 450mm from the bench. Half of what it was before:


Putting my hook at 225mm would not be a good position as the small results would be harder and less accurate to measure. To stop this I will put the hook at 675mm. This is halfway between my previous positions.


My results show that

The results show that the bigger the surface area, the greater the amount of oxygen produced in the reaction.

As the surface area increased, so to did the oxygen readings e.g. surface area = 4, average = 1.3, surface area 12 (4+8), average = 2.5.

The 3rd and 4th results increase the surface area less that the 1st to 2nd and so do the averages of them.

Also, the oxygen readings fluctuated so taking three recordings and averaging them out was a beneficial idea.

My prediction was correct.


The test could have been more reliable if the concentration of acid was kept the same each lesson as this caused higher or lower results depending on which concentration was used.

The pieces of potato were overlapping on top of each other in the test tube, which detracted from the accuracy of the result as the experiment was trying to make a correlation between surface area and oxygen produced. A conical flask with a thin mesh just above the bottom would have helped keep as much area exposed as possible.

The odd/fluctuating results were mostly due to the acid concentration problem as the overlapping happened every time.

I am more certain that my conclusions are correct because I have checked and recorded my results and found no unexplainable anomalies.

I could do more research into enzymes and how fast they convert their substrate into the components to enable me to work out the experiment using maths and practical experience.

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