I hung a spring from a clamp stand. I put ruler from the bottom of the loop on the spring across to a metre stick behind. I did this because I needed to be sure that although the spring wasn't touching the metre stick, I had the spring and the metre stick lined up properly. If you don't do this and end up looking across at the wrong angle, you'd get a different reading. Looking at a scale from the wrong angle is called PARALLAX ERROR.
I made sure that when there was nothing on the spring, the bottom of the loop lined up with 0cm. It means that this week we can read extension directly. We don't need to subtract the original length. I added 100ml of water at a time from a measuring jug in the kitchen. I was adding 100 grams of water every time - that is 1 Newton of weight.
The next picture shows 100 grams of water - that is 1 Newton of weight. You need to make a table for the following pictures where you have a column for Weight of water and a column for Extension. You must include units in your column headings. Read the extension each time from the photograph. Enlarging the photograph will help. The first extension is 3.0cm because you are reading to the top of the ruler.
Here is the reading for 200 grams of water which is 2N weight of water. The picture is a bit blurred but the extension is 6.1cm.
Next picture is 300 grams of water. You need to come up with the correct weight in Newtons and read the extension from the photograph.
Below is 400 grams of water.
Below is for 500 grams of water.
Before we go, here is a quick pause. After 500 grams I took the bottle off and found that the spring went back to 0 cm extension. This will be important later.
Then it was 600 grams of water.
700 grams of water.
800 grams of water.
900 grams of water
1000 grams of water
1100 grams of water
1200 grams of water
1300 grams of water
1400 grams of water
After 1400 grams, i took the water off and found that the spring did not go back to 0 cm. We had added too much weight and we had permanently stretched the spring.
Here's my graph. Notice that it is a straight line through the origin for a long time. We say that the
extension is directly proportional to the force. This is called
Hooke's Law. The the line starts to curve. The point at which the line starts to curve is called the LIMIT OF PROPORTIONALITY. If you remember, for 5N I took the weight off and the spring went back to its normal original length. We'd say that it is still elastic. In the end, at 14N, when I took the spring off it was permanently stretched by 2cm. That's what happens if you go beyond the limit of proportionality.
The points are not all perfectly on the line. They are close to the line but not perfect so we say that there is random error. Here is a picture of my measuring jug to help you to work out why.
