Paper 3-CPAC Model Answers

Question 1

Measuring spring constant of a string

Answer 1

Equipment:
spring, clamp, metre ruler, mass’
Method;
1)Measure the initial length (of the spring) Or record position of a ‘fixed point’ Or record the position of the bottom of the spring (with no masses on the spring)
2) Add mass/weight and record the new length/position
3) Repeat for a range of masses/weights

More accurate:
Reference to a precaution taken to ensure measurements were accurate
e.g. use of set square, method to reduce parallax, hang spring close to ruler, do not exceed proportional/elastic limit

Graph:
y=mx+c
F=kx+0
directly proportional with m=k

Question 2

Measuring refractive index of a substance

Answer 2

Equipment:
‡-protractor
‡-15 cm ruler
‡-laser light source
‡-pencil
‡-sheet of paper
-rectangular block of plastic (object)

Method:

1) Set up equipment as shown
2) Draw around the block
3) Mark incident and refracted light rays in three points
4) Measure angle of incident and refraction
5) Repeat 5 times

Limitations:
 blunt pencil,
 protractor divisions only to one degree,
 protractor of limited radius
 method requires rays to be marked and then drawn on limited precision