Magnet and Pivot Point

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endorsed by University of Cambridge International Examinations

Physics
for Cambridge IGCSE

Complete

seCond edition

Teacher’s Resource Kit

Ian Collins

2

PRACTICAL

Name:

2.2 Acceleration due to gravity using a ticker tape timer
Aim: To determine a value for the acceleration due to gravity, g. Equipment: n ticker tape timer n ticker tape n pulley mass n string n adhesive tape n

30 cm rule n scissors n glue
n

Introduction
As an object falls freely it will accelerate towards the centre of the Earth. This is due to the gravitational attraction between object and the Earth. The magnitude of this acceleration can be determined by analysing the motion of the falling mass with a ticker tape timer.

3 Record the motion of the trolley using the ticker tape timer.

Results and analysis
1 Cut, label and glue the tentick lengths side by side on the velocity–time graph on the results sheet attached. 2 Deduce the slope (gradient) of the plot.

Theory
1 The time interval between 10 dots is called a tentick length. 2 A tentick length corresponds to 0.2 seconds.

Conclusions
1 Determine a value for the acceleration due to gravity and compare this with the accepted value of 9.81 ms–2. 2 Is you value an overestimate or an underestimate? What might have caused this difference?

Method
1 Set up the apparatus as shown.
mass trolley frictionless slope

frictionless pulley

slotted masses

2 Allow the falling mass to pull the tape through a ticker tape timer via the string over the pulley.

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© OUP: this may be reproduced for class use solely for the purchaser’s institute

PRACTICAL

Name:

3.1 The principle of moments
Aim: To understand and apply the principle of moments using a lever. Equipment: n stand, boss and short metal rod to act as the fulcrum n 30 cm rigid metal strip with holes drilled and labelled every centimeter along its length to act as a balance bar n

6 × 20 g slotted or hanging masses

Introduction
The turning effect of a lever not only depends on the magnitude of the force but also the length of the lever arm. In other words, the length of the lever arm is the distance between the pivot point (fulcrum) and the point of application of the force.

The moment of a force about a pivot point is the force multiplied by the perpendicular distance from the pivot point. For two forces, F1 and F2, both acting downwards at perpendicular distances d1 and d2, either side of the fulcrum: F1 × d1 = F2 × d2 A lever is a simple machine that requires a rigid beam and a pivot point. It is used usually used to amplify a force to lift a load. The mechanical advantage, or amplification factor, is defined as the ratio between the load and the effort. load mechanical advantage = _____ effort

Method Theory
The turning effect of a force is called its moment. When an object is in equilibrium the sum of the clockwise moments is equal to the sum of the anticlockwise moments at that point. Experiment number 1 2 3 4 Load (masses to the left of fulcrum) 20 g at 10 cm 2 × 20 g at 5 cm 3 × 20 g at 4 cm and 1 × 20 g at 12 cm 3 × 20 g at 4 cm and 1 × 20 g at 12 cm Set up the balance bar with the fulcrum at its centre. The balance bar should be free to rotate about the pivot point.

Effort (masses to the right of fulcrum) 20 g at 10 cm 1 × 20 g at 10 cm 2 × 20 g at 12 cm

Is the lever in equilibrium?

Mechanical advantage of the lever

yes

If the balance bar is not horizontal (balanced) a small mass may attached to the back of the rule with adhesive tape to balance it. Add masses to the balance bar according to the following table and complete the empty boxes.

Question
Using the principle of moments, calculate the position of a 20g mass to counter-balance four masses: 1 × 50 g at 6 cm, 1 × 20 g at 8 cm, 1 × 10 g at 10 cm and 1 × 5 g at 12 cm. Show your working.

© OUP: this may be reproduced for class use solely for the purchaser’s institute

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