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Equilibrium (CIE IGCSE Physics)
Revision Note
Equilibrium
- In physics, the term equilibrium means:
A state of balance or stability
- In other words, a system in equilibrium keeps doing what it’s doing without any change
Conditions for equilibrium
- For objects in equilibrium:
- The forces on the object must be balanced
- There must be no resultant force
- The sum of clockwise moments on the object must equal the sum of anticlockwise moments
- There must be no resultant moment
- The forces on the object must be balanced
Examples of systems in equilibrium
When the forces and moments on an object are balanced, the object will remain in equilibrium
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Demonstrating equilibrium
Extended tier only
Aim of the experiment
- This experiment aims to demonstrate that there is no resultant moment for an object in equilibrium
Variables
- Independent variable = force, , and distance,
- Dependent variable = moment,
- Control variables:
- The length of the cotton loops should be equal on each side of the beam
Equipment
Equipment list
Equipment | Purpose |
Metre ruler with a small hole at the centre | To provide the beam on which to add masses |
2 × 100 g mass hangers | To attach the masses to the ruler |
8 × 100 g masses | To add the mass at different points along the ruler |
Clamp stand, boss & clamp | To secure the pivot in place |
Optical pin and cork | To act as the pivot |
Small piece of plasticine | To ensure the ruler is balanced at the start |
2 loops of cotton | To attach the mass hangers to the metre ruler |
Example set up of equipment to demonstrate equilibrium
The ruler acts as the beam with the pin as the pivot. Unequal masses are added at different distances until the beam is balanced and equilibrium is reached
Method
- Hang unequal loads on either side of the pivot; one person holds the beam while the other person hangs the loads
- Adjust the distances of mass 1, , and mass 2, , until the beam is balanced
- Adjust further to ensure the beam is perfectly horizontal with no resultant moment
- Record the distance from the pivot of masses and
- Repeat the process for different sized loads
Example results table
A results table should contain spaces for all the measurements taken and any calculations required
Analysis of results
- Force 1, , is providing the anticlockwise moment,
- Where:
- Where:
- Force 2, , is providing the clockwise moment,
- Where:
- Where:
- Remember to convert g to kg and cm to m for the calculations to give units of Nm for the moments
- The results should show that for all the systems tested, the anticlockwise moment is equal to the clockwise moment
- Therefore, there is no resultant moment when the system is in equilibrium
Evaluating the experiment
Systematic errors
- The cotton loops should be added to the ruler when viewed straight on to avoid a parallax error
- The cotton loops should be measured to ensure they are equal in length
- The experiment should be checked to ensure there is no friction between the metre ruler and the optical pin pivot so the ruler is balanced, only because of the added masses
Random errors
- The precision of the experiment is improved by:
- ensuring the experiment is done in a space with no draft or breeze, as this could affect the motion or position of the hanging masses
- using an electronic system or a spirit level that identifies the angle of the beam would improve the experiment, or using a flat rod with masses placed on top
- The accuracy of the experiment is improved by:
- taking more than five readings for each mass and position and then calculating the mean
- It is assumed that the mass of the cotton loops is negligible (zero)
- It is assumed that the mass of each mass and hangar is 100 g, this should be verified in advance using an electronic balance
Safety considerations
- Safety goggles should be worn because the cotton loops could snap and hit someone in the eye
- Use a G clamp to secure the clamp stand to the bench so it does not topple over and cause injury
- Stand up to carry out this experiment so you do not fall over when looking level with the metre ruler
- Place a mat or a soft material below the metre ruler to cushion any masses that may fall to the ground and to keep the area clear of feet and hands
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