Revision as of 17:51, 1 September 2009

Outcomes

• Pupils can identify the force, pivot and perpendicular distance to the pivot for a range of machines/tools and are able to calculate the moment in Nm.
• Pupils understand how the force and distance are related to the moment and can use the formula Moment = Force x perpendicular distance to pivot.
• Pupils are to understand the practical applications of changing the force and perpendicular distance for tools.
• Pupils can find the centre of gravity for a lamina using a pin and plumbline.
• Pupils can label the centre of gravity for a range of objects and can appreciate that it may not be located on the object itself.
• Pupils investigate the principle of moments for a balanced object.

Specification References

• 1.23 recall and use the relationship between the moment of a force and its distance from the pivot: moment = force x perpendicular distance from the pivot
• 1.24 recall that the weight of a body acts through its centre of gravity
• 1.25 recall and use the principle of moments for a simple system of parallel forces acting in one plane

Starter

• Here is a Moments Powerpoint to run along with the moments activities in this lesson.
• Pair the pupils up and have one pupil hold a metal rod out in front of them. (arm fully extended) Ask the partner to hang a 400g mass on the rod near the pupils hand. Now ask the partner to move the 400g mass away from thier hand and investigate how distance to pivot affects the turning force. Once the 400g reaches the end of the rod ask the pupils to add 4 more masses making the total mass 800g. What happens to the turning force when the force is increased?
• Using a large lever vary the distance to the pivot and use a set of masses to lift a very heavy object. From a range of readings get the student to predict how both the effort and perpendicular distance to the pivot is related to the moment or turning effect. Encourage a quantitative answer. If I double the weight of the heavy object what could I do to lift it? Double the weight or double the distance.
• Help the class derive the formula for moment and emphasise the idea of perpendicular distance. A large set square would help to visualise this.
• Have them determine the unit for moment.
• Question: Is Moment a vector quantity?
• Change the arrangement by placing the 'heavy object' on the same side as your effort. Ask the pupils how you would lift the object now. Introduce direction by discussing if the 'heavy object' has a clockwise or anticlockwise moment. What direction does the moment for your effort have? It is important that the pupils understand that the effort and load could both be on the same side as the pivot.

Main Body of Lesson

• Moment circus - Have the pupils circulate round the stations finding/calculating the moment for a range of tools. The 'effort' should be labelled with laminated arrows. Set squares and rulers can be given to help find the perpendicular distances. Here is a sheet for them to record their measurements. Moment Circus Sheet
• Pupils investigate the principle of moments by varying the force and distance of masses on a balanced ruler. They can calculate and show that the clockwise moment equals the anticlockwise moment.
• Introduce the idea of centre of gravity. Use several regular shaped objects and discuss where the weight would act if 'all' the weight vectors for every part of the object had to be replaced by one weight vector. Balancing the object on your finger on different sides could easily lead to the correct answer. Lead into irregular object and have the pupils estimate where the c. of g. would be, one object, such as a boom-a-rang with a c. of g. which is not located on the object would also be beneficial.
• Have the pupils cut a lamina out of the card and use the pin and plumbob method to find its centre of gravity.
• The powerpoint ends with a few practise questions. The first one can be left on the board and tried when they finish there principle of moments experiment. The next four can be set up on the large moments apparatus at the front of the class. Ask the pupils to work out the answer on the mini whiteboards before showing them. This is a quick and easy way to check for understanding.

Plenary

• You could end by showing a few more interesting centre of gravity senarios such as the balancing forks and match on the edge of a glass etc..
• Introducing the 'Challenges' for the next lesson would help. The class should be divided into groups of increasing ability. See next lesson

Homework

• Have pupils finish the write up for the principle of moments

Additional Information

Resources Required

• Starter - Large lever and pivot system to lift a pupil or very heavy object, large masses to counterbalance, large set square, thin rope tied in a loop, large Newtonmeter to lift one end of lever.
• Set of 6 different tools with a 'force vector' labelled (made out of laminated paper with the 'effort' labelled in Newtons, rulers
• Experiment - Class set of rulers and masses for principle of moments
• A few regular objects and irregular objects to help discuss centre of gravity including a boom-a-rang.
• Class set of card, scissors, retort stand, cork and pin, sting and plasticine.
• A few interesting balancing demos eg. match stick and forks on glass

Textbook References

• None

Website References

• None

Skills Addressed

Safety/Hazards

Notes

• None