Explore the motion of a weightlifter as they train at the gym. How can mechanical advantage help them to lift more weight?
- Review the online pupil material. Use a projector to share this material with your pupils during the lesson. Consider pre-building the Weightlifter model to use during the lesson to help pupils who are having trouble building.
- Make sure that you have covered the relevant concepts (i.e. gravity, gearing up and gearing down) in an earlier lesson.
- Consider the abilities and backgrounds of all your pupils. Differentiate the lesson to make it accessible to everyone. See the Differentiation section below for suggestions.
(Whole Class, 5 Minutes)
- Watch the pupil video here or access it via the online pupil material.
- Facilitate a quick discussion about which forces the pupils have seen in the different types of weightlifting in the video.
- Ask questions like these:
- Which forces help a weightlifter lift the weights? (Muscles pull to move our bodies, and they push or pull to lift and lower weights.)
- Which force makes the weights come back down? (Gravity)
- What is a pulley? What is a block and tackle? How can these help a weightlifter train? (Pulleys are wheels that a rope or a cable rolls over. A block and tackle is a system of 2 or more pulleys with a rope or cable threaded between them that can achieve a mechanical advantage in pulling a load.)
- Tell the pupils that they are going to build a model of a weightlifter and then experiment to explore balanced and unbalanced forces.
(Small Groups, 25 Minutes)
- Have the pupils work in pairs to build the Weightlifter model. Tell them to take turns, one partner searching for the bricks while the other builds, switching roles after each step has been done.
- Building steps 21-29 can be challenging for the pupils. In these steps, they will start to incorporate more LEGO® Technic elements into their building and demonstrate their observational skills in order to route the string correctly. Use the reference model that you have prebuilt to help the pupils to self-identify any building errors.
- You can find building help in the Tips section below.
- Have the pupils test the model using pulley position 1 from the building instructions to raise the barbell.
- Tell them to record their observations on their Student Worksheets (Teacher Support - Additional resources). (This model has no mechanical advantage because it does not generate an increase or reduction of required effort or speed. It only changes the direction of the motion.)
- The model is not ‘strong enough’ in this configuration to lift the barbell with all 4 weights (wheels) on. It can only lift fewer wheels.
- Ask the pupils to remove one wheel at a time from the barbell, repeat the experiment with fewer wheels, and record what happens.
(Whole Class, 10 Minutes)
- Gather your pupils together to share what they have learned.
- Ask: Was the force acting on the weightlifter balanced or unbalanced? (The force was balanced because the model didn't move until it was pushed/pulled up and down by hand.)
- Show the pupils how to change the model to pulley position 2.
- Have them test the model in this position and record their observations on their Student Worksheets.
(Whole Class, 5 Minutes)
- Gather your pupils together to review and discuss their experiments.
- Ask questions like these:
- What did you notice about the motion of the weightlifter when you changed the string position? (It was easier to move the weight, it moved by itself, and it didn’t move as high.)
- What makes the weightlifter raise the weights? (The weight [ i.e. the force of gravity] pulls down on the weighted bricks and the pulleys convert that downward motion into an upward pull.)
- Why does pulley position 2 have a different result from pulley position 1? (Position 2 gives the model a mechanical advantage of 2 to 1. This means that the model halves the effort that is needed to lift the load but also halves the height to which the load is lifted. You would need to pull twice the length of string to lift the load to the same height as pulley position 1.)
- Allow time for the pupils to disassemble their models, sort the bricks back into the trays and tidy up their workstations.
(Ongoing Throughout the Lesson)
- Ask guiding questions to encourage them to ‘think aloud’ and explain their thought processes and reasoning in the problem-solving decisions they have made when building their models.
- Measure your pupils’ proficiency in describing the balanced and unbalanced forces that are at work in the model.
- Establish a scale that suits your needs. For example:
- Requires additional support
- Can work independently
- Can teach others
- Have each pupil choose the brick that they feel best represents their performance.
- Green: I think I can describe balanced and unbalanced forces.
- Blue: I know I can describe balanced and unbalanced forces.
- Purple: I can describe balanced and unbalanced forces, and I can help a friend to understand, too.
- In their teams, have the pupils discuss their experiences working together.
- Encourage them to use statements like these:
- I liked it when you…
- I'd like to hear more about how you…
- The pulleys can be set up like this:
- Position 1
- Position 2
Simplify this lesson by:
- Having your pupils try the model without the ‘weights’ by pushing down on the yellow axle with one hand and resting the other on top of the weighted bricks
- Ask them to describe what they see and feel as they push down on the yellow axle
Increase the difficulty by:
- Having the pupils use bricks or the other wheels in the set to add extra weight to both the barbell and rear weights, and then challenge one another to rebalance the model
- Asking the pupils to use the bricks in the set to add character to their models
(Note: This will require additional time.)
To incorporate the development of maths skills, set up the Weightlifter model using pulley position 1 with the weight module lifted as high as it can go. The model will stay balanced. Have the pupils calculate how many wheels they would need to remove to allow the weightlifter to rise and the weight to fall (i.e. unbalanced force).
These are the weights for each part of the Weightlifter model:
- Weight module 121 g
- Weightlifter 40 g
- Wheels 23 g (each)
The pulleys' alignment and the force of friction on the pulleys will have a slight effect on the weights, so the weight of each side will vary.
National Curriculum Maths: Ma3/3.1 Measurement
The pupils will:
- Experiment and measure the effects of balanced and unbalanced forces on an object
- Explore the mechanical advantage of a block and tackle pulley system
- LEGO® Education BricQ Motion Essential Sets (one for every two pupils)
Understand and use mechanical systems in their products
Asking relevant questions and using different types of scientific enquiries to answer them
Setting up simple practical enquiries, comparative and fair tests
Gathering, recording, classifying and presenting data in a variety of ways to help in answering questions
Reporting on findings from enquiries, including oral and written explanations, displays or presentations of results and conclusions
Notice that some forces need contact between 2 objects
Ask relevant questions to extend their understanding and knowledge
Articulate and justify answers, arguments and opinions
Give well-structured descriptions, explanations and narratives for different purposes
Use spoken language to develop understanding through speculating, hypothesising, imagining and exploring ideas