Session 3: Measure a force

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Objective 1: Introduce the quantification of a force

Objective 2: Relate experimentally Force and Mass

 

Explanation

After their qualitative experiments on forces we will start quantitative experiments by measuring some forces

Material

• Scales
• Dynamometers of several ranges
• Objects of different weight
• Empty bag to fill with objects

Maximum duration

90’

Main questions to be asked

What do you think a force is? How do you know a force is present? How much weight can you lift? How can you measure a force?  What is the difference between Mass and Force?

Introduction/Starters

Ask to the children if they can imagine the forces they have applied on the ropes during the session 2 of experiments.

This question is possibly a bit too complex for them, at this stage the objective is just to listen their explanations and their representation.

Then ask them the weight they are able to lift.

Then you can relate both forces: traction on the rope and lift of a heavy weight. (Show a picture of this set-up).

The main activity will be to measure the forces.

Main activities

A.    Measuring the weight we can lift

• The empty bag will be filled successively with several objects of different weight;
• Give to each group a scale and ask the children to measure the weight of the bags containing the objects;
• Then ask each of them to measure the heaviest weight they can lift.

B.    Measuring the force we apply when we pull

• Referring to session 2 of experiments where they pulled the rope, ask your students if they can imagine how the pulling forces can be measured.
• You will then distribute the dynamometer, and they will measure the weight of the same objects already weighted with the scale on part A
• Dynameters are usually graduated in Newtons. Ask them to compute the ratio between the weight measured on the scale and the force measured with the dynamometer. The objective is to show evidence of the factor between Mass (kg) and Force (Newton). This factor is a constant (gravity).
• Optional: mathematical extension, you can also report the results of this last experiment on a graphical display.
• Attach a rope on a wall or something very solid, and attach to it the dynamometer corresponding to the range of force of the heaviest weight that children can lift.
• Ask them to pull and to measure the force they apply while  pulling

C Additional optional experiments: building a simple instrument to measure the force

• Do exactly the same experiment, attaching the elastic rope now, and the dynamometer. Then ask the students to apply several forces on the elastic rope.
• Students will measure the extension of the elastic ropes as a function of the force applied and measure it on the dynamometer
• If they are sufficiently advanced ask your students to represent their results on a graph
• You can use your elastic rope as a new instrument to measure forces
• The same can be done with small elastic bands, for smaller forces
• ….

Conclusion/Plenary

The students approach the way of measuring/quantifying a force.

These experiments have put in relation the quantification of the force they know the best, which is a weight.
Another instrument, the dynamometer has been introduced.
Depending on the age of the students, the gravity acceleration factor g can be measured and introduced. Ask students to write a paragraph about their understanding of weight, mass and gravity and how they could measure that (this can be a homework task or a task at the end of the lesson).