Session 4: Balance of forces – Velocity = 0

Objectives 1: To show that a static object (velocity equal to zero) is subject to balanced forces.

Explanation

 One of the common misconceptions: the static position is associated with the absence of forces, while in fact, it corresponds to a balance of equal forces.

Experiments with balanced forces will be performed to demonstrate the presence of forces while a balanced and static position is reached.

Material

• Buckets full of water
• Empty bottle of water (1.5 l)
• Scales
• String

Maximum duration

45 minutes

Main questions to be asked

What do you think a force is? How do you know a force is present? How do you know forces work on a stationary object?  How do you know forces work on you when you are seated in your chair?

Introduction/Starters

Everybody stands up without moving. Ask the question: “did you feel a force on your body ? “
Then jump in the air “What’s happening?”

When the students stands up again, ask them if a force is exerted on their body.

Main activities

Experiments: A fixed object is always subjected to balanced forces

A. Measuring our weight

Ask in each group, to one of the students, to stand up on the scale and to measure the forces. Depending on their age, ask them to write on a piece of paper or to draw the forces on the floor.

B. Weight of a bottle

1. Attach an empty plastic bottle to a string, let it fall. Again ask them how they understand what’s happening.
2. Fill the bottles with water, the ask them to gently (!) lower the bottle of water in the bucket full of water, keeping the string in their hands.
3. Ask them to write down or draw step by step what happens
4. I carry the bottle full of water, I feel …
5. I lowered the bottle of water in the bucket, and the string …
6. They can repeat the same operation measuring the weight of the bucket before and after that the bottle is within the water. Ask them to predict how the weight will vary.

Conclusion/Plenary

Forces are always present even if we don’t always “feel” them. A stationary body is always submitted to balance forces.

Ask students to predict which forces are working on a plane when it stands still on the runway before it starts moving?

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