Lesson plan

Balanced/unbalanced Forces

Experiment with things that balance, and discuss the forces involved when they are balanced and unbalanced.
Science content
Physics: Motion and Forces, Newton’s Laws, Gravity (K, 2, 6)

Do a selection of the activities.

Start or end with Balancing your body challenges.

Finding the balance point of a stick or ruler can be done:
1. large scale, using long dowel rods, with fewer students, or with student groups. Large balls of homemade play dough are added to each side of a long stick and it is balanced on a finger, to explore how the balance point moves as the mass is redistributed.
2. small scale, using a (paint) stick or ruler and modelling clay/lego. The ruler can be used for quantitative measurement and graphing.

Make a mobile, using the principles learned above to quickly assess where the balance point of each stick in the mobile is.

Experiment with the centre of mass with a balancing sculpture.
Discuss that without the weights the toothpick is unstable and falls because the mass is above the balance point (the tip of the toothpick on your finger). The sculpture moves til the mass (or the average of the distributed mass; centre of mass/gravity) is low as possible and below the balance point. Adding weight below the balance point lowers the centre of mass and makes it more stable.

A rocket has balanced and unbalanced forces alternating before, during and after launch.
At rest before and after launch, the force of gravity on the rocket balances the force of the Earth pushing back up on it, so it does not move.
When launched, the force of the propellant pushing the rocket up is greater than the force of gravity pulling down (they are unbalanced), so the rocket moves upwards.
At the top of its trajectory, the force from the propellant is running out and is balanced by the force of gravity, so the rocket is stopped for a moment.
Then as the rocket falls, there is no upwards force, so the forces are unbalanced again, with the force of gravity winning over.

Grades taught
Gr K
Gr 1
Gr 2
Gr 3
Gr 4
Gr 5
Gr 7