Year 12 Mechanics Newton's third law Experiments:


Purpose:

  • The purpose of these experiments is to clearly demonstrate Newton's third law "for every action there is an equal and opposite reaction"

Demo 1:

The balloon thingy (created by Phil English and Walt Rutgers)

  • Get two student volunteers and give them each a balloon (of the same size - preferably different color)
  • Complete the following 4 experiments and note that it does not matter who pushes on who both balloons will have the equal deformation.
Newtons_third_law1.jpg
For full instructions:

Demo 2:

Two scales being pushed together:

  • Get two bathroom scales and two student volunteers. Do essentially the same thing as was done in Demo 1, except this time an exact value can be read.
  • Get the class around to read the scale. I.e. when the person on the left pushes but the person on the right "doesn't", the scales will still read the same.
Newtons_third_law2.jpg
Do the above, except with 2 sets of scales inbetween them


Demo 3:

Two force meters being pulled apart:

  • Newton's third law not only applied to push forces but also to pull forces. Using a pair of force meters (digital or spring), get one student to pull their force meter while the other keeps theirs stationary - Both force meters will read the same force.
Newtons_third_law3.jpg


Demo 4:

  • Using low friction trolleys, skateboards, or office chairs push each other.
  • Do this first with people of the same approx. mass - they will both accelerate at the same speed
  • Do this with one larger person (or put extra masses on their trolley) and one lighter person. The lighter person will accelerate much faster than the heavier person.
Newtons_third_law4.jpg
This demo can be explained in terms of Newton's second law, but more correctly - the conservation of momentum - which states that momentum before = momentum after. As neither person 1 or person 2 initially has a velocity, the initial momentum (p=mv) must equal zero, therefore the final momentum must equal zero.
But both person 1 and person 2 have a velocity at the end you ask, the key is that the velocities are in opposite directions and therefore sum to zero.

There are many different applications of Demo 4 (this video is VERY good):



Demo 5:

Trolley Explosion:



Demo6:

Balloon Race:

Many teachers complete balloon races (and rockets etc.) when teaching Newton's third law - maybe best done in junior school.




References / other comments: