Newton's Third Law (Ray Oja)

Title: Newton’s Third Law, 8th Grade Physical Science


Principle Investigated: Newton’s Third Law – For every action there is an equal and opposite reaction.


California Science Content Standards Grade 8:

2. Unbalanced forces cause changes in velocity. As a basis for understanding this concept:

            a. Students know a force has both direction and magnitude.

            b. Students know when an object is subject to two or more forces at once, the result is the cumulative effect of all the forces.

            c. Students know when the forces on an object are balanced, the motion of the object does not change.

            e. Students know that when the forces on an object are unbalanced, the object will change its velocity (that is, it will speed up, slow down, or change direction).


Prior Knowledge:

    Students should know that an object that has a net force of 0 N acting on it will – if at rest remain at rest; if in motion remain in motion at a constant speed and in a straight line.

    Students should understand that the acceleration an object experiences depends on the mass of the object and the amount of force applied.

    Students should know that mass is a measure of inertia.

    Students should know that weight is a measure of the gravitational force on an object.




Demonstration 1-

Electronic balance

2 doughnut magnets

Stacking pole


Demonstration 2-




Activity (per student pair)-

Four pins

Styrofoam tray

Masking tape

Flexible straw


Marker pen

Small, round party balloon

Rocket car building instructions

Measured course




1) Perform Demonstration 1-

    a) place one magnet and stacking pole on balance

    b) have students record measurement

    c) place second magnet on stacking pole with north pole facing south pole, so second magnet is in contact with first

    d) have students record measurement

    e) cover scale readout and flip second magnet so that both north or both south poles of the magnets face each other causing the second magnet to hover

    f) ask students to predict the new measurement, noting that the second magnet is not resting on anything

    g) uncover readout, have students record measurement

    e) ask for explanations, discuss


            Demonstration 1 Explanation - When the second magnet is placed on top of and repelled by the first, there are two main forces at work once the second magnet is at rest. There is the force due to the acceleration of gravity acting on the second magnet in the downward direction, and there is the force created by the repelling magnetic fields (equal to the gravitational force, since the magnet is at rest) acting in the upward direction. If the magnetic fields push upward on the second magnet with a force equal to the downward gravitational force acting on the second, then, according to Newton’s Third Law, there must be an equal and opposite force. This means the first magnet is pushed downward with a force equal to the upward force, which is equal to the weight of the second magnet. So the total weight of this system is the same whether the second magnet is in contact with the first or just floats above it.


2) Perform Demonstration 2-

    a) blow-up balloon

    b) let it fly

    c) ask for explanations, discuss


            Demonstration 2 Explanation - When the air filled balloon is released, the air inside the balloon (which has been compressed by the balloon’s rubber walls) is pushed out of the balloon’s opening. When this happens the air pushes back on the balloon, propelling the balloon in the opposite direction. This simple demonstration of Newton’s Third Law at work also demonstrates the rocket principle.


3) Activity-

    a)Distribute rocket car-making materials

    b)Instruct student-pairs to build cars

    c)Have students test cars, record and graph data (make modifications and test again if time allows)

4) Discuss results of activity, discuss explanation - See the explanation for Demonstration 2 above (the only difference is the payload which in the activity is a Styrofoam car).
5) Discuss real-world application – rockets. (see link to NASA Rocket Principle page below)


Additional questions for discussion:


If for every action there is an equal and opposite reaction, why doesn’t the earth push back against me and move in the opposite direction when I jump upward?

Actually, it does. But the difference between my mass and the earth’s mass is so great the effect on the earth is imperceptible.


Where is the equal and opposite reaction force when a ball is dropped and falls toward the earth?

The earth exerts a pulling force on the ball which causes it to fall toward the earth’s surface. But the ball exerts an equal force on the earth, pulling the earth toward the ball. But as in the previous question, the masses of the two objects are vastly different, and the movement experienced by the earth is imperceptible.


Do action and reaction always occur in pairs?

Yes, forces always come in pairs known as action-reaction force pairs. Recognizing and describing action-reaction force pairs requires recognizing the two involved objects and describing what is pushing or pulling on what, and in which direction. As an example, when a bat contacts a pitched baseball the bat acts on the ball in one direction, the ball acts on the bat in the other. The ball appears to experience a greater force mainly because the batter continues to force the bat (and with it, the ball, since the two are in contact) in the original direction of the bat.


Rocket Car Activity:


NASA Rocket Principle Discussion: