Newton's Third Law of Motion
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McCoy, Carrol R. H. Chicago Board of Ed.
1. To introduce the student to basic concepts of Newton's Third Law of
2. To introduce the concept of thrust as one of the forces acting on
aircraft to make flight possible.
Rope, 2 spring balances, bucket with holes on opposite sides of
bottom, string, balloons, paper clips, straws, pencils, tape, scissors,
paper, wire, Xeroxed copy of Space Shuttle, rocket, calculator, carbon
dioxide pellet, firing pin.
Hook two spring balances together held by supports. Adjust the support
so that the balance at the right reads 5 lbs. The balance at the left
will also read 5 lbs. Increase the pull to the right to 10 lbs. The
pull to the left will also be 10 lbs. The pulls of the balances will be
found to be equal in magnitude and opposite in direction for all
settings of the support.
Fit a small toy airplane with a screw eye above its center of gravity
and fasten a pellet of compressed carbon dioxide underneath. Suspend
the plane from a wire passing through the screw eye and stretched
across the room. Puncture the pellet with a spring driven needle.
(Note: Firing pins for cartridges are available at hobby shops.) The
plane is propelled forward while the gas is discharged backward.
Attach the nozzle of a balloon to a straw with a 5 to 10 gram mass.
When blown up, the balloon will lift off a table with good stability.
The balloon without mass is erratic in its behavior. This experiment
shows that some control is needed to cause a rocket to behave as you
want it to do.
Powder-Propellant rocket. An inexpensive solid-fuel rocket motor (such
as Jetexengine) can be purchased at a hobby shop. A chemical pellet is
used as the propellant. (Caution: The fuel is inflammable and should
be operated outdoors.) Attach this rocket motor to a model airplane or
rocket that is suspended from a long horizontal wire. Fasten the
engine just below the center of gravity of the airplane or model. Then
ignite the rocket by lighting a fuse.
You may want to introduce the principle of action and reaction by
having a student demonstrate a model racing car driven by a carbon
dioxide cartridge. A good model will attain speeds close to 60 miles
Some Practical Applications of Newton's Third Law
1. An athlete executes a high jump by pushing against the earth which,
in turn, pushes him/her up in the air.
2. We swim by pushing against the water, which reacts, propelling us
3. A helicopter rises vertically because of the reaction to the
downward push against the air produced by its propellers.
4. The jet propelled plane and the rocket are thrust ahead by the force
of reaction on their own products of combustion enabling them to fly
at altitudes at which there is not enough air pressure for a
conventional propeller-driven vehicle to operate.
5. Water of high pressure hoses leaves the nozzle with great speed
resulting in a thrust which pushes the hose with such force that one
person cannot control it. Most people have experienced this to a
much lesser degree with the garden hose.