Newton's Third Law of Motion

McCoy, Carrol R. H. Chicago Board of Ed.

Objectives: 1. To introduce the student to basic concepts of Newton's Third Law of Motion. 2. To introduce the concept of thrust as one of the forces acting on aircraft to make flight possible. Apparatus Needed: 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. Recommended Strategy: 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 an hour. 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 ahead. 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.
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