Newtons 3rd Law (Justin Harris)

Title: Rocket Racing

Principle(s) Investigated: Newtons 3rd Law, Engineering practices of developing and modifying a proposed object

Standards:

MS-ETS1-4. Develop a model to generate data for iterative testing and modification of a proposed object, tool, or process such that an optimal design can be achieved.

Disciplinary Core Ideas:

Materials: Water Bottles, Balloons, Straws, plastic bottle caps, tape, scissors, wooden skewers, measuring tape, worksheet with data table, and areas to sketch.

Procedure: 1. Present a balloon racer and inquire about prior knowledge regarding the model.

2. Inquire about how it may work, what physics concepts may be involved?

3. Log responses in worksheet/quickwrite.

4. Ask class to design written/sketch of proposed concept model. Discuss scaling.

5. Build own balloon racer and test.

6. Race rocket racers.

7. Write brief evaluation of engineering design. State if you would keep the design the same, or change design for next round of testing.

Student prior knowledge: Students need to have a fundamental knowledge of forces and their ability to cause motion. Newtons 1st law will be helpful.

Explanation: The discrepant event or opening demonstration is used to provide a visual model of a rocket propelled object. The primary concept we are exploring here is Newtons Third Law of Motion via a balloon racer model. Within the construct of the racer, air is compressed within an expanded balloon. This balloon is attached to a straw nozzle. When the straw nozzle is released, Newtons Third Law takes effect and propels an action force of air out of the nozzle. The released air produces a reaction force against air molecules in the outside environment which pushes the racer in the opposite direction. Further concepts can be explored through this model such as Newtons 1st and 2nd law, P.E & K.E., torque, friction to name a few.

Questions & Answers: 1. How can we incorporate Newtons 2nd Law of Motion to investigate force using this model?

We can bring in other equipment into this investigation and use the model to calculate it's force a certain times. With a scale, tape measure, and stop watch we can find the mass of the vehicle and time its travel over a measured distance. If we time our vehicle from rest, we can calculate the acceleration using the equation Vf-Vo/Tf-To (Final velocity minus initial velocity divided by final time minus initial time.) If we find the mass of the vehicle with the scale, we can apply F=ma to calculage the force the vehicle would contact another object with over the measured distance.

2. If your racer is on the ground either at rest or moving, is there another example of Newtons 3rd Law in effect other than the action and reaction of the balloon propulsion?

Yes Newtons 3rd Law is still in effect, however the direction of the forces we need to account for are different. If the car is sitting on the ground and not moving, or moving and still in contact with the ground the car exerts an action force on the ground and the ground exerts a reaction force equal and opposite to the action force back on to the car.

3. If the racer does not move after releasing the nozzle, what is one possible cause for this in terms of a design or construction flaw and Newtons Third Law? (answers may vary)

There may be a leak in the nozzle or the balloon not allowing the action force to gain a reaction force from the air to propel the racer.

Applications to Everyday Life:

Newtons third law of motion is present constantly around us. Every day we walk, we push on the ground with our feet to propel ourselves forward. Any object that is sitting at rest is exerting an action force on the item in which it rests and the item in turn exerts a reaction force equal and opposite to the action. Thrown objects, mechanical objects in motion and at rest, struck objects all present examples of Newtons Third Law as each can be presented to have an action and reaction force equal and opposite to each other.

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