Page 1

Figure 1

Newton's Third Law is often stated as: For every action, there is an equal and opposite reaction. This statement means that when one body exerts a force on a second body, the second body also exerts a force on the first body. There is a pair of forces acting on the two interacting objects. The size of the force exerted by the first body is equal in size to the force exerted by the second body. The direction of the force exerted by the first body is opposite in direction to the force exerted by the second body. Forces always come in pairs – equal and opposite action-reaction force pairs. In this experiment you will use two Force sensors to demonstrate Newton's Third Law of Motion.

einstein™Tablet+ with MiLAB or Android/iOS Tablet with MiLAB and einstein™LabMate Force sensors (2) String Rubber band


Launch MiLAB (


2. 3.

Connect the Force sensors to ports on the einstein™Tablet+ or einstein™LabMate. Make sure that only the Force sensors are selected.

Program the sensors to log data according to the following setup: Force sensor

Force, Pull – positive (50 N) (N)

Set As Zero





50 Sec


Tie the two Force sensors together with a string about 20 cm long. Hold one Force sensor in your hand and have your partner hold the other Force sensor so you can pull on each other (see Figure 1).


Tap Run (


Gently tug on your partner’s Force sensor with your Force sensor, making sure the graph does not go off the scale. Also, have your partner tug on your sensor. You will have 50 seconds to try different pulls.


Save your data by tapping Save (


Repeat the experiment with different materials. Remember to save the data from each experiment with a unique name. What would happen if you used the rubber band instead of the string? Make a prediction using the Predict tool.


) on the upper toolbar to begin recording data.

) on the upper toolbar.


Open the Setup window (

) from the Sensor Control Panel.


Turn on the Predict tool and close the setup window. Tap the Predict tool (


instruction to sketch a prediction on the graph. Repeat steps ‎2-‎4Error! Reference source not found. using the rubber band instead of the string.

) and follow the

For more information on working with graphs see: Working with Graphs in MiLAB. 1.

2. 3. 4.

Examine the graphs. a. What can you conclude about the two forces (your pull on your partner and your partner’s pull on you)? b. How are the magnitudes related? c. How are the signs related? How does the rubber band change the results - or does it change them at all? While you and your partner are pulling on each other’s Force sensors, do your Force sensors have the same positive direction? What impact does your answer have on the analysis of the force pair? Is there any way to pull on your partner’s Force sensor without your partner’s Force sensor pulling back? Try it.


Fasten one Force sensor to your Lab bench and repeat the experiment. Does the bench pull back as you pull on it? Does it matter that the second force sensor is not held by a person? Use a rigid rod to connect your Force sensors instead of a string and experiment with mutual pushes instead of pulls. Repeat the experiment. Does the rod change the way the force pairs are related?

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Newton's Third Law  

Newton's Third Law  


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