Newton’s Third Law Experiment


Isaac Newton’s Third Law of physics states that for every action there is an equal and opposite reaction. This principle describes interactions between bodies, and an experiment has been conducted to study these relations. In the following, two Force Sensors were utilized in connection with computer software to observe and generate graphical data for such interactions.

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(1) Examine the two data runs. What can you conclude about the two forces? How are the magnitudes related? How are the signs related?
The graphs show that the forces on one sensor are the same as the force on the second sensor. Either force has equal magnitude. Setting the coordinate system correctly shows that these forces are opposite in direction: one is positive, the other negative.

(2) How does the rubber band change the results, or does it change them at all?
The rubber band graphs appeared much smoother, but the results are the same. Each sensor had equally large forces applied to them in opposite directions.

(3) While you and your partner are pulling on each other’s Force Sensors, do your Forces Sensors have the same positive direction? What impact does your answer have on the analysis of the force pair?
Initially the Force Sensors were calibrated oppositely, which made the graph appear as if each force was in the same direction. This was useful in determining their equal magnitudes, but for analysis the coordinate system was corrected in procedure step (11). It is apparent that the force pairs have opposite direction.

(4) Is there any way to pull on your partner’s Force Sensor without your partner’s Force Sensor pulling back?
Attempting to break Newton’s Third Law was a failure. There is simply no way to pull on one Force Sensor without the other pulling back; that is unless you cut the rope. Still, Newton’s general principle holds for systems without ropes, like in Moon-Earth interactions.

(5) What is really meant by equal and opposite in Newton’s law? Restate Newton’s third law in your own words, not using the words “action,” “reaction,” or “equal and opposite.”
All forces are an interaction between two objects and relative to the objects these forces sum to zero; there are no net-forces on the system of Force Sensors.

(6) Would answering these analysis questions have been more obvious if you had used the graphs from procedure 11, the reflected graphs instead of the graphs that traced each other?
The analysis would have been the same because we kept in mind that the Force Sensors did not have a standard coordinate system.



(1) You are driving down the highway and a bug splatters on your windshield. Which is greater: the force of the bug on the windshield, or the force of the windshield on the bug?
The magnitude of these forces are equal, though an illusion is created that the windshield exerts a greater force on the bug, simply because the bug has a tiny mass whereas the car has a great mass.

(2) Hold a rubber band between your right and left hands. Pull with your left hand. Does your right hand experience a force? Does your right hand apply a force to the rubber band? What direction is that force compared to the force applied by the left hand?
My right hand experiences the force of the rubber band pulling on it. My right hand applies a force to the rubber band in opposite direction that my left hand applies.

(3) Pull harder with your left hand. Does this change any force applied by the right hand?
The force applied by my right hand increases in magnitude.

(4) How is the force of your left hand, transmitted by the rubber band, related to the force applied by your right hand? Write a rule, in words, for the force relationship.
The applied pulling force of my left hand by way of rubber band causes my right hand to pull oppositely with the same force.


Discovering Forces

(1) What does the statement “the net force acting on an object mean?
Forces on an object can be of varying magnitude and directions. The net force on an object is a result from summing all the individual forces on the object.

(2) Can an object remain at rest when Fnet does not equal zero?
The answer is simply no. If my body didn’t have a normal force perpendicular to Earth’s surface, then I would literally be pulled to the core!

(3) Does an object need a net force acting on it in order to move at a constant velocity?
Any object need not have net forces acting on it in order to move in constant velocity. Constant velocity and rest are analogous.

(4) What is required for an object to change its velocity?
An object needs to have an applied net-force to change its velocity.

(5) If an object is moving to the right, can it have a net force to the left? Why?
Any object moving in any direction can have a force applied to it in any direction too. Velocity is independent of force. How could a spacecraft ever return to Earth if it could only exert forces toward space? Well, it couldn’t.

(6) What is required for an object to change the direction it is traveling?
An object must accelerate to change its direction of travel.

(7) When does a moving object have a zero acceleration?
An object in motion has zero acceleration when it’s motion or velocity is constant. That means its direction is constant too.

(8) When does a stationary object have a non-zero acceleration?
In the instance of throwing a ball in the air, whenever the ball reaches its maximum height it appears stationary for a split second, but it’s still experiencing gravitational acceleration.

(9) Can an object have a net force opposite to the direction of its acceleration?
By definition no object can have a net force opposite to the direction of its acceleration, but this is all dependent on the inertial-reference frame that such an object is depicted in. A remote controlled car on a train that is driving toward the back of the train has a net force relative to the passengers of the train. The train has acceleration relative to the train tracks. Logically, the answer is no.

(10) What can you say about the relationship between force and velocity?
There is no definite relation between force and velocity.

(11) What can you say about the relationship between force and acceleration?
Net forces have a direct relationship with acceleration.


In conclusion, this experiment has provided a deeper understanding of Newton’s Third Law, and showed agreement with the statement: for every action there is an opposite and equal reaction.

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