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Physics Lesson 4.7.4 - Examples of Action and Reaction Forces
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Welcome to our Physics lesson on Examples of Action and Reaction Forces, this is the fourth lesson of our suite of physics lessons covering the topic of Newton's Third Law of Motion, you can find links to the other lessons within this tutorial and access additional physics learning resources below this lesson.
Examples of Action and Reaction Forces
There are many examples of the application of action-reaction principle in daily life. Some of them include:
- Gravitational force of the Earth on objects near it - action; gravitational force of objects on the Earth - reaction
- Frictional force caused by the surface of the ground to a sliding object - action; frictional force caused by the surface of the sliding object to the ground - reaction
- The force of an object pushing down the ground - action; the force exerted by the ground when pushing up on the object - reaction, and so on.
Remark! The weight of an object is caused by the gravitational tug of earth, not the upward push of the table! Therefore, the reaction force that pairs with the object's weight is the tug on earth caused by the object. Usually this reaction force is irrelevant in solving problems but ought to be kept in mind to avoid the common error described at the beginning of the previous paragraph, i.e. taking weight as action and normal force as reaction.
Example 1
Identify as much pairs of action-reaction forces as possible in the figure below.
Solution 1
We can identify the following pairs of action-reaction forces in the above figure:
- Pulling force of the car to the rope - action; pulling force of the rope to the car - reaction
- Pulling force of the rope to the cart - action; pulling force of the cart to the rope - reaction
- Pulling force of the hanged object to the rope - object; pulling force of the rope to the hanged object - reaction
- Frictional force of the ground with the car's tires - action; Frictional force of the car's tires with the ground - reaction
- Frictional force of the ground with the cart tires - action; Frictional force of the cart tires with the ground - reaction
- Gravitational force caused by the Earth to the car - action; Gravitational force caused by the car to the Earth - reaction
- Gravitational force caused by the Earth to the cart - action; Gravitational force caused by the cart to the Earth - reaction
- Gravitational force caused by the Earth to the hanged object - action; Gravitational force caused by the hanged object to the Earth - reaction
- Pushing force of the car to the ground - action; pushing force of the ground to the car - reaction
- Pushing force of the cart to the ground - action; pushing force of the ground to the cart - reaction, etc.
It is not very helpful to draw any force diagram in this case, as it would be very confusing. So, please keep in mind only the force diagram used in Newton's Second Law of Motion.
You have reached the end of Physics lesson 4.7.4 Examples of Action and Reaction Forces. There are 4 lessons in this physics tutorial covering Newton's Third Law of Motion, you can access all the lessons from this tutorial below.
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