1. Newton’s 3rd law and Universal Law of Gravity
Unit V – Newton’s laws of motion

2. Newton’s 3rd law
Every force involves the interaction between two objects.
The two forces resulting in this interaction are called action and reaction forces.
Newton’s 3rd law states that for every action, there is an equal and opposite reaction.
The size of the force on the first object equals the size of the force on the second object.
The direction of the forces on the first object is opposite to the direction on the second object.
Force always comes in pairs.

3. Newton’s 3rd law
Example: fish swimming in water. The fish uses it’s fins to push the water backwards. The water must also be pushing the fish forwards.
Problem: For years, space travel was believed to be impossible because there was nothing that rockets could push off of in space in order to provide the propulsion necessary to accelerate. This inability of a rocket to provide propulsion is because ...
A. space is void of air so the rockets have nothing to push off of.
B. gravity is absent in space.
C. space is void of air and so there is no air resistance in space.
D. nonsense! Rockets do accelerate in space and have been able to do so for a long time.

4. Newton’s 3rd law – Identify action/reaction pairs
To identify the action/reaction make two statements describing who is pushing on whom and in what direction.
Identify the following action/reaction pairs: A. B.
C. Identify six action/reaction pairs.
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5. Universal Law of Gravity
The law of gravity is an inverse square law.
The inverse square law proposed by Newton suggests that the force of gravity acting between any two objects is inversely proportional to the square of the separation distance between the object's centers. Since the two quantities are inversely proportional, an increase in one quantity results in a decrease in the value of the other quantity. That is, an increase in the separation distance causes a decrease in the force of gravity and a decrease in the separation distance causes an increase in the force of gravity. Furthermore, the factor by which the force of gravity is changed is the square of the factor by which the separation distance is changed. So if the separation distance is doubled (increased by a factor of 2), then the force of gravity is decreased by a factor of four (2 raised to the second power). And if the separation distance is tripled (increased by a factor of 3), then the force of gravity is decreased by a factor of nine (3 raised to the second power).

6. Universal Law of Gravity
Equation for the Universal Law of Gravity:
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7. Gravity problems Sample Problem #1
Determine the force of gravitational attraction between the earth (m = 5.98 x 1024 kg) and a 70-kg physics student if the student is standing at sea level, a distance of 6.38 x 106 m from earth's center.
Sample Problem #2
Determine the force of gravitational attraction between the earth (m = 5.98 x 1024 kg) and a 70-kg physics student if the student is in an airplane at feet above earth's surface. This would place the student a distance of 6.39 x 106 m from earth's center.

8. Works Cited
The Physics Classroom. (2013). Newton’s Laws – Lesson 4. Retrieved from
The Physics Classroom. (2013). Newton’s Laws – Lesson 4. Retrieved from