PHS 100 - Self Assessment

Competency #1

Answer and Explanation

The correct answer is C.

This is an example of Newton's third law - whenever an object exerts a force on a second object, the second object exerts an equal but opposite in direction force on the first object.

That is to say, if the Sun pulls on the Earth, the Earth must pull on the Sun with the same force in an opposite direction.

Let's look at a drawing (not to scale)

gravitational diagram

Alternatively, you could use Newton's Law of Universal Gravitation to determine the size of the two forces.

Gravitational Pull of Sun on Earth		Gravitational Pull of Earth on Sun
mass 1 (Sun):	1.99x10³⁰ kg	mass 1 (Earth) :	5.98x10²⁴ kg
mass 2 (Earth):	5.98x10²⁴ kg	mass 2 (Sun):	1.99x10³⁰ kg
Gravitational constant G:	6.67x10^-30	G:	6.67x10^-34
distance between sun and earth:	1.55x10¹¹ m	distance between earth and sun	1.55x10¹¹ m

F_g = 6.67 x 10^-34 = 0.33 N (same either way)

In fact, without even using numerical values, we can see that the two forces are equal:

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