Magnitude of gravitinal force

Homework Statement

earth mass is approximately 81 times the mass of the moon. if earth exerts a gravitational force of magnitude f on the moon, the magnitude of the gravitional force of the moon on earth is

1) F
2) f/81
3) 9f
4) 81 F

Homework Equations

i think the equation should be 81f

The Attempt at a Solution

is it right that gravity*mass gives magnitude?

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If this is GSCE or higher level mechanics you should be using Newton's Laws of motion.

ideasrule
Homework Helper
What does Newton's third law say?

Doc Al
Mentor
is it right that gravity*mass gives magnitude?
You're thinking of gravitational force = mass*g, where g is the acceleration due to gravity at that point (or, equivalently, the strength of the gravitational field). Most often used near the surface of the earth, where g is known to be about 9.8 m/s^2.

Hint: That's not needed to answer this question.

new third law says The force exerted by Object A on Object B is equal in magnitude but the opposite direction so the magnitude should be the same as the one on the moon?

Doc Al
Mentor
new third law says The force exerted by Object A on Object B is equal in magnitude but the opposite direction so the magnitude should be the same as the one on the moon?
Exactly.

is it right that gravity*mass gives magnitude?
I think you are misunderstanding the term 'magnitude'.

If you know the differences between vector and scalar quantities then you should know that vector quantities have both magnitude and direction, whereas scalar applies only magnitude.

Displacement and Distance are examples of the different values using the same units of measurement.

For example;
Traveling around your house, from the front door clockwise and back to the front door in a circular path would give you a distance traveled of $$2 \pi r$$ where r is the distance from your starting point to the centre of your path of motion.
Your displacement in this situation would be 0, as you have made no overall net movement.

Another way of putting it is *how much of an impact has this journey had on your initial state*, so you see your magnitude of travel is obvious due to your legs getting tired, however if an observer where watching you at the start, closed their eyes and opened them once you had finished your cycle they would have no idea you had moved at all!

Hope that helps a little =]