# Ek physics 178

#### yingao88

An elephant and a feather are dropped from a height of 10 m in a giant vacuum
Chamber. Which of the following is correct?

I eliminate other answers because in vacuum there is no air resistant
I left with correct answer "they will strike the ground at the same time because both
Gravitational force and inertia are proportional to mass. " I understand
The first part of the sentence but what does it mean by gravitational
Force and inertia are proportional to mass? Isn't mass is independent factor?

#### philosophia

7+ Year Member
It sounds as though you know that from the Newtonian gravity expression Fg=mm/r^2, the masses of the objects cancel when you sum the forces on the masses accelerating. Ergo, both masses will fall at the same acceleration.

However, it is essential to understand that gravity is an attractive force between masses. The mass of the earth is acting on the objects, and the mass of the objects are acting on the earth (Newton's Third Law). Larger masses exert a larger gravitational field, and thus, they exert a larger gravitational force on the earth.

If you sum the forces on the earth you will find that the larger mass does exert a larger force on the Earth, albeit it minuscule in magnitude. Therefore, mass does play a factor in the problem, but it does so in a way that is not detectable by the human eye.

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#### mehc012

##### Big Damn Hero
7+ Year Member
An elephant and a feather are dropped from a height of 10 m in a giant vacuum
Chamber. Which of the following is correct?

I eliminate other answers because in vacuum there is no air resistant
I left with correct answer "they will strike the ground at the same time because both
Gravitational force and inertia are proportional to mass. " I understand
The first part of the sentence but what does it mean by gravitational
Force and inertia are proportional to mass? Isn't mass is independent factor?
Mass is independent of acceleration, yes...but that is because both gravitational force and inertia are proportional to mass.

Think of it this way: in order to produce the downward acceleration, the attraction of gravity must overcome the tendency to remain at rest (or at a lower rate of motion) aka inertia. The acceleration is the result of the relationship between those two properties. Therefore, it cancels out and though gravitational force is proportional to mass and inertia is proportional to mass, acceleration is not related to it.

Honestly, they were probably just trying to trip up anyone skimming through the question thinking "mass is irrelevant" by phrasing it that way.