# mass of apple & orange 20 pts

為甚麼一個蘋果的mass of inertia，是剛好等於它的mass of gravitation？

如果一個蘋果和一個橙的mass of inertia是0.1kg，它們雖屬不同物質，為甚麼它們的mass of gravitation 也剛巧是0.1kg呢？

physics8801,

Thanks. I am wondering why the inertial mass is the same as gravitational mass, irrespective of the material.

Inertial mass and electric charge are 2 different properties. There is not a direct relation between them. Somehow, inertial mass is same as gravitational mass. Strange.

franng2008,

Thanks.

Inertial mass is the 'reluctance' of an object to change in velocity.

Gravitational mass is about the attraction force between 2 ojects.

They appear to be different properties of an object. But they are the same. Strange.

### 2 Answers

- 追求自由Lv 51 decade agoFavorite Answer
The following information extracted from Wikipedia may help:

"The only difference there appears to be between inertial mass and gravitational mass is the method used to determine them.

Gravitational mass is measured by comparing the force of gravity of an unknown mass to the force of gravity of a known mass. This is typically done with some sort of balance scale. The beauty of this method is that no matter where, or on what planet you are, the masses will always balance out because the gravitational acceleration on each object will be the same. This does break down near supermassive objects such as black holes and neutron stars due to the steep gradient of the gravitational field around such objects.

Inertial mass is found by applying a known force to an unknown mass, measuring the acceleration, and applying Newton's Second Law, m = F/a. This gives an accurate value for mass, limited only by the accuracy of the measurements. When astronauts need to be weighed in outer space, they actually find their inertial mass in a special chair.

The interesting thing is that, physically, no difference has been found between gravitational and inertial mass. Many experiments have been performed to check the values and the experiments always agree to within the margin of error for the experiment. Einstein used the fact that gravitational and inertial mass were equal to begin his Theory of General Relativity in which he postulated that gravitational mass was the same as inertial mass, and that the acceleration of gravity is a result of a 'valley' or slope in the space-time continuum that masses 'fell down' much as pennies spiral around a hole in the common donation toy at a chain store. Dennis Sciama later showed that the reaction force produced by the combined gravity of all matter in the universe upon an accelerating object is mathematically equal to the object's inertia [1], but this would only be a workable physical explanation if the gravitational effects operated instantaneously."

2010-01-15 19:40:28 補充：

Strange? 是否代表我們無知呢?

牛頓發現 laws of motion 和 law of universal gravitation 的時候, 還以為個 mass 不會變, 但愛因斯坦 ......

說不定好快又會有新發現.

Source(s): http://en.wikipedia.org/wiki/Inertia - 天同Lv 71 decade ago
Mass is a physical quantity used to measure inertia. From Newton's Second Law, a force must be applied to overcome the inertia of an object in order to make it starts to move, i.e with an acceleration.

Hence, F = m.a

where F is the applied force , m is the mass (used to quantifying inertia), and a is the acceleration.

The mass m here is called the [inertia mass].

When under a grvitational field, an object has weight due to the graviational attraction on it. The weight of an object W is given by

W = m'g, where g is the acceleration due to gravity

here m' is the gravitational mass

When this object falls freely under gravity, it has an accleration and gains in speed. We could thus equally apply Newton's Second Law to this falling object, so we have,

F = m.a = W = m'g

But in free falling the acceleration a is equal to g,

hence, m.g = m'g

i.e. m = m'

The above theory will be equally applicable to any object, irrespective it is an orange or an apple.