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Answers to Readers' Questions on Physical Science:

Motion and Inertia

The following 4 comments and questions have been sent in. They are listed according to date.

List of first 10 items

• Mass is constant, while gravity determines weight.
• Effect of air resistance
• Calculating for falling objects
• Can a person parachuting fall at a constant speed?

Next 10

Mass is constant, while gravity determines weight.

Question

June 24, 2009

I have noticed that many of you have questions regarding your physics hw. One question involved an object on another planet with 1/2 of the gravity we experience on Earth. Anybody studying physics should know that MASS NEVER CHANGES. OnLY WEIGHT VARIES.

- USA

17943

Answer

It is true that mass is constant, while gravity determines weight.

I don't see anything on this page concerning an object on another planet and don't recall any question about 1/2 the gravity. Also, we have nothing concerning physics hardware. Are you sure you aren't thinking of another site?

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Effect of air resistance

Question

March 14, 2009

Counting air resistance, how can I calculate the time of of two different objects that are falling?

Kabilan - India

17457

Answer

The problem with air resistance is that it depends on the shape of the object, as well as its speed. There is no easy formula to calculate air resistance. It usually is don experimentally.

If the objects are not lightweight—like paper or cardboard—or they are not large flat objects, then air resistance is a small factor. For example, if you dropped two balls of different sizes, the effect of air resistance would be too small to be concerned with.

I hope that helps.

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Calculating for falling objects

Question

March 3, 2009

Two Objects with different weight are falling down from same height,
i want calculate the time and speed which object will reach ground first?

Kabilan - USA

17373

Answer

See our lessons on gravity, starting with:
http://www.school-for-champions.com/science/gravity.htm

Not counting air resistance, all objects fall at the same rate.

For calculating the time and speed, see:
http://www.school-for-champions.com/science/gravity_equations_falling.htm

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Can a person parachuting fall at a constant speed?

Question

November 12, 2007

let say a person is parachutting. the gravity pull is 700newtons. the air resistance and the mass of the person together is equal to 700newtons. would he decelerate at a constant speed or would he continue moving in a constant speed? because when a person is on a parachute gradually they begin to decelerate, dont they?

Pamela - Australia

14758

Answer

The gravity pull is the mass of the person and the parachute times the acceleration due to gravity constant (9.8 m/s^2). If the air resistance was equal to that force (700 newtons in this case), the person would no longer accelerate but would descend at a constant velocity.

This is the same thing that happens when you drop a coin from a high building. It accelerates until it reaches its terminal velocity because of the air resistance. Then it travels the rest of the way at a constant speed.

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