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Answers to Readers' Questions on Physics: Gravity
The following 180 questions have been answered. They are listed according to date.
List of first 10 items
- Trying to find distance after a given time
- Difference between F = mg and F = GMm/R2?
- Possible to have like a hotel in outer space?
- Question regarding the "sign" of the velocity
- Gravity is the intrinsic force of magnetism
- Why don't we fly off the Earth?
- Which has the greatest force of gravity?
- Explain how gravity is the same for all objects
- Gravity is the side effect of molecular force
- What is your weight on the Moon?
Trying to find distance after a given time
Question
December 13, 2009
Gravity Equations for Falling Objects
Formula:
Velocity for a given time => v = gt.
Velocity for a given distance => v = sqrt(2gd).
Problem:
Given time is 5 secs, find the velocity.
Formula: v = gt
v = 9.8m/s^2 * 5s
v = 49m/s
Therefore the distance after 5 seconds of falling is:
d = (9.8 x 1) + (9.8 x 2) + (9.8 x 3) + (9.8 x 4) + (9.8 x 5)
d = 147 meters
Given distance 147 meters, find the velocity.
Formula: v = sqrt(2gd)
v = sqrt(2 x 9.8m/s^2 * 147m)
v = 53.67m/s
Question:
As you can see from the above answers 49m/s vs 53.67m/s.
Why the two answers don't match? What's wrong with my calculation?
Note:
The formula "v = gt" simply saying "Velocity for a given time", so I assume that if the falling object stop after falling for 1 second, it's velocity is 9.8m/s which also means it has already traveled 9.8 meters after 1 second. So if the object has already fallen for 2 seconds, using the formula you'll get 19.6m/s which means the velocity from that second onward is 19.6m/s and within that second the object had fallen 19.6 meters. So you should add up 9.8 meters + 19.6 meters to know the total distances after 2 seconds. Please correct me if my assumption above is wrong. Thanks.
CH - Singapore
18840
Answer
Says the object is accelerating wallets falling, you cannot use the formula v = gt to determine the distance. Instead, you should use x = gt^2/2.
See: http://www.school-for-champions.com/science/gravity_equations_falling.htm
Thus, after five seconds, you would get: x = 9.8*5*5/2 = 118.75 m.
The gravity equation derivations can explain how the various formula were determined.
Difference between F = mg and F = GMm/R2?
Question
December 4, 2009
What is the difference between F=mg and F = GMm/R2?
Albert - Ghana
18786
Answer
F = GMm/R2 is the force of gravitation between objects in space. See our lessons on gravitation for more about that equation.
F = mg is the force of gravity relatively close to the Earth.
When you get close to the earth, the two equations become equal, and gravitation is then called gravity.
Possible to have like a hotel in outer space?
Question
November 26, 2009
Please i would like to know if it would ever be possible to have like an hotel outer space..
- USA
18742
Answer
Right now, the International Space Station is something like a small hotel, where astronauts live and work. In the future, it may be possible to build a large space station where people could visit and stay for a while. However, the trip there would be very expensive.
Question regarding the "sign" of the velocity
Question
October 7, 2009
I have a question regarding the "sign" of the velocity of an object when thrown upward.
When the object thrown upward, my math book says the velocity is positive, after reaching the highest point, the object starts going down, the direction of the velocity then negative. This seems to be opposite to your examples.
Thanks.
Kim - USA
18449
Answer
You have to be aware of the convention used in whatever book you use. Our logic is that when you drop an object, it is naturally increasing in speed. Thus, we called downward toward the ground as the positive direction for gravity. So, if you throw the object up, it is going in the opposite direction of gravity.
The most important thing is that your answer for the time, velocity and distance traveled is correct.
For your book's examples, use up as positive, but in our examples use up as negative. As long as you are consistent within an equation, the results should be the same.
Gravity is the intrinsic force of magnetism
Question
August 30, 2009
Gravity is the intrinsic force of magnetism, one lineral speed, and a variable of rotational speeds.
- USA
18235
Answer
I've never heard of any relationship between gravity and magnetism, especially since gravity affects non-magnetic matters too.
Why don't we fly off the Earth?
Question
August 21, 2009
If earth is revolving at very high speed then why should not we fall
YASHU SHARMA - India
18179
Answer
The rotates at only 1 revolution per day, which is very slow. Although there is some centrifugal force pulling us away from the ground, the gravity of the Earth is many times stronger, keeping us on the ground.
Which has the greatest force of gravity?
Question
June 29, 2009
Which one of the following objects will have the greatest force of gravity to the earth at its surface??.......
1) Alarge book
2)Ahouse
3)An average size car
4)A human bening
kaleem - KSA
17958
Answer
The force of gravity is greatest for the heaviest item.
Explain how gravity is the same for all objects
Question
June 12, 2009
If the acceleration of gravity is the same for all objects, how can you explain your results?
Leah - USA
17897
Answer
Try dropping two objects from the same height at the same time, you will see they hit the ground at the same time. Do not use a light object that is affected by air resistance. This type of experiment proves the rule.
Gravity is the side effect of molecular force
Question
May 18, 2009
Gravity is the side effect of the force that binds atoms/molecules together. This is one of the reasons it is a weak force, and a constant force, and also the reason why it cannot be identified as a separate force. It is also why, as one moves away from earth, the force of gravity diminishes arithmetically. Gravity does have an affect on light waves, speeding up light at the top of a wave if that light wave is within a certain distance of a sun or planet. This is one of the reasons light is a wave and a pulse. The 'speeded' up light moves into another dimension, and as the speed diminishes over time, it returns to our universe as DARK MATTER.
Want to learn more.... contact me.
Jacob - Canada
17765
Answer
Thanks for your theories on gravity. It is subject that has puzzled scientists and has resulted in a number of theories.
One theory of forces is that the exchange of gluons binds atoms together, while graviton particles are responsible for gravity. Gluons exhibit a strong force and gravitons represent a weak force. As far as I know, the theory does not say that gravity is a side-effect of molecular forces.
It is true that gravity affects light. The Theory of Relativity states that space curves near matter, causing they effect of gravity. But Quantum Mechanics shows that light particles--photons--can behave as matter. In either case, the effect of gravity on light has bee experimentally demonstrated.
What is your weight on the Moon?
Question
March 21, 2009
i had a question on my text book that-
"Your mass on the earth is 42 kg. What will be your weight on the moon? (g on moon = 1/6 X g, on the surface of the earth."
when i am writing the solution, like-
W=mg (on earth)
and W=mg1 (on the moon)
here i had problem, that what shall i write,
W=mg1 or W=mg.
If you would kindly clarify this problem!!!!!!!!!
Mahir - Bangladesh
17483
Answer
Mass is the same on both the Moon and Earth. Mass is how much matter an object has.
W = mg concerns weight. If W is your weight on Earth, then w = mg/6 is your weight on the Moon, since the gravity on the Moon is 1/6 that on Earth or g/6.
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Answers to Readers' Questions on Gravity