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Answers to Readers' Questions on Physics: Gravity
The following 182 questions have been answered. They are listed according to date.
List of first 10 items
- Changing the direction of a satellite
- What is the formula to calculate final velocity?
- 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
Changing the direction of a satellite
Question
May 8, 2010
if we reverse the direction of revolution of satellite against the direction of revolution of earth then then what will happen
abubakar - India
19685
Answer
Most satellites are put into orbit in the direction of the Earth's rotation, because going with the rotation makes it easier for the rockets to put the satellite in orbit. As long as the satellite is traveling at the proper speed for its altitude, it doesn't matter which direction it is moving.
Of course, if it is moving with the rotation of the earth, it appears to move slower with respect to the ground. If it is moving in the opposite direction, it will pass over the ground much faster.
What is the formula to calculate final velocity?
Question
April 7, 2010
If I know the height an object falls and I know that initial velocity of the object is zero. What is the formula to calculate final velocity?
Jack - USA
19512
Answer
See "Gravity Velocity Equations for Falling Objects" at:
http://www.school-for-champions.com/science/gravity_equations_falling_velocity.htm
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.
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Answers to Readers' Questions on Gravity