by Ron Kurtus

A total of 81 comments and questions have been sent in on Gravity. They are listed according to date.

## List of next 10 letters

### Country

Horizontal Motion Unaffected by Gravity re: Do objects hit the ground at the same time? Netherlands

Horizontal Motion Unaffected by Gravity Do objects hit the ground at the same time? Netherlands

Displacement Equations for Falling Objects Moving objects India

Work Against Gravity Applied force and force of gravity India

Gravity Throwing objects from different heights USA

Falling objects Terminal velocity of falling object USA

Center of Gravity Center of gravity for a one-axle trailer South Africa

Artificial Gravity Disagree with artificial gravvity Australia

Overview of Gravity Meaning of gravity constant "g" Pakistan

Center of Gravity Determine the center of gravity with plumb line Ghana

Next 10

First 10 letters

## re: Do objects hit the ground at the same time?

Topic: Horizontal Motion Unaffected by Gravity

### Question

April 11, 2015

Appreciate for your response but i would like you to refer from this discussiom "http://physics.stackexchange.com/questions/10773/how-does-gravity-affect-bullets". Referring to a long range gun, accordingly if im not mistaken, it is possible that an object may orbit the earth with the right velocity and force applied to an object. So it is possible that they will not hit the ground....

Priori - Netherlands

25910

For relatively short horizontal distances, the objects will hit the ground at the same time. But if the bullet travels far enough, the curvature of the Earth becomes a factor. If the projectile velocity is great enough, it can orbit the Earth or even go into space. See: Newton's Cannon for an example of this with illustrations.

## Do objects hit the ground at the same time?

Topic: Horizontal Motion Unaffected by Gravity

### Question

April 3, 2015

Sir, two object identical same mass in same altitude and position were release at same instance but other one horizontally applying certain force or greater, will it touch the ground at same time? Please answer. Aprreciate and thanks for effort in advance

- Netherlands

25884

Both objects will hit the ground at the same time. You can verify this by putting a coin near the edge of a table and holding another coin at the same height. Drop the one coin while flicking the coin on the table outward. Surprisingly, both coins will hit the ground at the same time.

They fall at the same rate, even though one is projected sideways,

## Moving objects

Topic: Displacement Equations for Falling Objects

### Question

March 11, 2015

1. An object is projected vertically upward with a certain velocity. After sometime it reaches to a height of 100 m and come back again to the point of projection. The total time taken in this journey is 8 sec. Calculate the distance covered and displacement of the body during this time.
2. An object moves from a certain position with uniform velocity of 20 m/sec. After 2 sec. another body moves from the same position. Calculate the velocity of the second body with which it has to move to get at the first body in 4 sec.

- India

25826

1. The object travels upward for 100 m and back downward for a 100 m. So the total distance covers is 200 m. Displacement is the change in position. Since the object ends up at the starting point, its displacement is 0.

2. After 2 sec, the first object has traveled 40 m. At a total of 6 sec (2 + 4) it travels 120 m. That means the second object must travel 120 m in 4 sec or 30 m/s.

## Applied force and force of gravity

Topic: Work Against Gravity

### Question

January 17, 2015

work done against gravity =mgh in which mg is the force acting on the object and h is the vertical displacement occured due to that applied force. but force of graviy is also mg . so my question is that how will the displacement occur if the applied force and force of gravity are equal

Reetinder Singh - India

25707

The applied force must be greater than the force of gravity for the object. Otherwise the object will not be lifted higher.

When you hold an object, the applied force equals the force of gravity. When your applied for is greater than gravity, you lift the object and do some work.

## Throwing objects from different heights

Topic: Gravity

### Question

January 12, 2015

If you throw two objects same size and speed from different heights would they hit the ground at the same time?

I assume the answer is no, because gravity does not effect horizontal direction.

Nichole - USA

25699

If you drop the objects or throw then downward from different heights, they would hit the ground at different times, according to the height. The same is true if you threw them in a horizontal direction.

## Terminal velocity of falling object

Topic: Falling objects

### Question

January 12, 2015

Let some object hang by a string. So no motion since weight was counter balanced no net force. Now string is cut. Object is falling under gravity and experiences air resistance. Terminal velocity is reached when weight is equal to the air resistance. So no net force and no net acceleration after terminal velocity. But why does the object continue to fall if there is no net force acting on it, why doesn't it just hang in mid air dead stopped ? Is this motion due to inertia ? If it falls a distance of 'x' at terminal velocity what is the work done in terms of W = F x d ?

thanks

MD - USA

25698

The force of gravity accelerates a free-falling object. Air resistance is a function of the velocity and it slows down the falling object.

At what is called the terminal velocity, the force caused by the air resistance equals the force of gravity. Thus the object will no longer accelerate. However, the object is traveling at the terminal velocity and has momentum according to that velocity, due to the Law of Inertia. Thus, it will continue to move at the terminal velocity.

Also, since the force on the object is the force of gravity minus the air resistance force, resulting in a no net force. Thus, the equation W = F x d cannot be used. However, the work on a falling object can also be defined as the difference in potential energy times the distance. Since PE = mgh, where "h" is some initial height, the work done at a distance "d" when moving at the terminal velocity is P = mgd.

See Work Done by Gravity Against Inertia and Air Resistance.

## Center of gravity for a one-axle trailer

Topic: Center of Gravity

### Question

January 1, 2015

Please could you help with formulae to calculate the horizontal & vertical centre of gravity of a single axle trailer/caravan with the only measurements are mass, length & width of trailer/caravan & distances from front/rear to axle & axle to front/rear - eg
Mass - 800kg
Total length - 5m
Total width - 2m
Length front to axle - 3m &
Length axle to rear - 2m

Only thing I possibly fathomed was

Xcg = (mass X length)/mass & Ycg = (mass X width)/mass
I am more than likely far off the mark.
Regards
Ian

Ian - South Africa

25677

There seems to be information missing in your problem.

Assuming the trailer is rectangular and has equally distributed weight, the CG would be at 1/2 the length and 1/2 the height.

If the trailer has a single axle, you would want to move the CG to be over that axle. That means redistributing the weight (or mass) such that 2/5 is on the side from the front to the axle and 3/5 of the weight is on the shorter side to the rear.

## Disagree with artificial gravvity

Topic: Artificial Gravity

### Question

December 6, 2014

I came across some incorrect information on your website and thought I should bring it to your attention. The page is the one dealing with Artificial Gravity

The topic is

Generating or Simulating Gravity in a Zero-G Environment:
In the movies, or even on some websites, we are led to believe that gravity can be created in space (in a zero-gravity environment), by spinning the chamber/ room so fast, that the people inside it are forced against the outside of floor of the spinning room (as per the figure in your article). This, however, will not work.

We've all been on the spinning ride at the carnival when we were young, and many of us have seen the spinning bucket experiment at some point, so most people just assume that the act of spinning in and of itself will create a gravitational force, exerting rimwards from the hub of the spinning room, aka centrifugal force. This can only work if there is a gravitational force already acting upon things in said room. As rotational force is applied and increased, gravitational force starts to compete with the rotational force, until "gravity" is applied -at most- at a 90 degree offset to its normal downward pull. I say again, this kind of gravitational simulation only works because it is being done in an environment where gravity already exists and is being exerted.

In a zero-gravity environment, a spaceship/ space station could have a room spinning at bone breaking velocities, but it cannot create gravity from nothing. An astronaut floating in the middle of the spinning room would continue to float in the middle of the spinning room, regardless of how fast the room spins, and no amount of careful maneuvering would create a gravitational force gluing him onto the floor of that room. Newtons first law of motion: An object in motion stays in motion unless acted upon by an outside force. Similarly, an object not-in-motion, will continue to remain not-in-motion, unless acted upon by an outside force. The only way a spinning room can simulate gravity in a zero-gravity environment, is if there are 'walls' that extend outwards in significant length and width, from the centre of the spinning room, just like spokes on a bike (but significantly wider than spokes, of course). These 'walls' will in fact become the 'floor'. As the room starts to spin, an astronaut floating in the centre of the room will soon find himself struck by one of the spinning 'walls'. As this 'wall' continues to push against the astronaut, it will apply force against him. The faster the room spins, the more force the wall will apply against the astronaut. When moving at a speed -which I've not yet worked out- the force applied by the 'wall' against the astronaut will be equivalent to the force normally applied by gravity, and the astronaut can then start using this 'wall' as his new 'floor'.

Thus a circular room is unnecessary, as the thing traditionally thought of as the floor, would actually become the wall, and what would traditionally be a wall, would become (when spinning) the floor. So, beyond the desire for the extra structural stability that might come from spinning a circular room vs. spinning a square room, the shape of the spinning room is entirely unimportant. So long as there are long and wide spokes that radiate outwards from the centre of the rotational force, then gravity can be simulated as these spokes become the floor.

So, the figure on the page is incorrect. Instead, if the circle were divided into 4 equal quarters (or it could be any denomination, really, I just thought 4 quarters would be easy), with the lines radiating straight outwards from the centre, to the circumference. The astronaut would stand on any one of these lines and would exert force against it equal to the force it exerts against him (Newtonian physics again). Eventually, when spinning fast enough, these spokes would simulate normal gravity for the astronaut and would become the new floor. If that doesn't make sense, think instead of a windmill. If you could take a windmill into space and have it spin there, then someone standing on one of the blades would feel that gravity was being simulated.

I hope this makes sense. I hope I've done a good enough job at translating my thoughts into words. I expect that this should all make sense to you, though. This is one of those things that I expect we all take for granted at some point, but as soon as we see the reality of it, it seems so obvious, that we can't believe that we ever thought otherwise.

Peace,

Raz - Australia

25618

Thanks for your comments an analysis of Artificial Gravity. I tried to be very careful in stating that motions such as acceleration and rotation (centrifugal force) only simulate the effect of gravity. They do not create gravity.

Centrifugal force is a function of inertia, as is the force created from acceleration, and does not require gravitational force to apply.

The spinning structure, with the curved wall acting as the floor would not have to spin at break-neck speed. The example I provided in the lesson stated that the rotation would be less than 5 revolutions per minute.

Anyway, it is good that you put some thought into the theory.

## Meaning of gravity constant "g"

Topic: Overview of Gravity

### Question

November 19, 2014

My question is about physical meaning og g=9.8 value.
Does it mean that an object having initial velocity as zero will cover a 9.8 meter distance in 1 second, falling under gravity near earth?
If yes then what if initial velocity of an object is not zero? Let say it 5 m/s then how much distance it will cover in 1 second falling under gravity?

Naveed - Pakistan

25563

"g" represents the acceleration of an object due to the force of gravity. g = 9.8 meters per second squared. It is correct that a dropped object will fall 9.8 meters in one second. The equation for distance with respect to time is y = (gt^2)/2. Thus y = 9.8*2*2/2 = 18.6 meters. See Gravity Displacement Equations for Falling Objects for more information.

If the initial velocity is not zero, it depends whether the object was thrown up or down. See Objects Projected Downward and Objects Projected Upward.

## Determine the center of gravity with plumb line

Topic: Center of Gravity

### Question

November 9, 2014

describe an experiment how to determine the center of gravity of an irregular shape using the plumb line method

eric - Ghana

25516

When an object is suspended so that it can move freely, its center of gravity is always directly below the point of suspension.

You can find the center of gravity in an object experimentally by hanging it from several points and using a plumb line to mark the vertical line. The intersection of two or more vertical lines from the plumb line is the center of gravity for the object.

For an irregular three-dimensional object, you need to carefully mark the path from the plumb line.

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