BF2S Forums Physics Help.

a = mG/r²
Mass of earth = 5.9742 × 10²⁴ Kg
Radius of earth = 6378 Km
G (gravitational constant) = 6.67300 × 10^-11 m³ kg^-1 s^-2
a = 7.8 ms^-2

7.8 = (5.9742 × 10²⁴ * 6.673 x 10^-11)/r²

r² = (5.9742 × 10²⁴ * 6.673 x 10^-11)/7.8

r² = 5.111 x 10¹³

r = 7149129m

But r is to the centre of earth.

r - r' = 7149129 - 6378000

= 771129m
= 771 km

Last edited by DrunkFace (2009-02-18 09:37:28)

TheunforgivenII wrote:

DrunkFace wrote:

a = mG/r²
Mass of earth = 5.9742 × 10²⁴ Kg
Radius of earth = 6378 Km
G (gravitational constant) = 6.67300 × 10^-11 m³ kg^-1 s^-2
a = 7.8 ms^-2

7.8 = (5.9742 × 10²⁴ * 6.673 x 10^-11)/r²

r² = (5.9742 × 10²⁴ * 6.673 x 10^-11)/7.8

r² = 5.111 x 10¹³

r = 7149129m

But r is to the centre of earth.

r - r' = 7149129 - 6378000

= 771129m
= 771 km

that answer didn't work....... so then I tried doing this:

g'/g = R^2/(R+h)^2 ,where

g is acceleration due to gravity at earth's surface = 9.8 m/s2,
g' is acceleration due to gravity at height h = 7.8 m/s2,
R is the radius of earth = 6400 km

7.8/9.8 = 6400^2/(6400+h)^2

h = 773.7 km

this answer was wrong too then I worked it again and tried:

g at h=g at earth/(1+h/R)^2
(g at h/ g at e)^1/2=1/(1+h/r)
(7.8/9.8)^1/2=1/(1+h/r)
0.88=1/(1+h/r)
h/r = (1/0.88)-1
h/6400000=0.136
h=870400m
h=870.4km

and this one is also wrong so I don't know what to do next........

Well if you say the radius of the earth is 6400 Km.

Then my answer is 749 Km.

I was going to say in my original post that all the figures in my original post are subject to some debate and rounding errors, so if the question has that data given it would have made it much easier.

TheunforgivenII wrote:

1) The gravitational acceleration at the surface of a planet is 27.0 m/s². Find the acceleration at a height above the surface equal to half the planet's radius.
_____________m/s²

This looks like a really easy problem, but I can't seem to get it right......

As you should know, the formula for acceleration due to gravity is g = GM / (R + h)²

GM (gravitational constant x mass of planet) remains constant, so you can make GM the subject of the equation then equate the two..



So, GM = g (R + h)²


You know when the radius equals "one" unit of radius, the acceleration is equal to 27.0m/s.

GM = 27.0 x 1² = 27.0


Now we find GM for when the radius is one and a half radius units.

GM = g x (1 + 0.5)² = 2.25 g


Equate the two to eliminate GM and you get..

27.0 = 2.25 g

g = 27.0/2.25

g = 12.0m/s

Story problems like this always made me concoct smartass answers chock full of what-ifs.

Last edited by unnamednewbie13 (2009-03-02 22:15:54)

TheunforgivenII wrote:

http://img187.imageshack.us/img187/3175/31530503.jpg

okay this question is very easy but for some reason I can't the answer.....
τ = f(r) which in turn τ = (270N)(.95m) = 256.5 N·m but as you can see on the image that's not the answer. So I have no idea what to do and I need help.

have you tried 0? cause maybe the frictional force is cancelling out the rotational force

or it could be negative 256.5

Last edited by argo4 (2009-03-30 14:22:54)

I know that school is over right now but I'm in summer school taking physics again and need help on one problem

A block is supported on a compressed spring, which projects the block up in the air with a velocity of V = V_oyj. The spring and the ledge it sits on retracts. You can win a prize by hitting the block with a ball. When should you throw the ball and in what direction to be sure the ball hits the block? (Assume the ball can reach the block before the block reaches the ground and that the ball is thrown at a height equal to the release position of the block.)

(This is all the information I have with no diagram or pictures with it. Which is why I'm sort of confused on what this problem is asking for so I really need some help with this.)

TheunforgivenII wrote:

I know that school is over right now but I'm in summer school taking physics again and need help on one problem

A block is supported on a compressed spring, which projects the block up in the air with a velocity of V = V_oyj. The spring and the ledge it sits on retracts. You can win a prize by hitting the block with a ball. When should you throw the ball and in what direction to be sure the ball hits the block? (Assume the ball can reach the block before the block reaches the ground and that the ball is thrown at a height equal to the release position of the block.)

(This is all the information I have with no diagram or pictures with it. Which is why I'm sort of confused on what this problem is asking for so I really need some help with this.)

Throw the ball at the same velocity from the start as the block in any direction tending towards the block (Assuming that the ball shall hit the block at this angle before it hits the ground) Horizontal velocity doesn't affect vertical, hence it would eventually hit at the point where the horizontal velocity puts it into the path of the block.

TheunforgivenII wrote:

Hakei wrote:

TheunforgivenII wrote:

I know that school is over right now but I'm in summer school taking physics again and need help on one problem

A block is supported on a compressed spring, which projects the block up in the air with a velocity of V = V_oyj. The spring and the ledge it sits on retracts. You can win a prize by hitting the block with a ball. When should you throw the ball and in what direction to be sure the ball hits the block? (Assume the ball can reach the block before the block reaches the ground and that the ball is thrown at a height equal to the release position of the block.)

(This is all the information I have with no diagram or pictures with it. Which is why I'm sort of confused on what this problem is asking for so I really need some help with this.)

Throw the ball at the same velocity from the start as the block in any direction tending towards the block (Assuming that the ball shall hit the block at this angle before it hits the ground) Horizontal velocity doesn't affect vertical, hence it would eventually hit at the point where the horizontal velocity puts it into the path of the block.

thanks hakei, does distance have a factor as in does it matter where you throw the ball?

As I said, as long as the horizontal velocity of the ball will allow it to reach the block before both of the objects hit the block it won't make a difference. If the velocities are matched then both objects will rise/fall at the exact same rate, and eventually they will collide.