Momentum and Newton's Gravitation force question (2 stars)by physics1311 Tags: gravitational force, kinetics, momentum, stars 

#1
Feb712, 12:43 PM

P: 8

At t = 0 a star of mass 5.0×1030 kg has velocity < 6.0×10^4, 7.0×10^4, 7.0×10^4 > m/s and is located at < 1.00×10^12, 4.00×10^12, 4.00×10^12 > m relative to the center of a cluster of stars. There is only one nearby star that exerts a significant force on the first star. The mass of the second star is 3.5×10^30 kg, its velocity is < 1.0×10^4, 2.0×10^4, 9.0×10^4 > m/s, and this second star is located at < 1.04×10^12, 3.94×10^12, 3.96×10^12 > m relative to the center of the cluster of stars.
At t = 1.0×105 s, what is the approximate momentum of the first star? (in vector coordinates) Was told to use Momentum principle, position update formula, and Newton's gravitational force law. I keep on getting <3E35, 3.5E35, 3.5E35>kg m/s The force of gravity I keep on calculating is not significant in that time to change the momentum of the particle. 



#2
Feb712, 02:54 PM

Mentor
P: 11,409

You'll have to show your attempt at a solution (how did you arrive at the momentum vector that you found) before we help. 



#3
Feb712, 05:08 PM

P: 8

First I used Newtons gravitational force equation. Fg= GM1M2/r^2
M1=5E30 kg is given M2 = 3.5E30 kg is given G = 6.66E11 Calculated R by using pythagorean theory for both coordinates and adding together I got R =1.14E13 Then using the force calculated I used the momentum principle, delta p = Fnet delta t 



#4
Feb712, 05:40 PM

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P: 11,409

Momentum and Newton's Gravitation force question (2 stars)EDIT: Also, you'll need to express Fnet as a vector, since the resulting momentum will also be a vector quantity. 



#5
Feb712, 06:26 PM

P: 8

Okay, thanks for that tip. I used the distance formula sqrt((x2x1)^2+(y2y1)^2+(z2z1)^2) equals distance.
I got r = 8.25E10m I then plugged it into the equation for gravitational force. F= GM1M2/r^2 I got Fg=1.72E29 Then using this force I did p(final)p(initial) = Fg delta(t) for each coordinate My answer was <3.17E35, 3.67E35, 3.33E35> and I tried making Fg negative which gave me <2.83E35, 3.33E35, 3.67E35> neiter answers were right, what am I doing wrong? 



#6
Feb712, 06:28 PM

P: 8

How do you express Fnet as a vector?




#7
Feb712, 06:43 PM

Mentor
P: 11,409

The gravitational force acts along a line connecting the centers of the two stars. You need to find a unit vector that lies on that line in the direction which the force is acting on the star in question (star #1 in this scenario). The force vector will then be the magnitude of the force multiplied by that unit vector. How do you find a unit vector that lies along the line connecting two points? 


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