Register to reply

Gravitational Force Vector exerted by 2 charged objects

Share this thread:
Nal101
#1
Sep16-09, 09:18 PM
P: 16
1. The problem statement, all variables and given/known data
Two thin hollow plastic spheres, about the size of a ping-pong ball with masses (m1=m2 = 2e-3 kg) have been rubbed with wool. Sphere 1 has a charge q1 = -4e-9 C and is at location < 40e-2, -20e-2, 0 > m. Sphere 2 has a charge q2 = -6e-9 C and is at location < -50e-2, 40e-2, 0> m

I've already calculated
Relative Position Vector: <-90e-2, 60e-2, 0>
Distance between q1 and q2: 1.0816653 m
Unit Vector: <-.8320502943, .5547001962, 0>
Magnitude of gravitational force exerted on q2 by q1: 2.29059829e-16
The Electrostatic Force between the two objects: 1.84615e-7

Questions:
1) What is the gravitational force (vector) exerted on q2 by q1?
2) What is the electric force (vector) exerted on q2 by q1?


2. Relevant equations
Fnet = G * [(m1 * m2)/r^2]
Felec = k * [(q1 * q2)/r^2]

3. The attempt at a solution
For question 1, I took each separate component of the unit vector and divided it by the magnitude.
i.e.
<(-.8320502943 / 2.29059829e-16), (.5547001962 / 2.29059829e-16), 0>
For question 2, I didn't know where to start, assuming I needed the answer to question 1.
Phys.Org News Partner Science news on Phys.org
Security CTO to detail Android Fake ID flaw at Black Hat
Huge waves measured for first time in Arctic Ocean
Mysterious molecules in space
Saladsamurai
#2
Sep17-09, 06:47 AM
Saladsamurai's Avatar
P: 3,015
Hey there NAl101

Why do you think that questions 1 and 2 are related?

You have already written the formula Felec = k * [(q1 * q2)/r^2]
which gives the electrostatic force bewteen two point charges (and I think it is safe to say that we can treat these as point charges).

Perhaps it is the sign of the charges that is troubling you? If so, I would simply treat the formula with their absolute values and then determine the direction by inspection.

That is: F_elec = k*[(|q1||q2|)/r^2]

Direction = "you tell me"
Nal101
#3
Sep17-09, 08:15 AM
P: 16
Thanks.

Why do you think that questions 1 and 2 are related?
Actually I do not know, it seems rather absurd at this point in time. I believe though at the time, since I couldn't figure out either one of them, they were seemingly related. I know this to be a false assumption though, seeing as they pertain to different aspects of the system.

The real problem I seem to be having is calculating the answers into a vector form. At first I thought it was simple, and for question 1, divide the unit vector by 2.29059829e-16 (magnitude of grav force) and on the same path, for question 2, divide the unit vector by 1.84615e-7 the electrostatic force between the two objects. This has apparently been disregarded as the correct answer, leaving me with no idea on how to calculate the answers.

Would I be correct in assuming the use of the two formulas towards my answer, having r = the values for x and y in the relative position vectors?

Saladsamurai
#4
Sep17-09, 08:33 AM
Saladsamurai's Avatar
P: 3,015
Gravitational Force Vector exerted by 2 charged objects

Hmm. I am not sure what the convention is here. We always used the "radial unit vector" to denote the direction. So the answer simply had a magnitude of |F_elec|=k*[(|q1||q2|)/r^2] and a direction of r_hat

If you want to write the answer in Cartesian Coordiantes, then you must take the Magnitude of the Vector:

|F_elec|=k*[(|q1||q2|)/r^2]

and then multiply it by the unit vector in the direction of r, which is what I think you are trying to say.

i.e. r_unit = (rxi+ryj)/|r|
Nal101
#5
Sep17-09, 08:48 AM
P: 16
So.. for problem 2..

1.84615e-7 <-.8320502943, .5547001962, 0>

and problem 1, change the scalar to 2.29059829e-16 ?

Awsome! it worked. Many thanks saladsamurai.


Register to reply

Related Discussions
Need help with Charged objects and Electron Force Introductory Physics Homework 4
Impulse exerted on objects Introductory Physics Homework 2
How to get the distance from the Sun when the net gravitational force exerted by the Introductory Physics Homework 2
Charged objects and the force between them? Classical Physics 2
The Gravitational Force (between two spherical objects) Introductory Physics Homework 6