Solving Vector Fnet: What is Fnet at Time t=0?

[ohheytai]

In the dark in outer space, you observe a glowing ball of known mass 1.2 kg moving in the xy plane at constant speed in a circle of radius 4.5 m, with the center of the circle at the origin (< 0, 0, 0 > m). You can't see what's making it move in a circle. At time t=0 the ball is at location < -4.5, 0, 0 > m and has velocity < 0, 70, 0 > m/s.

a)
At time t=0, even though you can't see what's causing the motion, what can you conclude must be the direction of the vector Fnet?

b)You learn that at time t=0, two forces act on the ball, and that at this instant one of these forces is F1 = < -261, 1764, 0 > N. What is the other force?

Homework Equations

Fperpendicular is m(v^2/r)

The Attempt at a Solution

for the perpendicular force i got 1306.666667
which is right so i thought Fnet would be that or <0,1306.66667,0> but its not can someone please help? and idk how to do b please someone help!

 

[Doc Al]

What is the direction of that net force? (Draw yourself a diagram of the motion.)

 

[ohheytai]

so would it be <0,j,0>?
and what about the 2nd questionnn?

 

[Doc Al]

so would it be <0,j,0>?

No.

and what about the 2nd questionnn?

First find the correct direction for F_net, expressed as a vector. Then you can just use the vector equation F_net = F₁ + F₂ to solve for the unknown force.

 

[ohheytai]

i don't get it how do you find the force then? it would be straight up would it be mass times velocity?

 

[Doc Al]

i don't get it how do you find the force then? it would be straight up would it be mass times velocity?

Did you draw a diagram of the motion, which is in the x-y plane?

You found the correct magnitude for the force, you just need the correct direction.

 

[ohheytai]

yes it would go straight up become Fnet is in the same direction of the momentum and velocity. so the direction would be <0,mv,0> right?

 

[Doc Al]

yes it would go straight up become Fnet is in the same direction of the momentum and velocity. so the direction would be <0,mv,0> right?

No. For uniform circular motion, what's the direction of Fnet?

 

[ohheytai]

Perpendicular? so it would be <mv,0,0>?

 

[Doc Al]

Perpendicular? so it would be <mv,0,0>?

Momentum has nothing to do with it. Centripetal force always acts towards the center of the motion.

 

[ohheytai]

okay so it is <mv,0,0> since the center is in the right?

 

[Doc Al]

okay so it is <mv,0,0> since the center is in the right?

You have the correct direction, but 'mv' has nothing to do with it.

 

[ohheytai]

ohhh wait wouldn't it be <Fperpendicular,0,0>?

 

[Doc Al]

ohhh wait wouldn't it be <Fperpendicular,0,0>?

If by Fperpendicular you mean the centripetal force that you calculated, then yes.

 

[ohheytai]

okay thanks you!