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

• ohheytai
In summary, a 1.2 kg ball is moving in a circle at constant speed, with the center of the circle at the origin. You can't see what's causing the motion, but you can conclude that the direction of the vector Fnet must be the same as the vector of the ball's momentum and velocity.
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!

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

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

ohheytai said:
so would it be <0,j,0>?
No.
and what about the 2nd questionnn?
First find the correct direction for Fnet, expressed as a vector. Then you can just use the vector equation Fnet = F1 + F2 to solve for the unknown force.

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

ohheytai said:
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.

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?

ohheytai said:
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?

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

ohheytai said:
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.

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

ohheytai said:
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.

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

ohheytai said:
ohhh wait wouldn't it be <Fperpendicular,0,0>?
If by Fperpendicular you mean the centripetal force that you calculated, then yes.

okay thanks you!

Last edited:

## 1. What is a vector and how is it different from a scalar?

A vector is a mathematical quantity that has both magnitude (size or value) and direction. This is different from a scalar, which only has magnitude and no direction. For example, velocity is a vector because it has both magnitude (speed) and direction (e.g. north or south).

## 2. How is Fnet (net force) calculated for a system of multiple forces?

Fnet is calculated by adding all the individual forces acting on an object, taking into account their magnitudes and directions. This can be calculated using vector addition, where the individual forces are represented as vectors with an arrow indicating their direction and the length of the arrow representing their magnitude. The sum of these vectors will give the net force acting on the object.

## 3. What is the significance of time t=0 in solving for Fnet?

Time t=0 is the starting point or initial time for the system. It is the moment at which the net force is being calculated. It is important to specify a time, as the net force acting on an object can change over time due to the addition or removal of forces.

## 4. How does the direction of Fnet affect the motion of an object?

The direction of Fnet determines the direction of the object's acceleration. According to Newton's Second Law, Fnet is directly proportional to the acceleration of an object. So, if Fnet is in the same direction as the object's motion, it will cause the object to speed up. If Fnet is in the opposite direction, it will cause the object to slow down.

## 5. Can Fnet ever be zero? What does this mean for the object's motion?

Yes, Fnet can be zero. This means that all the forces acting on the object are balanced and there is no acceleration. The object will either be at rest or moving at a constant velocity (if it was already in motion). This is known as equilibrium, and it can occur when the individual forces are equal in magnitude but opposite in direction, or when there are no forces acting on the object at all.

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