Can you find the applied force with just the initial and final velocties?

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Homework Help Overview

The discussion revolves around a physics problem involving a ball being pushed down a carpeted incline at an angle of 24°. The original poster seeks to determine the applied force on the ball given its initial velocity of 1 m/s and final velocity of 5 m/s over a distance of 6 meters.

Discussion Character

  • Exploratory, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • Participants explore the implications of not providing the mass of the ball, questioning how this affects the ability to calculate force. Some discuss dimensional analysis and the necessity of mass in force calculations. Others consider the role of rolling friction and energy conservation in the context of the problem.

Discussion Status

The discussion reflects a lack of consensus on whether the problem can be solved with the given information. Some participants suggest that the problem is unsolvable without additional data, while others propose that it might be approached through energy considerations, albeit with uncertainties regarding the mass and other variables.

Contextual Notes

Participants note that the problem's constraints, such as the absence of mass and the nature of the incline, limit the ability to derive a solution. The discussion also touches on the appropriateness of the chosen object (a ball versus a block) in the context of the problem.

glenohumeral13
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Homework Statement


I'm trying to create a problem for my physics standards that is hard. I gave myself a ball being pushed down a carpeted decline at 24° from the horizontal. The balls starts with an initial velocity of 1 m/s and ends the 6 meter decline at 5 m/s. I want to find the applied force on the ball.

Homework Equations


Perhaps "1/2vf2 = g*hi"?

The Attempt at a Solution


Found a stalemate because I don't want to give the mass, or else the problem is too easy. So, I can't use "Fg = m*ag".
 
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Do you know about dimensional analysis? This would tell you that force involves a mass dimension, so you cannot calculate a force from only distance and time dimensions.
(Why a ball? Wouldn't a ball roll?)
 
I assume that you mean the rolling friction force of the ball with the carpeted incline... (?)
 
haruspex said:
Do you know about dimensional analysis? This would tell you that force involves a mass dimension, so you cannot calculate a force from only distance and time dimensions.
(Why a ball? Wouldn't a ball roll?)
Okay. Thanks.
Yeah, it probably should have been a block.
 
NTW said:
I assume that you mean the rolling friction force of the ball with the carpeted incline... (?)
Could I find that with the given information?
 
glenohumeral13 said:
Could I find that with the given information?
No. The problem is unsolvable - in the same way the question "I am in a car and accelerate. It is 2 pm. Find the mass of the car" is not solvable.
 
glenohumeral13 said:
Could I find that with the given information?

I'm not sure... Let's see... The final velocity of the ball will be the same in free fall or rolling with no resistance along the incline. That velocity would be v = SQR (2 * 9,81 * 6 m * sin 24º) = 6,91 m/s. You state that there is an extra initial velocity of 1 m/s, so we would have a total of 7,91 m/s.

But 5 m/s is mentioned in the problem as final velocity. Thus, there is a braking force. It could come from rolling resistance and from rotational kinetic energy acquired by the ball during its run... Rolling resistance is a function of the mass of the ball, g, the angle and a coefficient mu. Rotational kinetic energy is a function of the angular velocity w, itself a funcion of v and the ball's radius r, and of the moment of inertia of a sphere 2/5 * m* r^2

Mass is not given. It might cancel away, I'm not sure, but the problem could perhaps be solved 'by energies', deriving the solution also in terms of the unknown magnitudes, maybe m (if it doesn't cancel away), a coefficient of rolling resistance mu, and the ball's radius r...

That, in case it can be solved at all... I am myself a solver of easy problems only...
 
NTW said:
I'm not sure... Let's see...
As I wrote in post #2, dimensional analysis proves there is not enough information. You have to be given a quantity with a mass dimension - could be mass, force, energy, momentum..., but distances times and accelerations by themselves cannot do it.
 
mfb said:
No. The problem is unsolvable - in the same way the question "I am in a car and accelerate. It is 2 pm. Find the mass of the car" is not solvable.
Haha. Okay. Thanks everybody. I'll just do something with μk and the coefficient then.
 

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