How to Determine the Location and Velocity of a Rolling Ball Down a Smooth Hill?

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Discussion Overview

The discussion centers around determining the location and velocity of a rolling ball down a smooth, differentiable, and frictionless hill. Participants explore the theoretical framework for analyzing motion in this context, including the implications of rolling versus sliding, and the necessary parameters for calculations.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions the feasibility of rolling a ball down a frictionless hill, suggesting it would only slide down.
  • Another participant emphasizes the importance of the hill's angle and gravitational acceleration in determining the ball's motion.
  • A different viewpoint argues that radius and moment of inertia are necessary for a rolling ball, indicating a distinction between rolling and sliding cases.
  • One participant provides a detailed energy conservation equation for a rolling ball, including expressions for both rotational and linear kinetic energy, leading to a derived formula for velocity.
  • Another participant challenges the concept of a one-dimensional hill, asserting that without friction, only linear acceleration is relevant, and discusses the implications of dimensionality on rolling resistance.
  • There is a reiteration of the need for radius and moment of inertia when considering the rolling case, while also noting that without friction, there can be no angular acceleration.

Areas of Agreement / Disagreement

Participants express differing views on the nature of motion (rolling vs. sliding) and the necessary parameters for analysis. There is no consensus on whether the discussion should focus on rolling dynamics or sliding dynamics, and the implications of friction are contested.

Contextual Notes

Limitations include assumptions about the hill's dimensionality and the role of friction in the analysis. The discussion does not resolve how these factors influence the motion of the ball.

exmachina
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newbie question, say I'm rolling a ball down a smooth , differentiable, and frictionless 1-Dimensional hill V(x) from a point x_i

find the location x(t) and the velocity dx/dt(t) of this ball at some arbitrary time t.

what would be the general approach towards such a problem?

note that for x_i such that dV/dx(x_i) = 0, dx/dt=0
 
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You can't roll a ball down a frictionless hill, you can only slide it down a frictionless hill.

You can use the angle of the hill and geometry to find the acceleration. You need no other information besides the angle of the hill and the value of g.
 
I disagree with russ watters. Don't you need Radius and moment of Inertia, Or that you are telling about sliding case. At that case you are right.
 
In the rolling case for a hard solid ball of uniform density without friction;

m*g*h = rotational K.E + linear K.E

mgh = (1/2)*I*w^2 + (1/2)*m*v^2
I = moment of inertia
w = angular velocity of ball
v = linear speed

now a rolling ball of diameter r turning at frequency has a linear speed v of;
v = 2*pi*r*f = w*r

so w = v/r

Therefore:
mgh = (1/2)*I*w^2 + (1/2)*m*v^2
becomes:
mgh = (1/2)*I*(v/r)^2 + (1/2)*m*v^2
= (1/2)*((2/5)*m*r^2)*(v/r)^2 + (1/2)*m*v^2
g*h = (1/2)*((2/5)*r^2)*(v/r)^2 + (1/2)*v^2
= (7/10)*v^2
v= sqrt((10/7)*g*h

So if h = 1m, d = 1m
v = 3.72 m/s
 
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You can't have a one dimensional "hill". You can have a one dimensional line and some constant force or constant acceleration.

If there is no friction, then there is no angular acceleration, so only the linear acceleration matters.

If the hill has 2 or 3 dimensions, isn't frictionless, then the rolling resistance will be greater than m*g*cos(θ)*(b/r), because rolling resistance is related to the total force between surfaces, not just the normal component.
 
thecritic said:
I disagree with russ watters. Don't you need Radius and moment of Inertia, Or that you are telling about sliding case. At that case you are right.
Radius and moment of inertia are needed if the ball is to be spun, but if there is no friction, there is no tangential force applied to the ball and thus no way to spin it.
 
To keep the responses to my questions in one place I will post in the other thread.
 
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