Solving Frictionless Slides: No Mass Required!

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Homework Help Overview

The problem involves a frictionless water slide where a slider starts from rest and descends a 30 m long incline set at a 25-degree angle. The original poster seeks to determine the speed of the slider at the bottom and to understand why the mass of the slider is not necessary for the calculations.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning, Assumption checking

Approaches and Questions Raised

  • Participants discuss using both force equations (F = ma) and conservation of energy to approach the problem. There is uncertainty about how to incorporate mass into the calculations, with some questioning the need for mass in determining acceleration and velocity.

Discussion Status

The discussion is active, with participants exploring different methods to find acceleration and velocity. Some have proposed using gravitational components, while others express confusion about the calculations and the role of mass. There is no clear consensus on the best approach, but various lines of reasoning are being examined.

Contextual Notes

Participants note the challenge of having multiple unknowns in their equations and the implications of using different methods, which leads to varying results. The discussion reflects a mix of attempts to clarify concepts and resolve the problem setup.

runningirl
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Homework Statement



A water slide can be though of as being frictionless. Let's say a slider starts from rest and goes down a slide that is 30 m long and is set at a 25 degree incline.

a) determine the speed of the slider at the bottom of the slide.

b) Explain why you don't need to know the mass of the slider.

Homework Equations



f=ma

The Attempt at a Solution



sin(25)*30=a
a=12.7 m/s/s.

i don't know if that's right.
would i not need the mass for the acceleration?
 
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hi runningirl! :smile:
runningirl said:
sin(25)*30=a
a=12.7 m/s/s.

you can solve this either by F = ma or by conservation of energy

i assume you want to use F = ma (in the direction of the slope) …

ok, if the mass is m, then what is F? :smile:
 
As the slide is frictionless, this is a fairly simple constant-acceleration problem. The only acceleration is in the y-direction, you can use a constant-acceleration equation to determine the downward velocity. Since you know the angle, you can then calculate/convert v_y to v.
 
tiny-tim said:
hi runningirl! :smile:


you can solve this either by F = ma or by conservation of energy

i assume you want to use F = ma (in the direction of the slope) …

ok, if the mass is m, then what is F? :smile:


uh... I'm not quite sure if i can find the acceleration since i have two unknown variables with f=ma.
 
why are you using distance?

use F = ma (in the direction of the slope)
 
but then i'll have two unknowns.
 
write it out anyway … something should cancel
 
uh...

f=m(12.7m/s/s)?
 
where does the 12.7 come from? :confused:

start from the beginning
 
  • #10
then it would just be f=ma because i don't have a force, mass, or acceleration.
 
  • #11
tiny-tim said:
where does the 12.7 come from? :confused:

start from the beginning

i think i figured it out.

i did ma=m(9.8)(sin25)
a=4.14

but i did it another way and got a different answer...

m(h)=.5(ma^2)
30/sin(25)=.5ma^2

a=5.96 m/s/s.
 
  • #12
(just got up :zzz: …)
runningirl said:
i think i figured it out.

i did ma=m(9.8)(sin25)
a=4.14

yes that's completely correct :smile:

(an alternative way is simply to say that the acceleration is the component of the gravitaitonal acceleration in the direction of the slope, ie 9.8cos65°)
but i did it another way and got a different answer...

m(h)=.5(ma^2)
30/sin(25)=.5ma^2

a=5.96 m/s/s.

i think you're thinking of conservation of energy …

mgh = .5mv2

(i can't think of any formulas with a2 in them)
 

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