Finding the force of a person given mass.


by mooney82
Tags: force, mass, person
mooney82
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#1
Sep19-09, 09:24 AM
P: 13
1. The problem statement, all variables and given/known data

A 30-kg child on roller skates starts up a 10 degree incline at 15 km/h. Assuming she does not propel herself how far up the incline does she travel before stopping? Ignore frictional losses.


2. Relevant equations
F=ma W=mg

3. The attempt at a solution

I can do this problem if the force is given, but our book doesn't explain how to get force from mass, also would the 15 km/hr have anything to do with it? I've tried to work the problem out several times but I must be using the wrong value for force.
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Doc Al
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#2
Sep19-09, 09:28 AM
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Quote Quote by mooney82 View Post
I can do this problem if the force is given, but our book doesn't explain how to get force from mass,
The force acting is the weight, which you should be able to calculate. What's the component of the weight parallel to the incline?
also would the 15 km/hr have anything to do with it?
The faster her initial speed, the longer it takes her to come to rest.
mooney82
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#3
Sep19-09, 09:34 AM
P: 13
So the weight is just m*g, which is 294.3 N?

The x component of mg would be 294.3cos10?

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Sep19-09, 09:41 AM
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Finding the force of a person given mass.


Quote Quote by mooney82 View Post
So the weight is just m*g, which is 294.3 N?
Right.
The x component of mg would be 294.3cos10?
No, that's the component perpendicular to the incline (the y-component).
mooney82
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#5
Sep19-09, 09:53 AM
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Quote Quote by Doc Al View Post
Right.

No, that's the component perpendicular to the incline (the y-component).
But if i set the x axis on the incline wouldn't the x component be the adjacent side?
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#6
Sep19-09, 11:11 AM
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Quote Quote by mooney82 View Post
But if i set the x axis on the incline wouldn't the x component be the adjacent side?
You need to understand how force components are found on inclined planes. Read: Inclined Planes
mooney82
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#7
Sep19-09, 11:24 AM
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Quote Quote by Doc Al View Post
You need to understand how force components are found on inclined planes. Read: Inclined Planes
Ok, so does the force from the girl start parallel to the ground or the incline? I think that is where I might be getting it wrong. I've been setting it parallel to the ground.
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#8
Sep19-09, 11:27 AM
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Quote Quote by mooney82 View Post
Ok, so does the force from the girl start parallel to the ground or the incline? I think that is where I might be getting it wrong. I've been setting it parallel to the ground.
The weight acts downward--it has no horizontal component. Find the component parallel to the incline--that's the direction she's moving in.
mooney82
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#9
Sep19-09, 11:50 AM
P: 13
Quote Quote by Doc Al View Post
The weight acts downward--it has no horizontal component. Find the component parallel to the incline--that's the direction she's moving in.
Ahhh I get it, you see the only example my professor gave in class was with an outside force pushing at an object, here there isn't any other force besides her weight, or mg pointing straight down! Thanks, my professor sucks at explaining things, thank you very much!!!!!


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