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# Mechanics dynamics watch

1. Question:
A boy of mass 40kg slides from rest down a straight slide of length 5m. The slide is inclined to the horizontal at an angle of 20 degrees. the coefficient of friction between the boy and the slide is .1. find
a) acceleration of boy
B) speed of boy at bottom of the slide

I have resolved it and assume you will need f=ma amd friction = mu R

but cannot find r as we are not given the fprce acting

2. (Original post by monkeyytastic)
Question:
A boy of mass 40kg slides from rest down a straight slide of length 5m. The slide is inclined to the horizontal at an angle of 20 degrees. the coefficient of friction between the boy and the slide is .1. find
a) acceleration of boy
B) speed of boy at bottom of the slide

I have resolved it and assume you will need f=ma amd friction = mu R

but cannot find r as we are not given the fprce acting

To find the acceleration you have to find the resultant force acting on the boy: force acting down the slope - Friction acting up the slope.

R is the force, on the boy, acting perpendicular to the slope. It has to be worked out by using the weight of the boy, and the angle of the slope.

Sorry if it's not explained brilliantly, but I don't want to TELL you how to do it, I want to HELP you work it out.
3. (Original post by insane2642)
To find the acceleration you have to find the resultant force acting on the boy: force acting down the slope - Friction acting up the slope.

R is the force, on the boy, acting perpendicular to the slope. It has to be worked out by using the weight of the boy, and the angle of the slope.

Sorry if it's not explained brilliantly, but I don't want to TELL you how to do it, I want to HELP you work it out.

Hi,

thank you, but i'm still extremely confused. i've tried loads of ways but get an answer way out. i usually find r by resolving the weight and the force given and adding the verticle resolved parts. but in this case we are not given the force acting so don't know how to work out r which means i can't work out f. if you could please explain how u get r i think i can do the rest?

thanks once again
4. What do you mean by "the force acting"?

There are three forces here, gravity, the normal reaction from the slide (acts perpendicular to the slide) and the frictional force (acts parallel to the slide).

Since the boy stays on the slide, the net force perpendicular to the slide is 0. This lets you find the normal reaction.
And then knowing the normal reaction lets you find the frictional force.

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