# What is the magnitude of the upward force of friction

hi guys :surprised

can anyone help me with this?
a tree house has a vertical "fire pole" of smooth metal, designed for quick exits. a child of mass 35.7 kg slides down the pole with constant acceleration, starting from rest. the pole is 3.10 m high. the trip to the ground takes 2.00 s. what is the magnitude of the upward force of friction exerted by the pole on the child?

cristo
Staff Emeritus

unknown = a d = v1(time) + 0.5(a)(time)2 ....time is squared
known
time= 2.00s a = 1.55m/s/s
distance = 3.10 m
v1= 0 m/s

this is what i have so far. what do i do after this? plz help

can someone help me

hage567
Homework Helper
Use Newton's second law to sum up the forces. Solve for the frictional force.

help

what do you mean?

i dont understand because right now i have the net force and acceleration.

hage567
Homework Helper
What forces are present? Can you tell me that? And their direction.

umm.....there is net force and frictional force and that's it.

hage567
Homework Helper
No, the net force is the difference between these two other forces. What is causing the kid to go down the pole in the first place?

sorry for the wait. i dont understand what you're asking?

"a tree house has a vertical "fire pole" of smooth metal"

hage567
Homework Helper
I'm asking what forces are acting on the child as he goes down the pole. The is the frictional force, and there is one more. Can you tell me what it is? (What makes things go towards the Earth?)
Do you know how to draw free body diagrams?

hage567
Homework Helper
OK, so you have gravity acting downwards, right? The frictional force acts which way?

Eureka! gravity is the force and yes, i know how to draw FBDs.

frictional force is upwards away from the object

hage567
Homework Helper
Yes, that's right. So what is your net force then?

frictional force minus gravity (not sure)

hage567
Homework Helper
Yes, that's the right idea. So use that in Newton's second law, solving for the frictional force.

hage567
Homework Helper
You don't know Newton's second law? Does F=ma look familiar?

sorry i'm a little slow at this because it's very new to me

yes but how is being used with this situation ?

doesn't f=ma have to do with acceleration and mass multplied together to give us fnet?

are you still there?

hage567
Homework Helper
You said that "frictional force minus gravity" is your net force. I would change this to say "gravitational force - frictional force" (since I would call "down" the positive direction).
So on one side of your equation you will have "gravitational force - frictional force" and the other you will have "ma". Do you know how to calculate the gravitational force? You already know m and a. So isolate the frictional force (call it "f"), and solve for it.

It asks " what is the magnitude of the upward force of friction exerted by the pole on the child? " would that have direction and would it be in newtons?

cristo
Staff Emeritus
kumar, your replies seem to just be whatever you're thinking at that time. Consider the question before composing a reply.
kumar_23 said:
It asks " what is the magnitude of the upward force of friction exerted by the pole on the child? " would that have direction and would it be in newtons?

What do you think? Does it have a direction (or is the direction already specified in the question?). What is force measured in?

hage567
Homework Helper
Yes, it would be in newtons. Usually if a question just asks for the "magnitude" it does not care about the direction. In this case, it is stated right in the question that it is an upwards force, so I would assume you could just leave it at that.