Homework Help: What is the magnitude of the upward force of friction

kumar_23 · Mar 7, 2007

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?

please help im really confuzzeled

cristo · Mar 7, 2007

Since this is a homework question, please show some work before we can help you.

kumar_23 · Mar 7, 2007

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

kumar_23 · Mar 7, 2007

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

kumar_23 · Mar 7, 2007

can someone help me

hage567 · Mar 7, 2007

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

kumar_23 · Mar 7, 2007

help

what do you mean?
what am i adding together?

kumar_23 · Mar 7, 2007

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

kumar_23 · Mar 7, 2007

can u please reply? thanks so much for the help =)

hage567 · Mar 7, 2007

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

kumar_23 · Mar 7, 2007

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

hage567 · Mar 7, 2007

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?

kumar_23 · Mar 7, 2007

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

kumar_23 · Mar 7, 2007

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

hage567 · Mar 7, 2007

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 · Mar 7, 2007

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

kumar_23 · Mar 7, 2007

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

kumar_23 · Mar 7, 2007

sorry about the repost.
frictional force is upwards away from the object

hage567 · Mar 7, 2007

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

kumar_23 · Mar 7, 2007

frictional force minus gravity (not sure)

hage567 · Mar 7, 2007

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

hage567 · Mar 7, 2007

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

kumar_23 · Mar 7, 2007

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

kumar_23 · Mar 7, 2007

yes but how is being used with this situation ?

kumar_23 · Mar 7, 2007

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