Find the frictional force acting on the block

Graif

After seeing some of the problems that you are asking help on, i feel really stupid, since mine seem so simple compared to yours. but here it is anyway

I am really having trouble with Friction here, how exactly do u get the coefficient of kinetic friction? I have 2 problems here:

1)A box slides down a 31.5° ramp with an acceleration of 1.28 m/s2. Determine the coefficient of kinetic friction between the box and the ramp.

Any hints? or something? i know some of the formulas, but i dont really know what to do. tried many things, and it gives me a wrong answer

2)A 3.20 kg block starts from rest at the top of a 30.0° incline and slides 2.20 m down the incline in 1.50 s.

(a) Find the acceleration of the block.
(b) Find the coefficient of kinetic friction between the block and the incline.
(c) Find the frictional force acting on the block.
(d) Find the speed of the block after it has slid a distance of 2.20 m.

I did find 2 of the answers in #2 but again, friction is killin me here

Now I also have a normal dynamics/kinematics problem which i though i knew, but webassign keeps marking it wrong. here it is:

3) A block with a mass of 20 kg is held in equilibrium on an incline of angle = 30.0° by the horizontal force, F, as shown in Figure 4-31. Find the magnitude of F.
Find the normal force exerted by the incline on the block. (Disregard friction.)

Figure 4-31 is just a ramp (a right triangle) with a cube above it, and the F arrow is horizontally poiting to the right above the cube.

in this one I drew my free body diagram, all pretty and stuff, and calculated the Fn to be 169.9 and 170, i well, drew the vectors down Fg and Fy, connected them to form a triangle, applied

(acceleration)(mass) = F

i got Fgy (Fy poiting down) =-169.9, so the Natural force should be 169.9 right? but the thing says im wrong.. :uhh:
This assigment is due today at 7am, if anybody can help me, please anwer this threat, PM me, or IM me to aim(cronowarrior007) or yahoo (coolkegcomputer) or MSN (coolcomputer7@hotmail.com).

Thanks so much

Related Introductory Physics Homework News on Phys.org

arildno

Homework Helper
Gold Member
Dearly Missed
Welcome to PF, Graif!
1)
Write Newton's 2.law out in the two components:
Tangential to the ramp and normal to the ramp.
Identify the terms you know, and remember the relation between frictional force and normal force.

Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving