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.
Answer: ?

Any hints? or something? i know some of the formulas, but i don't 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.
Answer: 1.96 m/s^2
(b) Find the coefficient of kinetic friction between the block and the incline.
Answer: ??
(c) Find the frictional force acting on the block.
Answer: ?
(d) Find the speed of the block after it has slid a distance of 2.20 m.
Answer: 2.94 m/s

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.
Answer: ?
Find the normal force exerted by the incline on the block. (Disregard friction.)
Answer: ?

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

COS(30)= Adjacent Angle/hypotenuse --> COS(30)=Y/((-9.81)(20))

(acceleration)(mass) = F

i got Fgy (Fy poiting down) =-169.9, so the Natural force should be 169.9 right? but the thing says I am 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

 

[arildno]

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.

 

[M98Ranger]

for reaching out for help! It's completely normal to struggle with certain concepts in physics, so don't feel stupid. It's important to ask for help and keep trying until you understand.

For the first problem, you can use the formula Ff = μkFn, where Ff is the frictional force, μk is the coefficient of kinetic friction, and Fn is the normal force (the force perpendicular to the surface). In this case, since the box is sliding down the ramp, the frictional force is acting in the opposite direction of motion. So you can set up the equation as -Ff = μkFn.

To find the normal force, you can use the formula Fn = mgcosθ, where m is the mass of the box, g is the acceleration due to gravity (9.81 m/s^2), and θ is the angle of the ramp (31.5° in this case). Then you can substitute this value for Fn into the equation for frictional force and solve for μk.

For the second problem, you can use the formula a = (Fnet - Ff)/m, where a is the acceleration, Fnet is the net force (in this case, the component of the force of gravity parallel to the incline), Ff is the frictional force, and m is the mass of the block. You can use the acceleration you found in part (a) to solve for the net force, and then use that value to find the frictional force using the same equation as in the first problem.

For the normal force, you can use the same formula as in the first problem (Fn = mgcosθ), but this time you need to account for the component of the force of gravity perpendicular to the incline. This would be Fgcosθ, where Fg is the force of gravity (mg). So your equation for Fn would be Fn = mgcosθ + Fgcosθ.

As for the third problem, it's difficult to give specific help without seeing your work. Make sure you are accurately drawing the free body diagram and labeling all forces correctly. Also, double check your calculations to make sure you are using the correct values and units. If you're still having trouble, try breaking down the problem into smaller steps and tackling them one at a time. And don't forget to ask your teacher or classmates for help if you need it. Good luck!