# easiest physics problem

tomtom232
Posers and Puzzles 14 Nov '07 00:38
1. 14 Nov '07 00:38
A 0.500 kg mass on a 3.20 meter string moves 2.5 radians/sec (1 revolution = 2 x pie x radians) in a horizontal circle. What angle does the string make with the vertical?
2. 14 Nov '07 00:451 edit
Originally posted by tomtom232
A 0.500 kg mass on a 3.20 meter string moves 2.5 radians/sec (1 revolution = 2 x pie x radians) in a horizontal circle. What angle does the string make with the vertical?
heres another one

A sled weighing 200N rests on a 15 degree incline, held in place by static friction. The coefficient of static friction is 0.5. (a) What is the magnitude of the normal force on the sled? (b) Whatis the magnitude of the static friction on the sled? (c) The sled is now pulled up the incline at a constant speed by a child. The child weighs 500N and pulls on the rope with a constant force of 100N. The rope makes an angle of 30 degrees with the incline and has negligible weight. What is the magnitude of the kinetic fricition force on the sled? (d) What is the coefficient of kinetic friction between the sled and the incline?
3. 14 Nov '07 07:40
first question is simple conical pendulum stuff:

let tension=T, weight = mg, angular velocity = w, length of string l, angle x.
resolve forces vertically:
Tcos[x] = mg
horizontally:
Tsin[x] = m(w^2) x lsin[x]
so T = m(w^2)l

subsituting into the first equation:
cos[x] = g/(w^2)l

and so x = 1.1, about 61 degrees.
4. PBE6
Bananarama
14 Nov '07 14:43
Originally posted by etotheipi
first question is simple conical pendulum stuff:

let tension=T, weight = mg, angular velocity = w, length of string l, angle x.
resolve forces vertically:
Tcos[x] = mg
horizontally:
Tsin[x] = m(w^2) x lsin[x]
so T = m(w^2)l

subsituting into the first equation:
cos[x] = g/(w^2)l

and so x = 1.1, about 61 degrees.
I agree, but x here is the angle the rope makes with the vertical. The "dip" is the additive inverse, 90 - 61 = 29 degrees.
5. 14 Nov '07 18:48
Originally posted by PBE6
I agree, but x here is the angle the rope makes with the vertical. The "dip" is the additive inverse, 90 - 61 = 29 degrees.
Huh? ðŸ˜• Doesn't the question ask for the angle with the vertical?
6. 14 Nov '07 19:01
for the second one:

let normal reaction force = R, frictional force up plane = F, coeff. of friction = m.

a)
resolving parallel to the plane:
R = 200cos[15] = 193N

b)
limiting equilibrium implies F = mR
so F = 97N

c)
the frictional force now acts down the plane, so
resolving along the plane:
F = 100cos[30] = 87N

d)
resolving parallel to the plane:
R + 100sin[30] = 200
so R = 150N
limiting equilibrium implies F = mR
so m = 0.58

Last part im not so sure about -- can you use limiting equilibrium in this case??
7. PBE6
Bananarama
14 Nov '07 22:10
Originally posted by etotheipi
Huh? ðŸ˜• Doesn't the question ask for the angle with the vertical?
Sorry, you're right.
8. 14 Nov '07 22:23
Wow, ignore my parts c and d from above as I have ignored a force ðŸ˜²

c)
frictional force now acts down the plane
resolving along plane:
F + 200sin[15] = 100cos[30]
so F = 35N

d)
resolving parallel to the plane:
R + 100sin[30] = 200cos[15]
so R = 143N
limiting equilibrium implies F = mR
so m = 0.24

I hope ðŸ™‚