- #1
Adoniram
- 94
- 6
Question 1:
1. Homework Statement
A bug of mass m crawls radially outwards with a constant speed v’ on a disc that
rotates with a constant angular velocity ω about a vertical axis. The speed v’ is relative to the
center of the disc. Assume a coefficient of static friction μ, find out where on the disc the bug
starts to slip.
F = ma = ΣF_i
The question asks when the force of friction is finally overcome, so I think:
ma = 0 = F_friction - F_cent.
Or
F_friction = F_cent.
μmg = (mv2/r)
v = wr
Solving for r:
r = (1/ω) √(2μg)
Seems too easy... Is this right?Question 2:
1. Homework Statement
A particle is placed on top of a smooth (frictionless) sphere of radius R. If the particle is slightly
disturbed, at what point will it leave the sphere?
Same as first question, just
F = ma = ΣF_i
Similarly, we want to know when the normal force of the sphere on the particle is overcome:
F_norm = F_cent
mg CosΘ = (mv2/r)
CosΘ = y/R (where y is the height above the center of the sphere)
So:
y = v2/g
Finding v2:
Using conservation of energy, PE_initial = PE_final + KE_final
mgR = mgy + mv2/2
Solving for v2
v2 = 2g(R-y)
Placing into equation for y:
y = 2g(R-y)/g = 2(R-y)
Solving for y:
y = (2/3) R
Correct? Or am I making a horrible mistake?
1. Homework Statement
A bug of mass m crawls radially outwards with a constant speed v’ on a disc that
rotates with a constant angular velocity ω about a vertical axis. The speed v’ is relative to the
center of the disc. Assume a coefficient of static friction μ, find out where on the disc the bug
starts to slip.
Homework Equations
F = ma = ΣF_i
The Attempt at a Solution
The question asks when the force of friction is finally overcome, so I think:
ma = 0 = F_friction - F_cent.
Or
F_friction = F_cent.
μmg = (mv2/r)
v = wr
Solving for r:
r = (1/ω) √(2μg)
Seems too easy... Is this right?Question 2:
1. Homework Statement
A particle is placed on top of a smooth (frictionless) sphere of radius R. If the particle is slightly
disturbed, at what point will it leave the sphere?
Homework Equations
Same as first question, just
F = ma = ΣF_i
The Attempt at a Solution
Similarly, we want to know when the normal force of the sphere on the particle is overcome:
F_norm = F_cent
mg CosΘ = (mv2/r)
CosΘ = y/R (where y is the height above the center of the sphere)
So:
y = v2/g
Finding v2:
Using conservation of energy, PE_initial = PE_final + KE_final
mgR = mgy + mv2/2
Solving for v2
v2 = 2g(R-y)
Placing into equation for y:
y = 2g(R-y)/g = 2(R-y)
Solving for y:
y = (2/3) R
Correct? Or am I making a horrible mistake?