# Homework Help: Static Equilibrum

1. Oct 30, 2016

### Tchao

1. The problem statement, all variables and given/known data
Which cases are examples of static equilibrium?

1. A box sliding down a ramp.
2. A baseball after being hit.
3. A box sliding along a horizontal surface.
4. A perfectly upright freely spinning top
5. A freely swinging simple pendulum
6. A spinning astronaut floating out in deep space.
7. A satellite orbiting Earth

2. Relevant equations

ΣF = 0
Στ = 0

3. The attempt at a solution
I believe it is options 4, 6 and 7.

2. Oct 30, 2016

### Student100

What's the definition of static equilibrium? Why do you believe 4, 6, or 7 apply?

(Hint, is torque zero for 4 and 6? Are the forces on 7 0? Is this theoretically possible under certain conditions?)

3. Oct 30, 2016

### TomHart

What is the definition of static equilibrium? Here is one I found:
If an object is at rest and is in a state of equilibrium, then we would say that the object is at "static equilibrium."

And another one:
"Static" refers to the object being motionless while "equilibrium" refers to the object either having no net forces acting upon it or having all of its net forces balanced.

So based on that, it isn't clear to me whether the spinning top and spinning astronaut are in static equilibrium. Maybe they are. But any object that is accelerating certainly is not in a state of equilibrium. Which of the 7 items above are undergoing acceleration?