# Homework Help: HELP free fall

1. Sep 28, 2008

### garva1

an object is thrown upwards, at the top of the path it has
a) zero acceleration
b) maximum momentum
c) maximum kE
d) is at equillibrium
e)none of the above

i know accelarion is 9.8m/s^2
i also know is not KE since at the top is potential
im confused of the other two

2. Sep 28, 2008

### LowlyPion

b) what is formula for momentum?
d) what is equilibrium?

3. Sep 28, 2008

### garva1

those are the ones that i dont know i think momentum is = mv

4. Sep 28, 2008

### chislam

Ok, so if you know that P = mv, and the object is at the max height of its trajectory, then what would be it's velocity and momentum?

5. Sep 28, 2008

### garva1

v is zero

6. Sep 28, 2008

### garva1

ok vo is zero but the final velocity =v + gt but im confused

7. Sep 28, 2008

### gabbagabbahey

yes, v=0 at the top, so what is the momentum at the top?

8. Sep 28, 2008

### garva1

9. Sep 28, 2008

### gabbagabbahey

look up the definition of mechanical equilibrium

10. Sep 28, 2008

### garva1

i dont get the definition is says something about rest but so does the free fall at top have equllibrium but then we only have one force gravity since air is neglected

11. Sep 28, 2008

### LowlyPion

OK. Is it at rest at the top?

12. Sep 28, 2008

### gabbagabbahey

http://en.wikipedia.org/wiki/Mechanical_equilibrium" [Broken]

"A rigid body is in mechanical equilibrium when the sum of all forces on all particles of the system is zero, and also the sum of all torques on all particles of the system is zero."

What forces are at play here? Is the net force on the particle zero at the top of its path?

Last edited by a moderator: May 3, 2017
13. Sep 28, 2008

### garva1

there is only gravity

14. Sep 28, 2008

### garva1

i dont get equllibrium sorry :(

15. Sep 28, 2008

### chislam

Ok, try and think fundamentally. At the very top, the instantaneous velocity is zero. If the velocity is zero then the net acceleration must also be zero at that very instant. So if F = ma, then the net force must be zero too. If you are still confused, I frankly do not know how else to explain it.

16. Sep 28, 2008

### garva1

so at the top there is equllibrium even though there is only one force the sum will be concidered zero

17. Sep 28, 2008

### garva1

"If the velocity is zero then the net acceleration must also be zero at that very instant. So if F = ma,"
but how does velocity applied in the equation f=ma

18. Sep 28, 2008

### garva1

but i thought that the acceleration was gravity and gravity was throughout the whole path

19. Sep 28, 2008

### gabbagabbahey

That's incorrect, the velocity will be zero, but it will still be accelerating downwards; the net force will still just be the force due to gravity, which is nonzero.

20. Sep 28, 2008

### gabbagabbahey

This thought is correct; see above^^^.