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Homework Mass Question

  • Thread starter XPX1
  • Start date
55
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1. Homework Statement
On a planet far, far away, an astronaut picks up a rock. The rock has a mass of 4.40 kg, and on this particular planet its weight is 40.0 N. If the astronaut exerts an upward force of 49.5 N on the rock, what is its acceleration?

Magnitude _____m/s2
Direction
up
down


2. Homework Equations
Acceleration = Velocity/Time


3. The Attempt at a Solution

I know that the rock is moving in the upward direction, because the force applied to the rock is more than its mass. One thing that I do not understand however, is how to find the acceleration. I am given the velocity, 49.5 but I do not get the time. How can I figure out the time knowing that the velocity is 49.5 and the rock weighs 40 N?
 
55
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anyone got any ideas on this?
 
Think about it this way.
The key law that you're supposed to use is that the net force on an object = its mass times its acceleration, or F = ma...
Now, you have 2 forces, one acting up and one acting down...
Read between the lines. :)
 
55
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Well if F = ma then

49.5 = 40a

49.5/40= a

a = 1.2375 but the answer is not correct, what did I do wrong!?
 
How much force would the astronaut have to exert to keep it in equilibrium?
 

PhanthomJay

Science Advisor
Homework Helper
Gold Member
7,115
456
Well if F = ma then

49.5 = 40a

49.5/40= a

a = 1.2375 but the answer is not correct, what did I do wrong!?
No, it's F_net =ma. The 49.5N force acts up, and the gravity force (weight)acts down. So what is F-net??
 
55
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So 49.5-40! = Fnet Fnet = 9.5

9.5/4.40 = a?
 
55
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Ah I see thanks I got it right!
 

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