- #1

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F=ma

F=(10kg)(9.81m/s2) = approximately 100N

So in order to slow the mass at a rate equal to g it should be a force less than 100N?? Im not sure how to figure this out. Help!!!

Thanks

Jerry Zink

- Thread starter jerryez
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- #1

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- 0

F=ma

F=(10kg)(9.81m/s2) = approximately 100N

So in order to slow the mass at a rate equal to g it should be a force less than 100N?? Im not sure how to figure this out. Help!!!

Thanks

Jerry Zink

- #2

rock.freak667

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There are three forces acting or well two since we are ignoring air resistance. What are these two and what direction are they going in (up or down)?

Since the body is moving downwards, the resultant force, ma, is downward. Can you formulate an expression for the resultant force ma?

- #3

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1. gravitational force on the 10kg mass "going down"

2. The upward force "going up"

So the upward force must go against the force of gravity plus the force of the mass.

The mass has a force of F=ma = 100N downward

Since the upward force is in the opposite direction it has to counteract the downward force and slow it down to = g

Im just having trouble formulating the expression for the upward force.

- #4

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Would the upward force be :

F=m2a ?

F=(10kg)(2)(9.81m/s2) = 196N

is this correct?

F=m2a ?

F=(10kg)(2)(9.81m/s2) = 196N

is this correct?

- #5

rock.freak667

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Correct.The two forces are

1. gravitational force on the 10kg mass "going down"

2. The upward force "going up"

Right but remember, the force of gravity is the 'force of the mass'.So the upward force must go against the force of gravity plus the force of the mass.

So you will have one going down and one going up (don't worry with the formulas for now, just use U for upwards and W for downward). If the mass is moving downwards, what direction should the resultant force be in?

- #6

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The resultant force should be in the upward direction

- #7

rock.freak667

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The mass is falling in which direction?The resultant force should be in the upward direction

- #8

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OH! So the resultant force is in the downward direction.

- #9

rock.freak667

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Right so your resultant isOH! So the resultant force is in the downward direction.

ma = W-U

and you identified the downward force as the force of gravity. So what is W equal to?

They want the

- #10

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a = 9.81m/s2

so W is equal to the downward force of 100N?

So this means u = W - ma

U = 100N - (10kg)(9.81m/s2)

U = 1.9N

Is this right?

Are you sure its not this "ma = U-W"

This would make more sense because then U is = to around 200N

- #11

rock.freak667

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If your upward force is greater then your object is moving upwards and not falling.

a = 9.81m/s2

so W is equal to the downward force of 100N?

So this means u = W - ma

U = 100N - (10kg)(9.81m/s2)

U = 1.9N

Is this right?

Are you sure its not this "ma = U-W"

This would make more sense because then U is = to around 200N

Normally what you have is

ma = W-U-F

Sorry though, I interpreted one part wrong. They want the mass to fall at 'g' right? Which is essentially with the force of gravity, which is constant. Since it's acceleration is constant, the resultant force is ?

- #12

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Is the resultant force still the force of the mass 100N downwards?

- #13

rock.freak667

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No, the resultant force is the resultant of W and U. Since the mass is not accelerating anymore, the resultant force is zero.Is the resultant force still the force of the mass 100N downwards?

- #14

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?? So if the resultant force is zero how do we find the upward force?

- #15

rock.freak667

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If we have ma=W-U and ma=0, then wouldn't W-U=0 ??? So if the resultant force is zero how do we find the upward force?

- #16

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Fw = (10kg)(9.81m/s2)

W = 100N

U = 100N

This doesnt seem right?

- #17

rock.freak667

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Why not?

Fw = (10kg)(9.81m/s2)

W = 100N

U = 100N

This doesnt seem right?

- #18

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Then wouldn't that implicate that the mass is not moving?

- #19

rock.freak667

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Not in this case. The body is falling. Which means it is traveling at a constant velocity.Then wouldn't that implicate that the mass is not moving?

- #20

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I really appreciate your help rock.freak667

Can you make any sense of the 200N answer?

Thanks again

Jerry Zink

- #21

vela

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This is correct.The resultant force should be in the upward direction

The fact that the mass is moving downward only tells you that the velocity vector points down. It doesn't tell you anything about the direction of the acceleration vector.The mass is falling in which direction?

For 1D motion, if the speed of the object is increasing, the acceleration points in the same direction as the velocity. If it's slowing down, the acceleration points in the direction opposite to the velocity.

- #22

ehild

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ma=mg=F-mg

ehild

- #23

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downward Force = mg = 100N

ma=mg=F-mg

(10kg)(9.81m/s2) = F - (10kg)(9.81m/s2)

F = (98.1)+(98.1)

F = approximately 200N

So the upward force is twice as great as the downward force this makes the downward mass fall at g? Can someone explain the theory behind this?

- #24

ehild

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The upward force will make the falling body slow down at a rate of g till it loses all its downward velocity. After that, it will rise.S

So the upward force is twice as great as the downward force this makes the downward mass fall at g? Can someone explain the theory behind this?

ehild

- #25

vela

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You're not taking into account the direction of the acceleration. Let's use the usual convention of positive being the upward direction and negative being the downward direction. If only its weight mg acts on the mass, you getSo the upward force is twice as great as the downward force this makes the downward mass fall at g? Can someone explain the theory behind this?

ma = -mg ⇒ a = -g

The negative sign indicates the mass accelerates in the downward direction at a rate of 9.8 m/s

ma = F - mg = 2mg - mg = mg ⇒ a = +g

Again the mass is accelerating at a rate of 9.8 m/s

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