Oh_Gee

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From GS-7

I thought "force acting parallel to the plane" meant the force pushing the block up which is why i did

F= 10 x 2 x sin 30


can somebody explain the correct answer to me
 

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Chrisz

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What you calculated is the net force. In order to calculate the total force applied, you also have to include the portion that is necessary to counter the gravitational force, which is mgsin(theta)
 

Oh_Gee

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What you calculated is the net force. In order to calculate the total force applied, you also have to include the portion that is necessary to counter the gravitational force, which is mgsin(theta)
in the answer why to they add the portion that is necessary to counter the gravitational force, instead of subtracting it
 
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in the answer why to they add the portion that is necessary to counter the gravitational force, instead of subtracting it
take it step by step
break down the weight vector into vx and vy components
the vx component is the one we care about mgsin(ø) -> 10*10*sin(30) = 50
that is the force that is parallel to the plane

there is acceleration .:. there is a net force
therefore ma = force applied - weight vector (they are subtracted b/c the force applied is to the right whereas the weight vector is to the left .:. they are acting in opposite directions)
ma = 10*2

20 = x - 50
.:. the force applied is 70 N
 
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Chrisz

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Feb 18, 2014
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in the answer why to they add the portion that is necessary to counter the gravitational force, instead of subtracting it
Let's say the total force applied parallel=F
F-mgsin(thetha)=ma
F=ma+mgsin(theta)

If you apply a force equal to mgsin(theta), ma will be zero, How do you expect increase velocity with net force equal to zero.
mgsin(theta) is in the direction opposing the movement of the object, which decelerates the velocity when no force is applied. You want to accelerate the speed, so you must apply a force that is more than necessary to counter the effect by gravitational force
 

Oh_Gee

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take it step by step
break down the weight vector into vx and vy components
the vx component is the one we care about mgsin(ø) -> 10*10*sin(30) = 50
that is the force that is parallel to the plane

there is acceleration .:. there is a net force
therefore ma = force applied - weight vector (they are subtracted b/c the force applied is to the right whereas the weight vector is to the left .:. they are acting in opposite directions)
ma = 10*2

20 = x - 50
.:. the force applied is 70 N
hmm i think i got it

the force mentioned in the first sentence is the total force (due to the push and gravity) on the block. but they want just the force due to the push
 
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