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lax400

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A boulder with a mass of 2,500kg on a ledge 200m above the ground falls. What is the speed of the boulder just before it hits the ground.

 

 

To find velocity you need to use the kinetic energy formula right? But when I tried to plug in everything, it wouldn't work. IT only looks as if the GPE formula would work... could someone help me please?

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You might be able to do it with just the kinematic equations,

 

you know: u,a,s, and want to find v....so you have/want 4 of the 5 variables so yep the kinematic equations can be used here. Do you know what they are?

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When it is motionless, it is at it's highest point. Here, the total mechanical energy is solely composed of potential energy. It's fastest point is when it is at height=0 and thus the total mechanical energy is solely composed of kinetic energy. If we disregard air friction, we can say that all of the potential energy was converted to kinetic energy in the fall. You can then set the initial potential energy equal to the final kinetic energy. Then you can solve for the velocity.

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You might be able to do it with just the kinematic equations,

 

you know: u,a,s, and want to find v....so you have/want 4 of the 5 variables so yep the kinematic equations can be used here. Do you know what they are?

 

umm no sorry im only in 9th grade =\

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You can use kinetic and potential energy.

 

[math]PE = mgh[/math]

 

where m = mass, g = the acceleration due to gravity (9.8m/s2) and h = the height. All of that energy will be converted to kinetic energy by the time the object hits the ground.

 

[math]KE = \frac{1}{2} mv^2[/math]

 

where m = mass and v = velocity. So make the kinetic energy equal the potential energy you found in the first step and solve for velocity.

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The potential energy it gains by dropping 200m is found by using:

 

PE = mgh

 

You know the mass, g is acceleration due to gravity (you'll use 10, or 9.8 or 9.81) and h is the distance it falls.

 

Then you know that all of that potential energy went in to kinetic energy, and you know the formula: KE = ½mv².

 

So you say that all the PE goes to KE, therefore:

 

PE = KE

 

mgh = ½mv²

 

gh = ½v²

 

2gh = v²

 

v = √(2gh)

 

That should help. You understand now?

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A boulder with a mass of 2,500kg on a ledge 200m above the ground falls. What is the speed of the boulder just before it hits the ground.

 

 

To find velocity you need to use the kinetic energy formula right? But when I tried to plug in everything, it wouldn't work. IT only looks as if the GPE formula would work... could someone help me please?

 

Remember that mass does not at all enter into solving this problem.

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Perhaps a little misunderstanding here but I was referring to the equations of velocity!

 

What do you think causes the "a"? The only reason you don't need mass is because we know the acceleration at sea level due to gravity.

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