I'm having a problem with a physics question from a exam I'm currently training for. I have no way of knowing if this is correct so I'm asking for help to review my answer!

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This box is on a 20degree incline and the spring constant K is 2500 n/m. Before the box was attached to the spring the spring was 500mm and after 565 so a 65mm difference.

The question is what does the box weigh? DL= the difference of the spring before and after the box, 0.065 meters. K=2500n/ms

I thought abit about it and thought about hooks law which is F=DL K. Then I did this, MGsin20=DL K since the box is a 20 degree angle. then I just divided away g and sin 20 and came up with the box weighed 48.38kgs.

Please would appreciate some help if I'm in the right ballpark or not. Thanks for future answers!


closed as off-topic by sammy gerbil, Jon Custer, Yashas, John Rennie, Kyle Kanos Mar 9 '17 at 11:00

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Yes, your reasoning is correct - the spring constant literally tells you how many Newtons you need to stretch the given spring by a meter, so by Hooke's law (with an e) you can calculate the force needed to extend your spring by 65 mm.

Now all you have to do is equate that spring force by the force that is pulling the box down the slope - in the frictionless case that is just the plain old $\vec{F_g}$ times the sine of the slope angle.

  • $\begingroup$ Thanks so much for the fast answer. Oh yeah I forgot to mention yes the surface was frictionless. So FG is m g sine slop angle? $\endgroup$ – Kennelmaster Mar 8 '17 at 1:23
  • $\begingroup$ @Kennelmaster Yes, since $\vec{F_g} = m \vec{g}$ always pulls down, because that's where Earth is, the box also tries to move straight down. Since the slope is in the way, the box will move along the slope and that's where the sine reduces the force by an amount depending on the angle. $\endgroup$ – Wojciech Morawiec Mar 8 '17 at 7:54
  • $\begingroup$ @Kennelmaster If that is the case, feel free to vote on the answer and accept it by clicking on the check mark :) $\endgroup$ – Wojciech Morawiec Mar 8 '17 at 12:09
  • $\begingroup$ Can't vote on it due to having less the 15 but I pressed the check mark! $\endgroup$ – Kennelmaster Mar 8 '17 at 12:48

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