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M2 acceleration problem Watch

1. http://www.edexcel.com/migrationdocu...e_20080521.pdf

Question 3)b). I used SUVAT to work out the acceleration as -20/7 ms^-2. However, this results in the wrong value for the coefficient of friction. However, if I use 20/7 for the acceleration I get the right answer. Surely that isn't right? Since it is decelerating down the plane.
2. Well friction is in the opposite direction
3. (Original post by ViralRiver)
http://www.edexcel.com/migrationdocu...e_20080521.pdf

Question 3)b). I used SUVAT to work out the acceleration as -20/7 ms^-2. However, this results in the wrong value for the coefficient of friction. However, if I use 20/7 for the acceleration I get the right answer. Surely that isn't right? Since it is decelerating down the plane.
It all depends on the direction you state as positive when you're resolving the forces on the block. If you resolve up the slope, the acceleration should be positive but if you resolve down the slope, then the acceleration should be -20/7ms^-2.
4. I resolved down the slope, v^2 = u^2 + 2as, getting the negative answer for acceleration and then my final answer was incorrect. Does that mean I had to resolve up the slope? ><
5. (Original post by ViralRiver)
I resolved down the slope, v^2 = u^2 + 2as, getting the negative answer for acceleration and then my final answer was incorrect. Does that mean I had to resolve up the slope? ><
Post your working, you've probably made a mistake when resolving the forces. Friction is acting up the slope.
You should be solving this if you have resolved down the slope:
.
6. Ahhh I had the friction the other way around after misreading the answer ^^' - thanks .

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