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A Level Maths mechanics question-projectiles

https://www.quora.com/profile/Bravewarrior/p-160659267
Here is the question with its solution. I am stuck on part b. In the mark scheme, they put a minus sign in front of the coeff of friction x R when resolving horizontally. Can someone please tell me why they did that? Thank you!
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Original post by pigeonwarrior
https://www.quora.com/profile/Bravewarrior/p-160659267
Here is the question with its solution. I am stuck on part b. In the mark scheme, they put a minus sign in front of the coeff of friction x R when resolving horizontally. Can someone please tell me why they did that? Thank you!

The particle is decelerating (negative acceleration) as friction acts against the direction of motion.
Original post by mqb2766
The particle is decelerating (negative acceleration) as friction acts against the direction of motion.

Ah that makes sense, thank you! 🙂

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