- #1
mklein
- 43
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Dear physicists
I am a physics teacher in London. Whilst I can understand exam style questions on circular motion (you would hope so !) I do have something which is bugging me.
It is regarding an object completing a "loop-the-loop" inside a track. (like a roller coaster, but let's keep this simple and imagine a block on a smooth surface).
My problem is this:
http://www.mpklein.co.uk/loop_the_loop.png
- At the beginning (travelling horizontal) there is no resultant force on the block as the weight and normal reaction cancel. Newton's 1st Law says the block continues at it's steady speed (I am fine with this)
- As the incline begins, the normal reaction tilts to the left. The weight is still vertically downwards. If we think about the RESULTANT of these two forces, they no longer cancel. In fact, if we put both forces tip-to-tail, the resultant is down the slope (so the speed of the block should decrease as it follows the path)
So, how does the block perform circular motion? To perform circular motion we need a RESULTANT force towards the centre of the circle. Which there isn't. I appreciate that the normal reaction points towards the centre of the circle, but this is not a resultant. I cannot even understand why the block moves upwards. I appreciate it has to follow the track which climbs upwards, but surely we should be able to explain this upward motion in terms of forces? The magnitude of the normal reaction is less than that of the weight - there is definitely no resultant force upwards.
Please could somebody enlighten me in simple steps?
Thank you
Matt
I am a physics teacher in London. Whilst I can understand exam style questions on circular motion (you would hope so !) I do have something which is bugging me.
It is regarding an object completing a "loop-the-loop" inside a track. (like a roller coaster, but let's keep this simple and imagine a block on a smooth surface).
My problem is this:
http://www.mpklein.co.uk/loop_the_loop.png
- At the beginning (travelling horizontal) there is no resultant force on the block as the weight and normal reaction cancel. Newton's 1st Law says the block continues at it's steady speed (I am fine with this)
- As the incline begins, the normal reaction tilts to the left. The weight is still vertically downwards. If we think about the RESULTANT of these two forces, they no longer cancel. In fact, if we put both forces tip-to-tail, the resultant is down the slope (so the speed of the block should decrease as it follows the path)
So, how does the block perform circular motion? To perform circular motion we need a RESULTANT force towards the centre of the circle. Which there isn't. I appreciate that the normal reaction points towards the centre of the circle, but this is not a resultant. I cannot even understand why the block moves upwards. I appreciate it has to follow the track which climbs upwards, but surely we should be able to explain this upward motion in terms of forces? The magnitude of the normal reaction is less than that of the weight - there is definitely no resultant force upwards.
Please could somebody enlighten me in simple steps?
Thank you
Matt