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# Help stuck on a higher physics question Watch

1. relate the changes in gravitational potential energy to changes in kinetic energy.
2. How do i do that though

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3. Change in GPE=Change in KE
4. I don't understand

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5. Your picture is not showing and i don't understand most statements in physics could you give me a formula or something which i could use
6. (Original post by Hollybollybee109)

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Set potential energy (mgh) equal to kinetic energy (1/2mv^2) and rearrange for v.

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7. If you mean to do this is doesnt give the right answer

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8. (Original post by Hollybollybee109)

If you mean to do this is doesnt give the right answer

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Also btw for future reference the masses cancel out so it should actually be v=sqrt2xgh
9. So i actually got one of my friends who do mechanics solve and they got 22. Something is that along the right line?
10. It gives the same wrong answer... I need help

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11. (Original post by Hollybollybee109)
It gives the same wrong answer... I need help

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12. For a the answer is 18.8 and for b the answer is 9.9

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13. This is what I got. Hope it's right and I didn't waste my time lol.

Ignore it's wrong I didn't see that it started off at 12 m/s.

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14. I dont understand why the answers are that
15. My teacher gave me the answers so maybe she got it wrong, thank you for your help

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16. (Original post by Hollybollybee109)
My teacher gave me the answers so maybe she got it wrong, thank you for your help

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I think 22.3 is correct, both my friends who studied mechanics and I (who did a level physics) got the same results. Please do update me im really curious as how the answer is 18 x
17. (Original post by Hollybollybee109)
My teacher gave me the answers so maybe she got it wrong, thank you for your help

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Yeah I have no idea how to do it, sorry. I tried using momentum, energy equations and kinematics.

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18. --
19. (Original post by Hollybollybee109)
For a the answer is 18.8 and for b the answer is 9.9

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I think I may know how your teacher arrives the "answer" of 18.8 m/s at Y.

I suggest that you attempt to understand the statements in physics instead of just the formula or equations to use.

Using conservation of energy and consider roller-coaster and Earth as the system.

PE at X + KE at X = PE at Y + KE at Y
---(1)

It can be simplified to

---(2)

If equation (2) is used, the answer would 22.3 m/s as shown below:

Your teacher's answer did not consider the speed at X. If the speed at X is zero, we would arrive 18.8 m/s for the speed at Y using equation (2) as shown below:
Attachment 579632579634

But the question stated that the speed at X is 12 m/s, so your teacher answer is incorrect.

Similarly, we would arrive the speed at Z to be 9.9 m/s using equation (2) and consider the speed at X to be zero. Test it yourself.

It may be good that you inform your teacher privately.
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Updated: September 12, 2016
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