Maths help mechanics!!!

Hi
Could someone please help with question 5 b.. I'm not sure why suddenly A is moving under gravity and i also really don't understand why we multiply the answer by two...
Any help is greatly appreciated!
Thank you!
http://pmt.physicsandmathstutor.com/download/Maths/A-level/M1/Papers-Edexcel/June%202014%20(R)%20MA%20-%20M1%20Edexcel.pdf

(edited 5 years ago)

Reply 1

vc94

12

Original post by examstudy

Hi
Could someone please help with question 5 b.. I'm not sure why suddenly A is moving under gravity and i also really don't understand why we multiply the answer by two...
Any help is greatly appreciated!
Thank you!
http://pmt.physicsandmathstutor.com/download/Maths/A-level/M1/Papers-Edexcel/June%202014%20(R)%20MA%20-%20M1%20Edexcel.pdf

When B hits the ground the string becomes slack i.e. no tension, so A will continue to move up slightly under gravity. A will then come down by the same distance so that the string becomes taut again, hence multiply distance by 2.

Reply 2

examstudy

OP

10

Original post by vc94

When B hits the ground the string becomes slack i.e. no tension, so A will continue to move up slightly under gravity. A will then come down by the same distance so that the string becomes taut again, hence multiply distance by 2.

why does a continue to move up if its slack and theres no tension? And how do you know it will come down by the same distance as it went up?

Reply 3

Sinnoh

22

Basically, gravity is the only force now acting on A, because the weight of B is no longer a factor as it has hit the ground and is therefore not pulling on A.

Now, I'm fairly sure that the reason it's multiplied by 2 is because A is initially still rising at the instant B hits the ground, so it has some vertical upwards motion and it still moving upwards but accelerating downwards due to gravity. So it reaches a maximum height and then drops back down a bit, hence the total actual distance is 0.6 because the maximum change in displacement is 0.3

Reply 4

examstudy

OP

10

Original post by Sinnoh

Basically, gravity is the only force now acting on A, because the weight of B is no longer a factor as it has hit the ground and is therefore not pulling on A.

Now, I'm fairly sure that the reason it's multiplied by 2 is because A is initially still rising at the instant B hits the ground, so it has some vertical upwards motion and it still moving upwards but accelerating downwards due to gravity. So it reaches a maximum height and then drops back down a bit, hence the total actual distance is 0.6 because the maximum change in displacement is 0.3

OK thank you, but what rule or fact tells you that the distance it falls will be identical to what it rose by?