M1 need help with a confusing suvat q

hi, for this question, I got an answer of 2s however after checking the markacheme, it is 1/7<t<2 ... It also involves some quadratic equation. I get that the inequality signs are there as the questions asks for time more than 2m. But I only got one answer for this tho.

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Reply 1

coconut64

Here is the markschme

thx

Reply 2

Kevin De Bruyne

Original post by coconut64

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The problem with your method is the left hand side - it starts at a height of ... so it is...

Reply 3

samb1234

Original post by coconut64

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You need to work out the two different times the ball is at 2m, and then you should be able to calculate the length of time it is above 2m from that

Reply 4

coconut64

Original post by SeanFM

The problem with your method is the left hand side - it starts at a height of ... so it is...

Hi, thanks for helping! I don't really know what you mean by that... The lhs method is to find out how long it takes for the ball to reach Max height. Whats wrong with that? Thx

Reply 5

coconut64

Original post by samb1234

You need to work out the two different times the ball is at 2m, and then you should be able to calculate the length of time it is above 2m from that

This is what the markscheme says as well. However I don't understand why the time taken to reach Max height isn't considered. Also I don't get why the distance is simply 2-0.6. This really confuses me. Thanks.

Reply 6

Kevin De Bruyne

Original post by coconut64

Hi, thanks for helping! I don't really know what you mean by that... The lhs method is to find out how long it takes for the ball to reach Max height. Whats wrong with that? Thx

Oh yes, I kind of see what you mean. I guess it is confusing you because you only have 1 answer and t takes a range of values. Your 2s is fine (as you can see) but for the first part of motion it's not always 2m or more above the ground so you need a lower bound for t as well.

Reply 7

Zacken

Original post by coconut64

2-0.6: what height does the football start off at?

(edited 8 years ago)

Reply 8

samb1234

Original post by coconut64

Well if the object starts at 0.6m above the ground, a height of 2m above the ground means that the vertical displacement from the starting point is 2-0.6 =1.4m. This gives you the two values of t where it is at 2m, and we know that it is above 2m for this whole time inbetween those values so can therefore work out how long it is above 2m

Reply 9

Fadel

When you plugged in the values in the formula. ut is not 0 because the starting velocity is 10.5 m/s. And a is -9.8. S in this case is the distance the ball should go up. If the initial position of the ball is 0.6m above ground and they want the ball to be 2m or more then the ball should travel 1.4m to reach 2m. You used the formula S=1/2gt^2 which is used ONLY for free fall.

Reply 10

tiny hobbit

Original post by coconut64

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You need to subtract the time that it took getting from the starting place up to 2m in the first place. Your method is more complicated than the one in the mark scheme, but it is fine provided that you subtract this early time.

Reply 11

Fadel

Original post by coconut64

Remember that the mark scheme doesn't show all the possible answers to a question. This question can be solved in a less confusing method. You can find the time it takes the ball to reach height of 2m (S in this case is 1.6m). Then, subtract this time from the time taken to reach maximum height.

Reply 12

coconut64

Original post by Zacken

2-0.6: what height does the football start off at?

I have tried it again and got 13/7 s which I think is right? But this time I didn't get 2s...

Reply 13

coconut64

Original post by Fadel

Thats what I have done (see below). I have worked out 13/7 without using the quadratic equation. However I didn't get 2 or the inequality equation. Does it matter? Thanks.

Reply 14

Fadel

Original post by coconut64

Thats what I have done (see below). I have worked out 13/7 without using the quadratic equation. However I didn't get 2 or the inequality equation. Does it matter? Thanks.

Doesn't matter. Question cannot force you to use a method unless they specify to, which never happens in M1. I solved by this method in my mocks and earned the full credit.