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###### A-Level Mechanics question help

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2 weeks ago

A helicopter is hovering at rest above horizontal ground at the point H. A

Parachutist steps out of the helicopter and immediately falls vertically and freely

under gravity from rest for 2.5 s. His parachute then opens and causes him to

immediately decelerate at a constant rate of 3,9 m/s^2 for T seconds (T < 6), until

his speed is reduced to V m/s, He then moves with this constant speed V m/s

until he hits the ground. While he is decelerating, he falls a distance of 73.75 m.

The total time between the instant when he leaves H and the instant

when he hits the ground is 20 seconds.

The parachutist is modelled as a particle.

Find the Speed of the parachutist at the instant when his parachute opens

Find the value of T

I can't seem to figure it out, any ideas?

Parachutist steps out of the helicopter and immediately falls vertically and freely

under gravity from rest for 2.5 s. His parachute then opens and causes him to

immediately decelerate at a constant rate of 3,9 m/s^2 for T seconds (T < 6), until

his speed is reduced to V m/s, He then moves with this constant speed V m/s

until he hits the ground. While he is decelerating, he falls a distance of 73.75 m.

The total time between the instant when he leaves H and the instant

when he hits the ground is 20 seconds.

The parachutist is modelled as a particle.

Find the Speed of the parachutist at the instant when his parachute opens

Find the value of T

I can't seem to figure it out, any ideas?

Original post by JoshyBoi1912

A helicopter is hovering at rest above horizontal ground at the point H. A

Parachutist steps out of the helicopter and immediately falls vertically and freely

under gravity from rest for 2.5 s. His parachute then opens and causes him to

immediately decelerate at a constant rate of 3,9 m/s^2 for T seconds (T < 6), until

his speed is reduced to V m/s, He then moves with this constant speed V m/s

until he hits the ground. While he is decelerating, he falls a distance of 73.75 m.

The total time between the instant when he leaves H and the instant

when he hits the ground is 20 seconds.

The parachutist is modelled as a particle.

Find the Speed of the parachutist at the instant when his parachute opens

Find the value of T

I can't seem to figure it out, any ideas?

Parachutist steps out of the helicopter and immediately falls vertically and freely

under gravity from rest for 2.5 s. His parachute then opens and causes him to

immediately decelerate at a constant rate of 3,9 m/s^2 for T seconds (T < 6), until

his speed is reduced to V m/s, He then moves with this constant speed V m/s

until he hits the ground. While he is decelerating, he falls a distance of 73.75 m.

The total time between the instant when he leaves H and the instant

when he hits the ground is 20 seconds.

The parachutist is modelled as a particle.

Find the Speed of the parachutist at the instant when his parachute opens

Find the value of T

I can't seem to figure it out, any ideas?

Sketching the information usually helps, so upload what youve tried.

But its obviously a 3 stage suvat problem, where each stage represents a phase with constant acceleration. The speed when the parachute opens is the first phase and you should be able to do it as its a single suvat?

Reply 2

2 weeks ago

Original post by mqb2766

Sketching the information usually helps, so upload what youve tried.

But its obviously a 3 stage suvat problem, where each stage represents a phase with constant acceleration. The speed when the parachute opens is the first phase and you should be able to do it as its a single suvat?

But its obviously a 3 stage suvat problem, where each stage represents a phase with constant acceleration. The speed when the parachute opens is the first phase and you should be able to do it as its a single suvat?

Apologies, should have made it a little clearer, I can find the speed completely fine, it is with finding T that I'm struggling with.

I placed the speed part in there to give a little more context

Original post by JoshyBoi1912

Apologies, should have made it a little clearer, I can find the speed completely fine, it is with finding T that I'm struggling with.

I placed the speed part in there to give a little more context

I placed the speed part in there to give a little more context

What have you sketched/written down about the info you know - can you upload what youve tried. The aim is (obviously) to combine a couple of suvats involving T and V for the 2nd and 3rd phase to get T.

(edited 2 weeks ago)

Original post by mqb2766

What have you sketched/written down about the info you know - can you upload what youve tried. The aim is (obviously) to combine a couple of suvats involving T and V for the 2nd and 3rd phase to get T.

Surely you can find T from the information you've been given/calculated so far (you have u, a, and s) without needing V?

Original post by DFranklin

Surely you can find T from the information you've been given/calculated so far (you have u, a, and s) without needing V?

Looks like it. Was only 1/2 concentrating and missed the bit about the decelerating distance, though was half hoping the op would post some form of attempt.

Original post by mqb2766

Looks like it. Was only 1/2 concentrating and missed the bit about the decelerating distance, though was half hoping the op would post some form of attempt.

I understand not really wanting to get involved until the OP has shown some effort - I had some difficulty phrasing my comment so it wouldn't reward someone looking for the answer without putting in any effort!

Reply 7

2 weeks ago

Original post by DFranklin

I understand not really wanting to get involved until the OP has shown some effort - I had some difficulty phrasing my comment so it wouldn't reward someone looking for the answer without putting in any effort!

Yeah, apologies for that. Busy weekend. Will attempt tomorrow at college as I have a 2 hour gap, I shall upload my workings from then on. (Also I don’t use this site all that much so I kinda forgot to reply, apologies!)

Reply 8

2 weeks ago

No worries you two, managed to figure it out. T is 5 seconds.

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