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Kinematics and Dynamics

A slimming club is situated at the top of a building; to motivate its clientele, the club has installed its own lift which contains a weighing machine. The lift accelerates uniformly at 1.0ms-2 for 90% of its journey, both going up and coming down.

(iv) The reading (in kg) on the weighing machine when the 80kg person stands on it as the lift decelerates upwards at 2.0ms-2?

Not sure if I'm doing this right... F=mg+ma

So F=80x9.81+80x-2
F=784.8+-160
624.8 / 9.81 = 63.69 = 64KG???

Or

F=80x9.81+80x2
F=784.8+160
944.8 / 9.81 = 96.3 = 96KG???

Or am I doing this completely wrong??? Well, at least I tried! Thanks!
Original post by keevesygoesrawr
A slimming club is situated at the top of a building; to motivate its clientele, the club has installed its own lift which contains a weighing machine. The lift accelerates uniformly at 1.0ms-2 for 90% of its journey, both going up and coming down.

(iv) The reading (in kg) on the weighing machine when the 80kg person stands on it as the lift decelerates upwards at 2.0ms-2?

Not sure if I'm doing this right... F=mg+ma

So F=80x9.81+80x-2
F=784.8+-160
624.8 / 9.81 = 63.69 = 64KG???

Or

F=80x9.81+80x2
F=784.8+160
944.8 / 9.81 = 96.3 = 96KG???

Or am I doing this completely wrong??? Well, at least I tried! Thanks!



Deceleration up = acceleration down

So the acceleration and gravity are acting in the same direction

Meaning the second one is right
Reply 2
Avatar for saquzk
saquzk
5
Your First Answer is right. I'll show you how:


Notice that the acceleration is -2 because the lift is decelerating
New Doc 1-page-001.jpg423.6KB
Original post by saquzk
Your First Answer is right. I'll show you how:


Notice that the acceleration is -2 because the lift is decelerating


Thank you very much!
Reply 4
Avatar for saquzk
saquzk
5
You're very welcome!
Original post by saquzk
You're very welcome!


I had my Kinematics and Dynamics test on Monday.. I got 82% which is an A, quite proud of myself :smile:
Reply 6
Avatar for saquzk
saquzk
5
Good for you bro!!:smile:
I need help with the following would really appreciate it if one of you could help out!

You're asked to start from rest, go a distance L=105m and end up at rest again, all in a minimum time. You're vehicle has a max acceleration capability of a1= 7.5ms^-2 and max braking cabaility of a2= -10ms^-2. The min time solution is to use max braking and acceleration. Determine t1,t2 and the cars maximum speed. T1 is how long car accerates for and t2 decelerate.

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