could u help me w/ this stupid physics?

Question

ok. joseph is riding in an elevator which is accelerating downward at -20 meters per second squared. by summing the vertical forces, find the magnitude of the force that the cailing is exerting on joseph if he has a mass of 82 kg.

Answer 1

I am surprised by the magnitude of incorrect answers. The acceleration -20 m/s*2 in the downward direction. It simply means that acceleration is +20 m/s*2 in the upward direction. So the elevator is accelerating not downward but in "upward" direction. This is the whole catch of the problem.

And one more thing, If U literally need force on Joseph by the "ceiling" then it will be ZERO. Bcos ceiling is the roof and Joseph naturally stands on floor !

Now as acceleration is defined we can work simply from newtons second law. Working in the ground frame of reference.

contact Force of floor (not ceiling) on Joseph - Force of gravity on Joseph= Mass of Joseph X acceleration of Joseph in upward direction.

which comes as,

contact Force of floor (not ceiling) on Joseph - 82 X 9.8 = 82 X 20
Now U can calculate the force of ceiling.

and if U are not in a mood of converting negative sign. of acct. to positive then no problem, U can take net forces in the downward direction. answer would be same !

equation in that case will be,

-Force of floor (not ceiling) on Joseph + Force of gravity on Joseph= Mass of Joseph X acceleration of Joseph in downward direction (which is -20 So is negative)

And if U are confused by a variety of different answers then I suggest that take a copy and pen, and try to solve this problem again by applying Ur own logics. I am sure that U will get the right answer.
-------- --------- ---------- ---------- ------------ ---------- ----------- --------
EDIT-judy, does the acceleration in yr ques is +20m/s*2 downward. if it is then elevator will accelerate faster then Joseph in downward direction. so in this case Joseph will attach to the ceiling of the elevator and the statement given by U will be correct.plz check Ur answer and clear whether this is the case.

Answer 2

I disagree with Matthew I. And with mike1942f (his multiplication is wrong). And with pearlsawm... - somehow he subtracted 9.8 from 20 and got 2.2???

Fortitude has the right idea, and his signs and final answer are correct. The acceleration of the elevator is jamming joseph against the ceiling but gravity is taking away about 1/2 of the pressure.

Consider if the elevator was accelerating downward at 9.8 m/s^2, exactly equal to the standard acceleration due to gravity acting on joseph. Joseph would appear to be weightless inside the elevator car. Both falling together at the same rate. He would have zero acceleration with respect to the reference system of the car. 9.8 - 9.8 = 0. I think this shows that subtracting the accelerations is the right way to do it.

Answer 3

The downward acceleration is minus 20m/s^2.

In this problem the upward direction is taken as positive.

First let us reverse the direction, i.e. down ward direction as positive.

The acceleration is now plus 20m/s^2. This is inclusive of earth’s pull of g.

But for the earth’s pull, the elevator will have a downward acceleration of only 20 -9.8 = 2.2 m/s^2.

Similarly, but for earth’s pull, the man will be at rest and the elevator will push him down with an acceleration of 2.2m/s^2.

The ceiling of the elevator is pushing the man down ward with a force F = ma = 82 x 2.2 = 180.4N.

Answer 4

The earth's gravity is pulling Joseph down as well, so subtract the earth's acceleration from -20

-20 - (-9.8) = 10.2

Multiply that by his mass
F = (m) (a)

F = (82)(10.2) = roughly 837 N

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Answer 5

Since the earth's accelleration is
(32 ft) / sec(2) = 9.7536 m / sec(2)
the net downward A is 20-9.7536 = 10.2464
F=ma =82 kg x 10.2464 = -349.64857 newtons

Answer 6

Fortitude has the right idea, but his signs are incorrect. You are accelerating in the same direction as gravity if you are accelerating downward. So that should be addition, not subtraction. Other than that minor algebraic error, his answer is correct.

Answer 7

WHat???