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YAPT *updated with one more!* SOLVED

Fenixgoon

Fenixgoon

Lifer
*update* problem #2:

a car takes 80 meters to stop and the coefficient of kinetic friction is 0.8
what is the initial velocity of the car?






doing some physics homework, cant figure out how to do this crap! 😛

1)A 75-kg person stands on a scale in an elevator. What does the scale read (in kg) when a)the elevator is at rest,b)the elevator is climbing at a constant speed of 3 m/s, c)the elevator is falling at 3 m/s, d) the elevator is accelerating upward at 3/ms^2, and e) the elevator is accelerating downward at 3 m/s^2



go atot physics go! 🙂
 
at rest: N-mg=ma=0, n=mg, n=75*9.8
constant positive v: n-mg=ma=0, n=mg, n=75*9.8
constant negative v: n-mg=ma=0, n=mg, n=75*9.8
positive acc: n-mg=ma, n=m(g+a), n=75(9.8+3)
negative acc: n-mg=ma, n=m(g+a), n=75(9.8-3)
 
a b and c all have the same answer. When the elevator is moving at a constant speed (up, down, or zero) the scale will read the person's weight based upon their mass and their acceleration due to gravity.

In both d and e, you must account for the fact that the elevator is accelerating. The acceleration of gravity is 9.8 m/s^2.

That is all I will say towards solving your problem. Now, do your own homework!

R

EDIT: One more hint: You need to think about what the scale is reading versus what it is actually measuring...
 
so as long as you're not accelerating (but moving at a constant speed, either up or down), the scale isn't affected at all? interesting.. i guess it makes sense though, since gravity = acceleration, so you'd need to affect the acceleration to change the gravitational force.
 
N-mg=ma=0 when acceleration is 0, ie when elevator is at rest or at constant velocity
N-mg=ma where ma is not = to zero due to the precence of acceleration
 
*update* problem #2:

a car takes 80 meters to stop and the coefficient of kinetic friction is 0.8
what is the initial velocity of the car?
 
f=ma
Fy: N-mg=ma=0, N=mg
Fx: -Fk=ma, -uN=ma, -u(mg)=ma, -ug=a, -7.84=a
V^2=V0^2+2ax, 0=V0^2+2(-7.84)(80), v=35.4m/s

 
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