I need help with Physics Hw
Hello everyone. I'm having a bit of trouble with my HW about forces. We just got into the topic a few days ago and I'm kind of lost. o.o
Help is much appreciated.
Draw a FBD for a 40 kg person in an elevator, on a scale, moving at a speed of 5m/sec downwards. Find the values for all the forces. What would the scale read in kg? in N?
I pretty much just don't know what to do with the speed, usually we get acceleration in the problem instead.
October 25, 2012
7 Comments • Newest first
@Amuro Thank you so much! I actually understand it now.
[quote=PureIchigo]@RisingRain Is it 0 because the speed is the same which makes the acceleration zero?[/quote]
By Newton's 1st law of motion, a constant velocity means there's no net force.
Instead of taking Newton's word for it, you can also look at Newton's 2nd Law: F = ma.
Fnet = ma; Fnet = Fup - Fdown (up positive)
Assuming the weight of the elevator is negligible, Fdown = (40kg)(9.81m/s^2).
a = 0 (if constant speed), so Fup - Fdown = 0. => Fup = Fdown.
@RisingRain Is it 0 because the speed is the same which makes the acceleration zero?
If it's just speed, then the FBD should be mg up (normal force), and mg down, as net force in the y-direction is = 0. So the scale should read 40(9.8) N.
Is 5m/s constant speed or acceleration?
@FrostyGun Yeah, haha. It's pretty confusing sometimes.
Jesus so much "Physics HW Help" threads lately.