Easy physics question

For the discussion of the sciences. Physics problems, chemistry equations, biology weirdness, it all goes here.

Moderators: gmalivuk, Moderators General, Prelates

Adam Preston
WINNING
Posts: 102
Joined: Sat Mar 19, 2011 10:29 pm UTC

Easy physics question

Postby Adam Preston » Thu Sep 29, 2011 9:25 pm UTC

Okay so today I started exercising Newton's laws of motion, so there was a question about a lift, a man stood in the lift who weighed 70kg, his weight which pointed downwards was shown with w, an opposing force pointing upwards was labeled r. The question was what's the value of W when the acceleration is 2m/s^2 downwards. The teacher said it was 560N, R had to be higher since it would require more force to hold the elevator. But surely if the force is higher then there would be no movement. Or does it simply mean that it accelerates for a small amount of time before the forces become equal?
He who will not economize will have to agonize - Confucius

gorcee
Posts: 1501
Joined: Sun Jul 13, 2008 3:14 am UTC

Re: Easy physics question

Postby gorcee » Thu Sep 29, 2011 9:47 pm UTC

The opposing force R pointing upwards... was it drawn against the man or the elevator?

User avatar
idobox
Posts: 1591
Joined: Wed Apr 02, 2008 8:54 pm UTC
Location: Marseille, France

Re: Easy physics question

Postby idobox » Sat Oct 01, 2011 2:34 pm UTC

In this problem, the man doesn't move relative to the lift, so the force it applies on the lift, and the lift applies to him cancel each other.
But if the lift is accelerating, you have a new vertical force applied to the man, which can also be seen as a change of the value of g, and thus a change on both the force applied by the man on the lift and the one by the lift on the man.
If there is no answer, there is no question. If there is no solution, there is no problem.

Waffles to space = 100% pure WIN.

Adam Preston
WINNING
Posts: 102
Joined: Sat Mar 19, 2011 10:29 pm UTC

Re: Easy physics question

Postby Adam Preston » Sun Oct 09, 2011 7:34 am UTC

I think the force R was drawn to represent the elevator
He who will not economize will have to agonize - Confucius


Return to “Science”

Who is online

Users browsing this forum: No registered users and 16 guests