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Physics Mechanics- friction

Shah 72

Member
Apr 14, 2021
193
A box of mass 20 kg is at rest on a rough horizontal ground. Find the magnitude of the total contact force in each of the cases.
a) The box is pulled horizontally to the right by a force of 40N
b) the box is pushed to the left by a force of 50N at 15 degree above the horizontal .
c) the box is pushed to the left by a force of 50N at 15 degree below the horizontal.
 

Shah 72

Member
Apr 14, 2021
193
A box of mass 20 kg is at rest on a rough horizontal ground. Find the magnitude of the total contact force in each of the cases.
a) The box is pulled horizontally to the right by a force of 40N
b) the box is pushed to the left by a force of 50N at 15 degree above the horizontal .
c) the box is pushed to the left by a force of 50N at 15 degree below the horizontal.
I have worked out the ans for a) will be 204N.
b) iam getting the ans 213N, but the textbook says 193N
c) I get the ans 187N , the textbook ans is 218N.
 

skeeter

Well-known member
MHB Math Helper
Mar 1, 2012
935
A box of mass 20 kg is at rest on a rough horizontal ground. Find the magnitude of the total contact force in each of the cases.
a) The box is pulled horizontally to the right by a force of 40N
b) the box is pushed to the left by a force of 50N at 15 degree above the horizontal .
c) the box is pushed to the left by a force of 50N at 15 degree below the horizontal.
I have worked out the ans for a) will be 204N.
b) iam getting the ans 213N, but the textbook says 193N
c) I get the ans 187N , the textbook ans is 218N.
I don't get any of those values, yours or the text. I'm interpreting total contact force as the Normal Reaction Force the rough (assumedly horizontal) surface exerts upward on the box. Is that how your text defines it?

Is there more information that you haven't posted?
 

Shah 72

Member
Apr 14, 2021
193
I don't get any of those values, yours or the text. I'm interpreting total contact force as the Normal Reaction Force the rough (assumedly horizontal) surface exerts upward on the box. Is that how your text defines it?

Is there more information that you haven't posted?
The question is Q(3)20210420_100046.jpg
This is the exact info in the text book. And iam getting a different ans from the ans given in the textbook. 20210420_100046.jpg20210420_100046.jpg
 

skeeter

Well-known member
MHB Math Helper
Mar 1, 2012
935
(a) normal contact force = weight = 20g = 196 N

(b) normal contact force = weight - vertical component of the applied force = 20g - 50sin(15) = 183 N

(c) normal contact force = weight + vertical component of the appled force = 20g + 50sin(15) = 209 N

may help your understanding if you sketched free body diagrams for each situation