• Support PF! Buy your school textbooks, materials and every day products Here!

Calculating kinetic friction

  • Thread starter Ry122
  • Start date
  • #1
565
2
A 23.0 kg block is initially at rest on a horizontal surface. A horizontal force of 76.0 N is required to set the block in motion. After it is in motion, a horizontal force of 60.0 N is required to keep the block moving with constant speed. Find the coefficients kinetic friction from this information.

my attempt:
After the block is in motion 60N is required to prevent it from accelerating.
Therefore what is left over must be used to overcome friction which is 16N.
Friction force = weight x coefficient of kinetic friction
16N=23x9.8 x ck
ck=.070
But this is incorrect.
What am i doing wrong?
 

Answers and Replies

  • #2
Matterwave
Science Advisor
Gold Member
3,965
326
60N is the force required to prevent it from accelerating, this is the same force to overcome friction. 16N isn't anything in this problem, it's just the difference between the force required to overcome static friction and the force required to overcome kinetic friction.

Replace 16N with 60N in your calculations and try again.
 
  • #3
565
2
When friction force and the accelerative force are equal speed is constant. Anything extra will cause the body to accelerate so the accelerative force isn't overcoming friction.
60n is used to determine the static friction.
 
  • #4
2,017
85
as you say you need the same force as friction to stay at constant velocity.

The question tells you there there is 60 N of friction when moving.

76 N is used to determine static friction.
 

Related Threads for: Calculating kinetic friction

  • Last Post
Replies
5
Views
18K
Replies
5
Views
12K
Replies
2
Views
27K
Replies
1
Views
1K
Replies
3
Views
1K
Replies
1
Views
985
Replies
15
Views
2K
Replies
7
Views
13K
Top