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

Forces and Motion - Sliding Box Up Inclined Plane w/ Friction

  • Thread starter Scimitar
  • Start date
  • #1
2
0

Homework Statement


You are helping a friend move, and you need to load a 2.65x10^2 kg box of books. You slide the box up a ramp, which has an incline of 30 degrees and a coefficient of static friction of 0.45. You apply the force on the box at an angle of 39 degrees with respect to the ramp.

Calculate the minimum force needed to slide the box up the ramp.


Homework Equations



Net force = ma

The Attempt at a Solution


I'm not too sure how to draw the FBD for this situation since the static friction may be working in the same direction as the Fapp since the box would slide downwards if no Fapp was affecting it. But I don't really know.
 

Answers and Replies

  • #2
tiny-tim
Science Advisor
Homework Helper
25,832
250
Hi Scimitar! Welcome to PF! :smile:
… the static friction may be working in the same direction as the Fapp since the box would slide downwards if no Fapp was affecting it. But I don't really know.
Ah, I see what you mean …

before anyone pushes, the static friction is pointing up, since it's stopping the box from sliding down.

You have to remember that static friction is not equal to µN, it is ≤ µN.

In fact, it's a "left-over" force that you have to calculate without using µ at all … it's the force that balances all the other forces.

So when you start to pull the box, the static friction gets less, and eventually it becomes 0.

Then it gets bigger, but down the ramp, until it equals µN.

After that point, it's dynamic friction, and so it is exactly µN (different µ of course :wink:).

When you draw the FBD, you can forget about the history, of the static friction changing direction, and just apply the rule that the friction opposes the motion, which by definition (ie from what the question says) is that the box is about to move upwards
 
  • #3
1
0
I was wondering how you solve the problem. And also what the answer is. (what equations to use and how to arrange them)
 
  • #4
haruspex
Science Advisor
Homework Helper
Insights Author
Gold Member
33,214
5,278
I was wondering how you solve the problem. And also what the answer is. (what equations to use and how to arrange them)
Then you need to take the role of the original poster, i.e. post an attempt.
 

Related Threads on Forces and Motion - Sliding Box Up Inclined Plane w/ Friction

Replies
1
Views
16K
  • Last Post
Replies
1
Views
991
  • Last Post
Replies
1
Views
3K
  • Last Post
Replies
5
Views
2K
  • Last Post
Replies
5
Views
3K
  • Last Post
Replies
2
Views
1K
  • Last Post
Replies
16
Views
3K
Replies
1
Views
16K
Replies
3
Views
12K
Top