What is the Force Required to Keep a Block from Slipping Down a Wall in Physics?

In summary, the conversation discusses a physics problem involving a block of wood being pushed against a wall with a coefficient of friction of 0.60. The goal is to determine the force needed to prevent the block from slipping down the wall. The solution involves using Newton's laws of motion and the fact that the block is in equilibrium. The maximum value of static friction force must be used to find the least amount of normal force needed to hold the block up. The conversation also touches on the use of negatives and the possibility of ignoring friction between the hand and the block. Overall, there is a lot of helpful input given to understand the problem.
  • #1
Nx2
40
0
ok, so i got this assignment for the march break and our teacher hit us with this weird question.

You are pushing horizontally on a 3.0kg block of wood, pressing it against the wall. if the coefficient of friction between the block and the wall is μ = 0.60, how much force must you exert on the block to keep it from slipping down the wall?

so, i came to the conclusion that Fg = 29.4N, I am guessing Ff would be opposite to it, but i don't know if it will equal Fg. Also i think that the normal force will be oppsite to the applied force in this question, no? The thing is, i don't know how to solve for any of these values. would i assume that Ff equals Fg then use Ff = μ * FN and solve for FN? would that work?... i don't know...
... Any help would be very much appreciated. Thanks.

- Tu
 
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  • #2
Nx2 said:
would i assume that Ff equals Fg then use Ff = μ * FN and solve for FN? would that work?...
That is the solution. But you wouldn't 'assume' Fg = -μ * FN. You know that from Newton's laws of motion since the block does not accelerate: Fg + μFN = ma = 0

AM
 
  • #3
im sorry, I am kinda confused... so i would use Fg = -μ * FN?... i don't understand where the negetive came from.

- Tu
 
  • #4
Andrew Mason did not mean that you can't use it- his point was the you shouldn't say "assume" for something that MUST be true- if the block doesn't move then there must be no net force. There IS force Fg= -mg (the negative means downward) due to gravity so there must be a force upward of +mg. The only possible upward force is the friction force which is μ*FN: μ*FN= mg= -Fg. The reason for the negative on -Fg is because the two forces oppose: Fg is downward so μ*FN must be upward. If you are worried about the fact that μ*FN is upward, note that "negative times negative is positive" and Fg is itself negative.
 
  • #5
ooo... ic... sorry bout that... my teacher never uses negetives, he never told us anythiing bout it... but i read in the textbook that negetive means negetive direction. don't know why he doesn't use it. he always says don't worry about it... well anyways, thanks alot.

- Tu
 
  • #6
A few comments about this problem:
(1) As Andrew points out, you know the block is in equilibrium so: [itex]mg + F_f = 0[/itex]. The weight and friction force point in opposite directions.
(2) The maximum value of static friction force (for a given normal force) is given by [itex]F_f = \mu N[/itex]. Although the problem didn't state it, you are supposed to find the least amount of normal force (N) needed to hold the block up. (You are always welcome to push harder!)
(3) You are probably expected to ignore the friction between your hand and the block.
 
  • #7
thanks for all the input guys... i appreciate it. makes everything so much clearer.

- Tu
 

1. What is physics?

Physics is a branch of science that deals with the study of matter, energy, and their interactions.

2. What is horizontal force?

Horizontal force is a type of force that acts in a horizontal direction, parallel to the surface of an object.

3. How is horizontal force measured?

Horizontal force can be measured using a force meter or a spring scale. The unit of measurement for horizontal force is Newton (N).

4. What are some examples of horizontal force?

Some examples of horizontal force include pushing a book across a table, kicking a ball, or pulling a wagon.

5. How does horizontal force affect motion?

Horizontal force can cause an object to accelerate or decelerate, depending on the direction of the force. If the force is greater than the opposing force, the object will accelerate in the direction of the force. If the opposing force is greater, the object will decelerate in the opposite direction.

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