Push a Block against a Wall- Static Friction

In summary, the problem involves a block weighing 13.0 N being pushed against a wall with a horizontal force of 9 N. The coefficients of static and kinetic friction are given as 0.62 and 0.42 respectively. To determine the normal force exerted by the wall and the block's acceleration, Newton's laws must be used. In the horizontal direction, the two forces are equal and opposite, resulting in zero acceleration. In the vertical direction, the acceleration is not 9.8m/s^2 as the block is not in free fall. A free body diagram must be drawn to determine the direction of the acceleration, and Newton's laws must be used to solve for it.
  • #1
Phoenixtears
83
0

Homework Statement



A 9 N horizontal force F pushes a block weighing 13.0 N against a vertical wall (Fig. 6-21). The coefficient of static friction between the wall and the block is 0.62, and the coefficient of kinetic friction is 0.42. Assume that the block is not moving initially.


(Image attached)

Determine the normal force that the wall exerts against the block
9 N (It is the same amount pushing against the block- they should be equal)

Determine the blocks acceleration
m/s^2


Homework Equations



Fk= (Coefficient of K)*N
F=ma
Max= Push- normal
May= Static- mg

The Attempt at a Solution



Alright, I began drawing a force graph. On this graph weight faces down, normal force is exerted left (see attachement), static force is exerted up, and push force is exerted right. Determining the acceleration means horizontally, because the acceleration vertically is 9.8. Now, looking at my equations, only one contains 'a': F=ma. So, what do I know in this equation? I can get mass from the 13 N weight. 13/9.8= 1.32653 mass. That just leaves me to find the force. Going back, the acceleration I am trying to find is horizontal. So, I need to use the push-normal to get my total force. Here's my big problem, don't the push force and the normal force equal each other? So that would leave me with an Ma= 0. What does this mean? What have I done wrong?

Thank you in advance!

~Phoenix
 

Attachments

  • Block Against Wall.gif
    Block Against Wall.gif
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Last edited:
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  • #2
Hi Phoenixtears,

At this point I cannot view your attachment, but I can point out a couple of things about what you have written.

In the horizontal direction, if the two horizontal forces are equal and opposite, then they will give zero horizontal acceleration like you have found.

In the vertical direction, you seem to be assuming that the acceleration is 9.8m/s^2, which is not true. If an object is in free fall, its acceleration is 9.8m/s^2 downwards, but being in free fall means that gravity is the only force acting on it. However, here there are two forces in the vertical direction: gravity and the frictional force, so you have to use Newton's law to find out what the acceleration is.
 
  • #3
Draw a free body diagram, if using the textbook knight. Otherwise draw a simple diagram and just as alphysicist suggested use Newtons Laws. You are asked for an acceleration but what direction is it in. Which way do you think it would go, imagine lightly holding a block up against a wall which way would it likely go?
 

1. What is static friction?

Static friction is the force that prevents an object from sliding against a surface when it is at rest. It is the force that must be overcome in order to start an object's motion.

2. How is static friction calculated?

Static friction is calculated by multiplying the coefficient of static friction (μs) by the normal force (N). The coefficient of static friction is a constant that depends on the materials in contact, and the normal force is the force exerted by the surface on the object perpendicular to the surface.

3. What factors affect the magnitude of static friction?

The magnitude of static friction depends on the coefficient of static friction, the normal force, and the roughness or smoothness of the surface. A larger normal force or a rougher surface will result in a larger static friction force.

4. Can static friction be greater than the force applied?

Yes, static friction can be greater than the force applied. This is because static friction adjusts to match the force applied, up to a certain limit known as the maximum static friction force. If the force applied exceeds this limit, the object will start to move.

5. How does the angle of the surface affect static friction?

The angle of the surface does not directly affect static friction. However, it can indirectly affect it by changing the normal force and the coefficient of static friction. For example, if the surface is tilted, the normal force and the coefficient of static friction will change, which will affect the magnitude of the static friction force.

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