HELP problem involving force of friction and normal force

In summary: The magnitude of the normal force is F = 6.0 N.b) The magnitude of the normal force is F = 10.0 N.c) The magnitude of the normal force is F = 12.0 N.
  • #1
namit05
4
0
A 2.5 kg block is initially at rest on a horizontal surface. A horizontal force of magnitude F = 6.0 N and a vertical force of magnitude P are then applied to the block. The coefficients of friction for the block and surface are μ_s = 0.40 and μ_k = 0.25. Determine the magnitude of the frictional force acting on the block if ...
a) ...the magnitude of P is 8.0 N. (Hint -- is the block moving?)

b) ...the magnitude of P is 10 N. (Hint -- is the block moving?)

c) ...the magnitude of P is 12 N. (Hint -- is the block moving?)



Fn=-mg
Ff=μFn

I have been stumped with this problem for a long time now. I understand the force being applied is both horizontally and vertically and that I have to decompose each component but I have no clue on how to do it. I tried finding the normal force by multiplying the weight 2.5 kg by 9.8 m/s2 and then using the kinetic mu to find the force of friction but this was wrong. Please help! I am really stumped.
 
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  • #2
This is not clear, what is the direction of P?
 
  • #3
Sorry. P is going straight up on the box. It is going vertically up.
 
  • #4
And for that matter, what's the direction of F? It's not enough to say "vertical" or "horizontal". We need to know "up or down" and "right or left".
 
  • #5
Direction of F is right. Direction of P is up.
 
  • #6
You're on the right track. Don't forget to calculate the maximum force of static friction (using static mu). What might help is to draw a free body diagram. In many problems, the normal force will be equal in magnitude (but opposite in direction) to the weight. This isn't the case if the surface the object rests isn't horizontal. It's also not the case if something else is lifting (at least partially) your object. If you stand on a bathroom scale placed on a level surface, the bathroom scale will give you your weight, which would be equal to the normal force (the amount that the scale is pushing back up on you.) Now, imagine someone hanging on the ceiling attaches a rope to you and pulls up slightly... what will happen to the reading on the scale? This is what's going on in your problem - there's a vertical force to consider also.
 
  • #7
Can I get some real help with the problem please. I still do not understand how to find acceleration or normal force. If anyone could help me that would be great. Thanks
 
  • #8
To figure out the normal force, do this: Identify the vertical components of all forces acting on the block. (I count three vertical forces.)

What must these forces add up to? (Assume the block does not leave the surface.) The only unknown force is the normal force; solve for it in each case.
 

1. What is the definition of force of friction?

Force of friction is a force that opposes the motion of an object when in contact with another surface. It is caused by the microscopic interactions between the surfaces of the two objects.

2. How is the force of friction calculated?

The force of friction can be calculated using the formula: F = μN, where F is the force of friction, μ is the coefficient of friction, and N is the normal force. The coefficient of friction is a constant that depends on the types of surfaces in contact.

3. What is the relationship between force of friction and normal force?

The force of friction is directly proportional to the normal force, meaning that as the normal force increases, the force of friction also increases. This is because the normal force determines the amount of contact between the two surfaces and thus the strength of the microscopic interactions that cause friction.

4. How does the force of friction affect the motion of an object?

The force of friction acts in the opposite direction of an object's motion and decreases its speed. It also causes an object to come to a stop when the force of friction is greater than the applied force.

5. How can the force of friction be reduced?

The force of friction can be reduced by using lubricants, such as oil or grease, between two surfaces. Smooth surfaces also have less friction than rough surfaces, so polishing or using smoother materials can also reduce the force of friction.

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