Properties of Friction H.W Problem

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    Friction Properties
AI Thread Summary
The discussion focuses on a physics homework problem involving a 2.5 kg block subjected to horizontal and vertical forces. The key task is to determine the frictional force acting on the block for varying magnitudes of the vertical force P. Participants emphasize the importance of understanding how P affects the normal force and whether static friction can prevent the block from moving. It is clarified that P does not aid in moving the block but rather increases the difficulty of movement by contributing to the normal force. A free body diagram is recommended to visualize the forces and solve for the frictional force accurately.
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Homework Statement


I can't speak physics very well. So when you help me be a little patient please.
A 2.5 kg block is initially at rest on a horizontal surface. A horizontal force F of magnitude 6.0 N and a vertical force P are then applied to the block. Fig 6 shows a picture of a block at rest and the forces applied in there directions. The coeffictients of friction for the block and surface are static friction=.40 and kinetic friction=.25. Determine the magnitude of the frictional force acting on the block if the magnitude of P is (a) 8.0 N, (b) 10 N, and (c) 12 N.


2. Homework Equations [/b
Fk=Uk Fn
Fs,max=Us Fn
-Fk=ma
Fn=mg
Fs=ma
(Kinematic Equations table)



The Attempt at a Solution


Ok I have to find the magnitude of the friction force. Usually when I come across problems finding magnitude I usually use the equation Fn=√Fx^2 + Fy^2

do i have to find something first before i find the magnitude. Like the acceleration or something.
 
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Is force P pushing the block into the horizontal surface or away from it?
 
it's pushing into the horizontal surface
 
What you need to do is first figure out if the block is able to move at all at each given value of P (i.e. is static friction strong enough to stop the block from moving). Ask yourself how P contributes to the normal force. If the block is able to get past the static friction, then you are left with the simple equation Force of friction = coefficient of friction times the normal force. Acceleration does not appear in this equation, so you do not need to worry about that.
 
Shouldn't it move because they're all greater than the force of F.
 
Are you trying to say that p doesn't help the block move anywhere.
 
You are correct that P does not help the block to move anywhere. In fact, it is helping to prevent the block from moving by contributing to how difficult it is to move the block. Draw a free body diagram for the block and find out what the normal force is. Think about how static friction acts to prevent an object from moving. Does the equation for static friction force allow the block to move at all?
 
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