# Static and Kinetic Friction Forces (Finding Net Force)

• FAJISTE
In summary, the block slides due to the static and kinetic friction. The net force on the block is 21.3 N.
FAJISTE

## Homework Statement

The coefficient of static friction between a block and a horizontal floor is 0.414, while the coefficient of kinetic friction is 0.138. The mass of the block is 5.24 kg. A horizontal force is applied to the block and slowly increased. What is the value of the applied horizontal force at the instant that the block starts to slide?

b) What is the net force on the block after it starts to slide?

F=uN

## The Attempt at a Solution

a)Okay, so the answer to the first part is 21.3 N.

F = uN where us is .414 and N is mg.

b) I don't get how to find the net force. So the mg and Normal force cancel out, so I calculated the horizontal forces using F=uN. I subtracted the static force from the kinetic force and the answer was wrong. What am I doing wrong?

No information is given about the acceleration, which affects the answer. Some assumption must be made. You could assume the object is moving in uniform motion with no acceleration, in which case the net force is zero from F = ma. However, I suspect you are supposed to assume that the force you found in part (a) continues to be applied. Since the friction force is now reduced, the object will accelerate horizontally so there is a nonzero net force acting on it. Just add up the horizontal forces (not including F = ma which tells what acceleration the net force causes). The vertical forces add to zero as you said.

Yea what I'm not sure about is what forces they are.

Is it the static Friction force + the Kinetic Friction force? I really don't know what the horizontal forces are

bump sry but its due tomorrow

doesn't anyone know this?

bump bump bump

FAJISTE: For part (b), change the plus sign in post 3 to a minus sign.

Yeah i tried that too and the answer was wrong :(

FAJISTE: You said you tried that, but what number did you try? Show your work so we can make sure we are talking about the same equation and numbers. How many tries are you allowed? I agree with the post by Delphi51. In other words, the only other logical tries for part (b) I can think of (so far), if the above fails (and show your work for that), are zero, and "unknown." Do they subtract points from you for multiple tries? Also, are you sure you did not omit any words from the given problem statement in post 1?

n/m got it lol, i was putting 14.9 into the computer instead of 14.19

Thanks though!

## 1. What is the difference between static and kinetic friction forces?

Static friction force is the force that prevents an object from moving when a force is applied to it, while kinetic friction force is the force that opposes the motion of an object that is already in motion.

## 2. How do you calculate the net force when both static and kinetic friction forces are present?

The net force is equal to the difference between the applied force and the static friction force, or the sum of the applied force and the kinetic friction force, depending on the direction of motion.

## 3. What factors affect the magnitude of static friction force?

The magnitude of static friction force depends on the normal force between the surfaces in contact, the coefficient of static friction, and the roughness of the surfaces.

## 4. How can the coefficient of kinetic friction be determined experimentally?

The coefficient of kinetic friction can be determined by measuring the force required to keep an object moving at a constant velocity and dividing it by the normal force.

## 5. How does the angle of incline affect the friction forces?

The angle of incline affects the normal force and therefore, the magnitude of the friction forces. The steeper the incline, the greater the normal force and the greater the friction forces.

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