# Homework Help: Friction force problem

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1. Mar 31, 2015

### physaru86

A 1.2 kg block is resting on horizontal surface. The coefficient of static friction between block and
surface is 0.5. What is the magnitude and direction of force of friction on block when magnitude of external force acting of block in the horizontal direction is 9.8 N

fs = μs*N, N = mg

m = 1.2 kg, μs = 0.5, g = 9.8
fs = 0.5*1.2*9.8 N = 5.88 N

2. Mar 31, 2015

### Staff: Mentor

Is this really the full problem statement, exactly as presented, because something seems wrong about it.

Chet

3. Mar 31, 2015

### physaru86

please tell me what is wrong about it?

4. Mar 31, 2015

### Staff: Mentor

If the applied force is less than the normal force times the coefficient of static friction, then the friction force will equal the applied force and the body will not move. If the applied force is equal to the normal force times the coefficient of static friction, then the friction force will equal the applied force and the body will be on the verge of moving. But, in your example, the applied force is greater than the normal force times the coefficient of static friction, so the body will move, and kinetic friction will prevail; in this case, the coefficient of static friction is irrelevant, as is the normal force times the coefficient of static friction.

Chet

5. Mar 31, 2015

### physaru86

S
So can the force of kinetic friction be found? if so how to find

6. Mar 31, 2015

### Staff: Mentor

To find that, you need to know the coefficient of kinetic friction.

Chet

7. Mar 31, 2015

### physaru86

Yes it was the same doubt i had. can the coefficient of kinetic friction be found

8. Mar 31, 2015

### Staff: Mentor

Sure, it can be measured.

Chet

9. Mar 31, 2015

### physaru86

thanks

10. Apr 2, 2015

### joaopedro._

Oposed to motion

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