# Find the friction force impeding its motion

• 312213
In summary, the friction force impeding the motion of a 21.0 kg box released on a 38.0° incline is correctly calculated as 121N. To find the coefficient of friction, one should draw a diagram and identify the forces involved, including the component of gravity that accelerates the box down the plane and the component that is equal and opposite to the normal force. The equation used to find the coefficient of friction is Ffr = \muFN, and using the correct values, the coefficient of friction is calculated to be 0.747. Understanding the relationships between forces is important before using equations.
312213

## Homework Statement

A 21.0 kg box is released on a 38.0° incline and accelerates down the incline at 0.267 m/s2. Find the friction force impeding its motion.

Friction force is correctly calculated as 121N

## Homework Equations

Ffr = $$\mu$$FN (Friction force equals mu times normal force, in case the symbols are not in its commonly used form)
FN=mg (Normal force equals mass times gravity)

## The Attempt at a Solution

Ffr = $$\mu$$FN
121=$$\mu$$FN
121=$$\mu$$mg
121=$$\mu$$(21)(9.8)
5.7619=tex]\mu[/tex](9.8)
0.588=$$\mu$$

I see this as the correct method, but the answer came out as incorrect, so that means I'm wrong. Is this formula incorrect for finding mu? If so, what is supposed to be used/what am I supposed to do in order to find mu?

note: numbers are obtained using longer, more accurate numbers of 121.09613162202047395584132290722 / ((21)(9.8)) = 0.5879494655004859086491739552964 but shortened down to look better

312213 said:

## Homework Statement

A 21.0 kg box is released on a 38.0° incline and accelerates down the incline at 0.267 m/s2. Find the friction force impeding its motion.

Friction force is correctly calculated as 121N

## Homework Equations

Ffr = $$\mu$$FN (Friction force equals mu times normal force, in case the symbols are not in its commonly used form)
FN=mg (Normal force equals mass times gravity)

## The Attempt at a Solution

Ffr = $$\mu$$FN
121=$$\mu$$FN
121=$$\mu$$mg
121=$$\mu$$(21)(9.8)
5.7619=tex]\mu[/tex](9.8)
0.588=$$\mu$$

I see this as the correct method, but the answer came out as incorrect, so that means I'm wrong. Is this formula incorrect for finding mu? If so, what is supposed to be used/what am I supposed to do in order to find mu?

note: numbers are obtained using longer, more accurate numbers of 121.09613162202047395584132290722 / ((21)(9.8)) = 0.5879494655004859086491739552964 but shortened down to look better

Generally for inclined plane problems one draws a set of coordinate axes with one axis, say the x-axis along the plane and the y-axis perpendicular to it. Then you should draw in your forces: mg for gravity, N for the normal force between the plane and the block, f for the frictional force acting up the plane. You will need to identify the component of mg that accelerates the box down the plane and the component of mg that is equal and opposite to the normal force. You will have some sines and cosines floating around. (I'll leave it to you to figure that out.) The moral of this story: Don't start hunting for equations before you have drawn a picture and identified the forces involved and understand the relationships between them. Understand what's happening before you start using equations!

121=$$\mu$$(21)(9.8)(cos38)
0.747=$$\mu$$

I drew the diagram but forgot the the inclination affects the total force. Thank you.

Last edited:

## 1. What is friction force?

Friction force is a resistive force that opposes the motion of an object as it slides or moves across a surface.

## 2. How is friction force measured?

Friction force is typically measured in newtons (N) using a force meter or spring scale.

## 3. What factors affect the amount of friction force?

The amount of friction force depends on the types of surfaces in contact, the force pressing the surfaces together, and the roughness of the surfaces.

## 4. How does friction force affect an object's motion?

Friction force acts in the opposite direction to an object's motion, causing it to slow down and eventually come to a stop.

## 5. How can friction force be reduced?

Friction force can be reduced by using lubricants, making surfaces smoother, or reducing the force pressing the surfaces together.

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