# Kinetic Friction Lab Question

• jlewallen18
In summary, the conversation discusses a physics problem involving a block on an incline with a coefficient of static friction and kinetic friction. The angle of the ramp is gradually increased until the block begins to slide down. The time it takes for the block to slide a distance of 0.65 m starting from rest is found by using the equation Kf = (coef of frict)*m*g*cos(theta) and kinematics. The correct answer is determined to be 0.61 seconds. The student expresses confusion and gratitude for the help received from the forum.

## Homework Statement

A 4.5 kg block is on an incline with a coefficient of static friction of 0.50. The angle that the ramp makes with the horizontal is increased gradually, until the block begins to slide down the ramp. If you know that the coefficient of kinetic friction between the block and the plane is 0.10, find the time it will take the block to slide down the ramp a distance of 0.65 m starting from rest.

## Homework Equations

Kf = (coef of frict)*m*g*cos(theta)

## The Attempt at a Solution

tan^-1 (.5) = 26.6
theta = 26.6 degrees
F_g = mg
F_g = (4.5)*(9.81) = 44.1 N
F_gx = 44.1*sin(26.6) = 19.7
a_x = 19.7 N / 4.5 kg = 4.39 m/s^2
4.39 - (coef of frict)*m*g*cos(theta)
4.39 - (.1)(4.5)cos(26.6)
4.39 - .402 = 3.99 m/s^2
d = .65
d = 1/2at^s
.65 = 1/2 (3.99) t^2
.65 = 1.995t^2
t^2 = .326
t = .571 seconds.

I'm really confused, as we haven't covered this stuff in lecture, yet we have labs that deal with it. I'm sorry if I completely fudged up something because I don't know what I'm doing...

jlewallen18 said:

## Homework Statement

A 4.5 kg block is on an incline with a coefficient of static friction of 0.50. The angle that the ramp makes with the horizontal is increased gradually, until the block begins to slide down the ramp. If you know that the coefficient of kinetic friction between the block and the plane is 0.10, find the time it will take the block to slide down the ramp a distance of 0.65 m starting from rest.

## Homework Equations

Kf = (coef of frict)*m*g*cos(theta)

## The Attempt at a Solution

tan^-1 (.5) = 26.6
theta = 26.6 degrees
F_g = mg
F_g = (4.5)*(9.81) = 44.1 N
F_gx = 44.1*sin(26.6) = 19.7
ok so far
a_x = 19.7 N / 4.5 kg = 4.39 m/s^2
you went astray here. Do not calculate acceleration in x direction until you have first determined all forces acting on the block in the x direction. You should note your units become inconsistent when subtracting force units from acceleration units.

PhanthomJay said:
ok so faryou went astray here. Do not calculate acceleration in x direction until you have first determined all forces acting on the block in the x direction. You should note your units become inconsistent when subtracting force units from acceleration units.

ok, so I should subtract Kinetic force from my F_gx first before calculating acceleration?

Yes.

jlewallen18 said:
I'm really confused, as we haven't covered this stuff in lecture, yet we have labs that deal with it
I see things haven't changed much in the 40 years since my college days!

jlewallen18 said:
ok, so I should subtract Kinetic force from my F_gx first before calculating acceleration?

Yep, force due to kinetic friction.

Then, kinematics.

PhanthomJay said:
Yes.

I see things haven't changed much in the 40 years since my college days!

Haha I guess not! Thank you all for your help, .61 s is the answer. I will definitely come back to this forum for help (if I need it) again!

## 1. What is kinetic friction?

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

## 2. How is kinetic friction measured in a lab?

Kinetic friction can be measured by using a device called a friction meter or by using a force sensor connected to a computer. The force needed to keep the object in motion at a constant speed is measured and used to calculate the kinetic friction.

## 3. What factors affect kinetic friction?

The factors that affect kinetic friction include the types of surfaces, the force pushing the objects together, and the roughness of the surfaces. Other factors such as temperature and the presence of lubricants can also affect kinetic friction.

## 4. How do you reduce kinetic friction?

Kinetic friction can be reduced by using lubricants between two surfaces, making the surfaces smoother, or by reducing the force pushing the objects together. Additionally, using wheels or ball bearings can also help reduce kinetic friction.

## 5. Why is studying kinetic friction important?

Studying kinetic friction is important because it helps us understand how different materials interact with each other and how we can improve the efficiency and performance of machines and tools. It also has practical applications in various industries such as transportation, manufacturing, and sports.