# Friction Lab Report

## Purpose

To measure the friction between two objects and calculate the coefficient of friction (μ) for each surface (corkboard, rubber, sandpaper, particle board, dry towel, and wet towel), and understand frictional force based on various circumstances.

## Hypotheses

1.) If an object has a greater weight, then its frictional force will be greater as well.

2.) If a surface has more granules, then it will have a greater coefficient of kinetic friction.

3.) If a towel is wet rather than dry, then an object going across the towel will experience a greater friction.

## Materials

Corkboard surface, Rubber, Sandpaper, Particle board, Spring scale, Wooden blocks A and B with hooks, 2 Towels, Water

## Calculations

Frictional force = The amount of Newtons on the spring scale while the object is dragged across a surface.

Normal force = Weight

μ = The coefficient of kinetic friction for a surface.

Kinetic Friction Equation: μ = Frictional Force / Normal Force

Object A kinetic friction

Corkboard: μ = .35 / .85 = .41

Rubber: μ = .40 / .85 = .47

Sandpaper: μ = .50 / .85 = .59

Particle Board: μ = .10 / .85 = .12

Dry Towel: μ = .60 / .85 = .71

Wet Towel: μ = 1.5 / .85 = 1.8

Object B kinetic friction

Corkboard: μ = .45 / 1.15 = .39

Rubber: μ = .50 / 1.15 = .43

Sandpaper: μ = .60 / 1.15 = .52

Particle Board: μ = .15 / 1.15 = .13

Dry Towel: μ = .70 / 1.15 = .61

Wet Towel: 2.0 / 1.15 = 1.7

Accepted coefficients for kinetic friction

Corkboard: μ = Accepted value not available

Rubber: μ = .53

Sandpaper: μ = .613

Particle Board: μ = .2

Dry Towel: μ = Accepted value not available

Wet Towel: μ = Accepted value not available

Percent error equation:

((Accepted value - Average value from experiment) / Average value from experiment) * 100 = Percent Error

Rubber: ((.53 - .45) / .45) * 100.0 = 13%

Sandpaper: ((.613 - .555) / .555) * 100.0 = 10.5%

Particle Board: ((.2 - .125) / .125) * 100.0 = 60%

## Analysis

The first hypothesis was if an object has a greater weight, then its kinetic frictional force would be greater as well. The hypothesis is correct. In all the surfaces tested, the kinetic frictional force of object B, the object with greater weight, was greater than the kinetic frictional force of object A, the object with lesser weight. For example, on the corkboard surface, the kinetic frictional force of object A was .35 N, whereas the kinetic frictional force of object B was .45 N, and on the wet towel surface, the kinetic frictional force of object A was 1.5 N, and the kinetic frictional force of object B was 2.0 N. If an object has a greater weight, its kinetic frictional force must be greater as well because of the equation μ = Kinetic Frictional Force / Normal Force. μ is a coefficient constant, and stays the same for each surface. Therefore, when there was a greater weight (normal force), there was a greater kinetic frictional force. The greater the normal force is, the greater the kinetic frictional force has to be to keep μ constant.

The second hypothesis was if a surface has larger granules, then it will have a greater coefficient of kinetic friction. The hypothesis is correct. The surfaces with the largest granules from greatest to least were in this order - wet towel and towel, sandpaper, rubber, corkboard, and particle board. When the wooden blocks were dragged across the different surfaces with different granules, the towels had the greatest coefficient of kinetic friction, the wet towel having an average coefficient of 1.75, and the dry towel having an average coefficient of .66. The average coefficient of sandpaper was .555, rubber was .45, corkboard was .4, and particle board was .125. The surfaces which had the largest granules also had the greatest coefficients of kinetic friction. This is because of Newton's first law, which states that an object in motion will stay in motion unless an external force acts upon it. Surfaces with larger granules were able to stop an object more easily, as they were more obstructive, therefore increasing the coefficient of kinetic friction.

The third hypothesis was if a towel is wet rather than dry, then an object going across that wet towel would experience a greater kinetic frictional force. This hypothesis is correct. For both objects A and B, the kinetic frictional force for the wet towel (1.5 N, 2.0 N) was greater than the kinetic frictional force of the dry towel (.6 N, .7 N). The objects going across the wet towel had a greater kinetic frictional force because the water sticking to the towel caused the hairs of it to stand up, thereby blocking the object's movement and requiring more force to get across the kinetic frictional force. Also, the water on the towel may have caused the object to be more likely to stick to the towel, therefore increasing the kinetic frictional force.

μ does not have any units because of its equation "μ = Frictional Force / Normal Force". Both frictional force and normal force have the units N, or Newtons, so when they divide each other they cancel out, leaving μ with no units. The percent error for rubber was 13%, for sandpaper was 10.5%, and for particle board was 60%. The potential causes for error were that the experimenters may not have used enough force to pull the wooden block over each surface, therefore decreasing the coefficient of kinetic friction calculated. Another error may have been that the experimenters did not drag the wooden block across the surface at an exact 180 degree angle (horizontally), therefore the frictional force calculated may have changed and affected the calculated coefficient of kinetic friction as a whole for each surface.