28-March-2017: Lab 7: Modeling Friction Forces

Lab 7: Modeling Friction Forces

Author: Tian Cih Jiao

Lab Partners: Weisheng Zhang, Kitarou

Date: March 13, 2017

Purpose of the lab:

There are five different experiments involving friction. The purpose of the lab is to measure both the kinetic and static coefficients, make predictions on accelerations and compare them to experimental values.

Theory/Introduction:

(1)   Static friction: The original plan was to hang a cup of water and to keep adding water drops to it. However, the system is set up in way where the block is connected to a hanging mass with a string. Instead of adding water drops to the cup, we add to the hanging mass by 5 grams at a time. We stop adding mass to it until the block starts to slide. Record data and do calculations.

(2)   Kinetic friction: This part of the lab requires a calibrated and zeroed force sensor. By connecting the force sensor and the block using a tied string, we pull it horizontally at a constant speed and record the data. We repeat the same processes for different masses (a set of five) to have a better idea of the average of the coefficient of kinetic friction.

(3)   Static friction from a sloped surface: For this part of the lab, we slowly raise one end of the surface until the block starts to slide. Measure that data and determine the coefficient of static friction.

(4)   Kinetic friction from sliding a block down an incline: Different from the last part, we give a pretty small force to make it move at a certain acceleration. Record that data and determine the coefficient of kinetic friction.

(5)   Predicting the acceleration of a two-mass system: Based on the calculated results in previous parts, we can establish a model for the case where a hanging mass sufficiently heavy is connected to the block to accelerate the system. Compare the theoretically predicted values to our experimental results.

Summary of apparatus/experimental procedure:

This lab set up : 

LabPro, Force sensor, block, hanging masses, string, wooden surface

Measured data:

Part 1:

Part 2:

Part 3:

We use app to measure that the angle is 34 degree.

Part 4:

The angle of the incline is 

Part 5:

The model we establish for the system of the hanging mass for accelerating the block is:

Analysis:

Part 1:

The slope = coefficient of static friction.

Part 2:

The slope = coefficient of kinetic friction.

so it is smaller than static friction.

Part 3: Then we use the slipping angle to get the following equation.

Part 4:
The block slides along the surface at an angle of 21 degrees above horizontal.

We apply linear fit to the velocity v.s. time graph (y = mx + c). As shown in the graph, the slope of the graph (m) is the acceleration, which is 0.249 m/s^2.

Following is one of us calculation, using acceleration, angle and mass of the block to get coefficient of static friction.

Part 5:
Since we have established our own model, we can predict the minimum amount of mass needed to accelerate the system.

As the calculation shows, the minimum mass is 0.045 kg.

Conclusions:

From the lab, we know that coefficient of static friction is larger than that of kinetic friction.
And there is still some errors like different surface.