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7.2. Solved Examples: Force of Friction and Normal Force; Free Body Diagrams

Example 1

A [latex]17 \, \text{kg}[/latex] toolbox sits on a level floor. A worker uses a rope and pulls the toolbox with a force F (see diagram below). The coefficients of static friction and kinetic friction between the toolbox and the floor are [latex]0.65[/latex], respectively [latex]0.40[/latex].

  1. Find the pulling force needed to just set the toolbox in motion.
  2. If then, the toolbox continues to move with an acceleration of [latex]0.20 \,\text{m/s}^2[/latex], find the new force needed to maintain this acceleration.
Free-body diagram of a toolbox being pulled horizontally, showing applied force F, friction, weight, and normal force.
Figure 7.5. Free-body diagram of a toolbox being pulled with force F.
Given:
[latex]m = 17 \text{ kg}[/latex]
[latex]mu_s = 0.65[/latex]
[latex]mu_k = 0.40[/latex]
[latex]a = 0.20 \text{ m/s}^2[/latex]
Find:
[latex]\text{(a)} \quad F = ?[/latex]
[latex]\text{(b)} \quad F_1 = ?[/latex]

Solution:

  1. The pulling force must overcome the friction for the toolbox to start moving.

    [latex]F > F_f[/latex], but [latex]F_f = \mu_s F_N[/latex]

    Analyze the motion on the axis (axes). Ask the question: Is this a situation of equilibrium?

    Since [latex]F_N[/latex] is positioned on the y axis, we need to ask the equilibrium question about the [latex]y[/latex] axis.

    On the y axis we have a situation of equilibrium because the toolbox is not moving on the [latex]y[/latex] axis. This means that:

    [latex]F_{\text{net}, y} = 0[/latex]

    [latex]F_{\text{net}, y} = F_N - F_g = 0, \quad \text{or} \quad F_N = F_g[/latex]

    On the other hand,

    [latex]F_g = m \times g[/latex]

    Substitute the expressions into [latex]F_f[/latex] :

    [latex]F_f = \mu_s \times m \times g = 0.65 \times 17 \times 9.8 = 108.29 \text{ N}[/latex]

    Answer:

    [latex]F > 108.29 \text{ N}[/latex]


  2. Analyze the motion on the axis (axes).
    Ask the question: Is this a situation of equilibrium? Since F is positioned on the x axis, we need to ask the equilibrium question about the [latex]x[/latex] axis.On the [latex]x[/latex] axis we do not have a situation of equilibrium because the toolbox is moving with an acceleration.In this case we use Newton’s second law and the net force on the x axis to solve the question:

    [latex]\text{(a)} \quad F_{\text{net}} = m \times a[/latex]

    [latex]\text{(b)} \quad F_{\text{net}} = F - F_f[/latex]

    Since expressions (a) and (b) represent the same net force, we can equal them:

    [latex]F - F_f = m \times a[/latex]

    Solving for [latex]F[/latex]:

    [latex]F = F_f + m \times a = \mu_k \times m \times g + m \times a = 0.40 \times 17 \times 9.8 + 17 \times 0.20 = 70.04 \text{ N}[/latex]

    Answer:

    [latex]F = 70.04 \text{ N}[/latex]

Notes

  1. Recall: Equilibrium occurs when either the object is at rest, or it moves at a constant speed.
    Equations to use:

    [latex]F_{\text{net}, x} = 0[/latex]

    [latex]F_{\text{net}, y} = 0[/latex]

  2. Recall: An object moving with an acceleration is not in a situation of equilibrium.
    Equations to use:

    [latex]F_{\text{net}} = m \times a[/latex]

    [latex]F_{\text{net}} = \sum \overline{F}[/latex]

Try it!

  1. A sled is being pulled by a child with a rope that makes a [latex]30°[/latex] angle with the vertical. The sled is moving horizontally at a constant speed. Which one of the following statements are true?
    1. Equilibrium situation on both [latex]x[/latex] and [latex]y[/latex] axes
    2. Equilibrium situation only on [latex]y[/latex]
    3. Equilibrium situation only on [latex]x[/latex]
  2. The normal force is always directed:
    1. Parallel to the contact surface
    2. Opposing the motion of the object
    3. Perpendicular on the contact surface
  3. If an object moves horizontally at a constant speed due of a horizontal pulling force of [latex]50 \text{ N}[/latex], what is the force of friction?
    1. Not enough information to calculate it
    2. [latex]50 \text{ N}[/latex]
    3. [latex]490 \text{ N}[/latex]

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College Physics – Fundamentals and Applications Copyright © by Daniela Stanescu, Centennial College is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.