Friction & Forces Concepts - PREP FOR PHYSICS UPCOMING TESTS

Info on Friday’s Test

The focus will be on the concepts of static/kinetic friction that we investigated in our 2 labs.  Knowledge of Newton's 3 Laws and how to solve Sum of Forces problems on horizontal and inclined planes is also essential.

A full list of topics we covered this unit has been posted (see 4.16) in the Forces folder.  Everything EXCEPT "Systems of Masses" is considered fair game for Friday.

Forces

• Newton’s 3 Laws of Motion:

  • First Law

    • If in equilibrium, F_NET = 0, therefore a = 0

    • If in accelerated motion, F_NET ≠ 0, therefore a ≠ 0

  • Second Law

    • ∑F_x = ma_x

    • ∑F_y = ma_y

  • Third Law

    • Action/Reaction

    • F_B￫A = -F_A￫B

• Free-body Diagrams (FBD)

• Weight

  • F_g = mg (where g = 9.8 m/s²)

  •  ≠ MASS

  • “Apparent Weight” = F_N

    • In plain speak, what the scale reads when systems accelerate up/down

• Equilibrium & Accelerated Motion Problems

• Static vs. Kinetic Friction

  • F_fk = μ_k*F_N

  • F_fs ≤ μ_s*F_N

  • μ_s / μ_k - coefficient of friction (static/kinetic)

  • ANYTHING from the Static vs. Kinetic Friction Lab

• Horizontal and Inclined Planes

• Force Vectors & Finding Components

  • (see diagram, right)

• Steps for Solving Force Problems

1. Draw an FBD and set of appropriate axes

2. Write out the Sum of Forces equations for both axes according to your FBD

3. Write out the Sum of Forces equations for both axes according to Newton’s 2nd Law

4. Set the respective Sum of Forces equations equal to each other and solve for the unknown

• Systems of Masses

  • Atwood Machine

  • Inclined planes & pulley systems, with & without friction

• Note: knowledge & use of kinematics and its respective equations are still fair game

Projectiles

List of Topics:

• Free-fall & Projectile Motion Concepts

  • Free-fall - when an object is only under the influence of gravity

  • Vertical Motion

    • a_y = - g = - 9.8 m/s²

    • “g” = acceleration on the surface of the Earth due to gravity

    • At maximum height, v_y = 0 m/s

  • Horizontal Motion

    • a_x = 0 m/s²

    • Constant velocity horizontally

      • v_ix = v_fx = v_x

• Problem-solving

  • Vertical Projectiles

    • Released from rest; v_iy = 0 m/s

    • Thrown upwards; v_iy = + (positive)

      • When landing at the original height

        • Time to rise = time to fall (t_up = t_down)

    • Thrown downwards; v_iy = - (negative)

  • Horizontal Projectiles

    • v_iy = 0 m/s

    • Height of launch (Δy) determines time in the air

  • Angled-launch Projectiles

    • Initial velocity is a combination of vertical and horizontal velocities

      • Initial Velocity = v_i

      • Angle of launch = Θ (“theta”)

        • v_ix = v_i * cos(Θ)

        • v_iy = v_i * sin(Θ)

      • Angle of launch is measured relative to the horizontal plane

• When landing at the original height

  • Time to rise = time to fall (t_up = t_down)

• At maximum height, v_y = 0 m/s

• Techniques to consider

  • Quadratic formula: when solving for t (time) and you can create an equation in the form of: At² + Bt² + C = 0

  • 2 Equations, 2 Unknowns: when you have 2 unknowns, consider how you might write equations with these unknowns in both the X- and Y-directions

Study Guide: https://drive.google.com/file/d/1YREHZbfy9uGRHzW9UaLQ7AZE7oiDvbxL/view?usp=sharing 

Vectors - Motion and Forces in Two Dimensions Section of Physics Classroom: https://www.physicsclassroom.com/class/vectors 

1. Review the content on the following pages

   1. Lesson 2 (A through G) - Projectile Motion

   2. Lesson 1 (D, E) - Finding Vector Components

2. As you go through each of the pages, complete any “Check Your Understanding” or sample problems for each of the above pages (NOTE: not every page has problems at the bottom)