Physics of Sport Midterm 1 - Lesson 3: Newton's Laws of Motion

Key Concepts

• The interaction of forces results in equilibrium or acceleration

• No Net Force = No Acceleration → No Change in Motion

  • If there is a net force on an object, then it will accelerate in the direction of the net force 

What is Force?

• Intuitively → a force is a push or a pull

• In physics → a force is an interaction between a body and the environment

• Forces are vectors → they have magnitude and direction

• Forces can be long-range or contact forces

Types of Forces

1. Weight (W) → gravitational attraction between Earth & object

2. Normal Force (F_N) → support force from surfaces

3. Tension (T) → support force by ex. ropes

4. Kinetic Friction (F_K) → contact force between 2 surfaces (tangent to surface & opposing motion)

5. Static Friction (F_S) → stronger than kinetic friction (opposes possible movement)

6. Drag Forces (F_D) → fluids resist motion

Equilibrium - Newton’s 1st Law, F₁ = F₂

• Equilibrium exists when the net force of an object is ZERO (object at rest or has constant

  • Ex. the weight of a man on a floor is supported by the floor, OR running at a constant speed, OR no motion!

  • Constant velocity = zero acceleration = zero force

Free Body Diagrams

• Special example of vector diagrams

• All forces acting on an object are drawn from it’s center 

• The size of the arrows drawn is related to their magnitude (weight) & the direction is the direction the force acts

  • Force pulling down = adding more weight

  • Force pulling up = taking away weight

Newton’s 2nd Law of Motion

• The acceleration of a body is directly proportional to the net force acting on it and inversely proportional to its mass

  • a = F_net ÷ m

Friction

• A force that resists relative motion of objects in contrast

• Friction is due to small irregularities that result in “adhesion” between the surfaces → this “stickiness” is due to electrostatic forces

• Friction is what allows us to walk

2 Types of Friction

1. Static Friction (F_s) → NOT moving, equilibrium

   • has a max value after which the object starts moving

   • F_s > F_k

   • F_s = F_applied → always since not moving

     • Ex. 400N of applied force to start it moving

2. Kinetic Friction (F_k) → Moving

   • F_k = F_applied → when moving at a constant velocity

     • Ex. 200N of force to keep it moving at a constant speed

Coefficient of Friction 

• A measure of how “sticky” 2 surfaces are 

• Measures from 0.0 → 1.0

• Has NO units

• It’s a ratio of the frictional force to the normal force on the object 

• Symbol = u (with a long tail)

  • u = F_a ÷ F_N

  • F_s = u_sF_N

  • F_k = u_kF_N

• F_{f (Friction)} can be found by measuring the applied force to start or continue sliding movement

Newton’s 3rd Law

“For every action, there is an equal and opposite reaction 

• Action = Reaction

• F₁ = F₂

Action - Reaction Forces

• If A pushes B, then B pushes A

• Pushes by A & B are equal in magnitude

• Pushes by A & B are opposite directions

• Pushes by A & B are on different objects, as a result THEY DO NOT CANCEL EACH OTHER OUT

  • F_A = -F_B

Centripetal Motion

• Moving around a circle at a CONSTANT SPEED

• Change in direction = change in velocity = acceleration 

• Based on Newton’s Laws → all acceleration requires a Net Force

  • An object moving in a curved path is FORCED

    • These forces that cause movement in a curved path = centripetal forces

    • Centripetal Motion results in Centripetal Force

Centripetal Force

• “Center Seeking”

• Greater force → Greater force

• Greater velocity → Greater force

• Smaller radius → Greater force 

• **Direction of the centripetal force is radial, towards the center of the motion

•  F_c = m*v² ÷ r

Centrifugal Force

• Force is an APPARENT force

  • Force is in a rotating framework

    • Feels like a force is throwing you out