Newton's Laws and Motion Review

Surface Area and Falling Speed

  • Increasing the surface area of an object decreases its falling speed due to increased air resistance.
  • Objects with larger surface area experience more air resistance because there is more area for air to push against.

Newton’s Second Law of Motion

  • When a force greater than zero is applied to an object (like kicking a kickball), the object accelerates or changes direction.
  • The mass of the object affects the force needed to move it: a heavier object requires more force to accelerate.
  • Key relationships:
    • More force results in more acceleration.
    • More mass requires more force to achieve the same acceleration.

Trick Triangle for Force, Mass, and Acceleration

  • Force (N) = Mass (kg) x Acceleration (m/s²)
  • Mass (kg) = Force (N) / Acceleration (m/s²)
  • Acceleration (m/s²) = Force (N) / Mass (kg)

Practice Problems on Force

  1. If a 1000 kg car accelerates at 5 m/s², what force is applied?

    • Formula: Force = Mass x Acceleration
    • Force = 1000 kg x 5 m/s² = 5000 N

  2. If someone pushes that car with 110 N of force, what is its acceleration?

    • Formula: Acceleration = Force / Mass
    • Acceleration = 110 N / 1000 kg = 0.11 m/s²

Understanding Acceleration

  • Acceleration is defined as a change in an object’s velocity (speed or direction).
  • Factors affecting acceleration:
    1. Amount of force applied: More force results in faster movement.
    2. Direction of force: Determines the direction of movement.
    3. Mass of the object: Heavier objects require more force to move.
  • Net Force: The total force acting on an object; calculated by:
    • Subtracting opposing forces.
    • Adding forces in the same direction.
  • Example: An object will not move if forces balance out (net force = 0).

Identifying Forces and Directions

  • Use arrows to indicate the direction of forces.
  • Example situation: If two forces act on an object, the object moves in the direction of the greater force.

Net Force Practice Problems

  • Example: 10N right force vs. 7N left force.

    • Net Force = 10N - 7N = 3N right.

  • Example: 5N right vs. 3N left.

    • Net Force = 5N - 3N = 2N right.

  • Example: Forces balance (5N right and 5N left).

    • Net Force = 5N - 5N = 0N.

Speed Equations

  • Speed is defined as the distance traveled over time.
  • Using the trick triangle:
    • Speed = Distance / Time
    • Time = Distance / Speed
    • Distance = Speed x Time
Practice Problems on Speed

  1. Car travels 150 km in 3 hours:

    • Speed = 150 km / 3 hr = 50 km/hr.

  2. Plane flies at 200 km/hr for 9 hours:

    • Distance = 200 km/hr x 9 hr = 1800 km.

  3. Ship travels 500 km at 50 km/hr:

    • Time = 500 km / 50 km/hr = 10 hr.

Understanding Velocity

  • Velocity is speed in a specific direction.
  • Example comparisons:
    • Top skater’s velocity = 60 km/hr East.
    • Bottom skater’s velocity = 60 km/hr West.
  • Same speed but different velocity due to direction.

Frictional Forces

  • Friction slows down acceleration:
    • For friction to slow a moving object, it targets the motion direction by acting opposite to it.
Types of Friction
  1. Static Friction: Force preventing movement between stationary objects.
  2. Sliding Friction: Created when surfaces slide against each other.
  3. Fluid Friction: Occurs when moving through a fluid (liquid or gas).
  4. Rolling Friction: Created when objects roll over a surface.
  • Friction occurs when surfaces interact.
  • Strength depends on:
    • Surface texture.
    • The force with which two surfaces are pressed together.