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.