Force and Motion

Force and Motion

  • Understanding the relationship among force, mass, and motion is essential.

Forces

  • Definition: A force is a push or pull exerted on an object.

    • It has both size and direction, making it a vector quantity.

    • Represented using arrows to indicate magnitude and direction.

  • Types of Forces in Nature:

    • Gravitational force

    • Electromagnetic force

    • Strong nuclear force

    • Weak nuclear force

    • SI unit for force: Newton (N)

  • Forces act in pairs and do not always result in motion.

    Net Forces: The total force acting on an object when all individual forces are combined. A positive net force indicates motion in the direction of that force, while a net force of zero implies that an object is either at rest or moving at a constant velocity. Forces act in pairs and do not always result in motion.

Gravitational Force

  • Gravitational force pulls objects downward towards the Earth's center.

  • An upward force can accelerate objects against gravity.

Contact Forces

  • Include various types such as:

    • Friction

    • Air resistance

    • Spring force

    • Normal force

    • Applied force

    • Tension

  • Sir Isaac Newton's Observations:

    • An object in motion stays in constant motion unless acted upon by a force (First Law).

    • To move an object, a force is required; greater force leads to greater acceleration.

Motion

  • Distance: Total ground traveled, a scalar quantity.

    • Can NEVER be negative! Only positive!

      • Scalar Quantity: Only has magnitude

      • Requires two things:

        • Value

        • Appropriate units

      • Ex. Mass, Temperature, and speed

  • Displacement: Distance with direction, a vector quantity.

    • It can be positive or negative

      • Positive = north and/or east direction

      • Negative = south and/or west direction

        • Vector quantity: Has magnitude AND direction

        • Requires 3 things:

          • Value

          • Appropriate units

          • DIRECTION!

          • Ex. Acceleration and Velocity

        • On graphs:

          • Vector represents by arrow

          • Length = magnitude

          • Arrow faces direction of motion

          • A vector can be “picked up” and moved on the paper as long as the length and direction its pointing does not change

    • If an object moves in a single direction, the displacement = distance + direction

      • However, if an object moves in two opposing directions, the displacement is the difference between the two

      • Also, if an object moves in two directions, a triangle will be formed.

        • If the angle is 90 degrees, use the Pythagorean Theorem.

Speed

  • Definition: The distance traveled over time.

    • Calculated using a specific formula.

    • SI unit for speed: meters per second (m/s).

  • Types of Speed:

    • Average speed: Total distance divided by total time.

    • Instantaneous speed: Speed at a specific moment.

Velocity

  • Definition: A vector quantity that includes both speed and direction.

    • SI unit for velocity: meters per second (m/s).

    • Changes if either speed or direction changes.

Acceleration

  • Describes how an object's velocity changes over time.

    • Can involve speeding up, slowing down, or changing direction.

    • SI unit for acceleration: meters per second squared (m/s²).

Motion Graphs

  • Used to visualize an object's motion over time, including:

    • Position vs. Time graphs

    • Velocity vs. Time graphs

    • The graph's slope indicates speed:

      • Steeper slope = faster motion

      • Gentle slope = slower motion

Newton's Laws of Motion

  • First Law: An object at rest stays at rest, and an object in motion stays in motion unless acted upon by an unbalanced force.

    • Inertia: Resistance of an object to change its state of motion.

  • Second Law: Motion of an object depends on the magnitude and direction of acting forces and mass.

    • Formula: Force = Mass x Acceleration.

    • Greater mass requires greater force for motion or direction change.

    • Unbalanced forces cause acceleration.

  • Third Law: For every action, there is an equal and opposite reaction.

    • Example: If Object A exerts a force on Object B, then Object B exerts an equal and opposite force on Object A.

Falling Objects

  • Gravitational force acts between objects with mass.

    • It is non-contact and can travel through space.

  • Factors affecting gravitational force:

    • Mass: Greater mass increases gravitational force (direct relationship).

    • Distance: Increasing distance decreases gravitational force (indirect relationship).

  • Humans and objects on Earth experience minimal effect from each other's gravitational force due to their comparatively small masses.

    • Mass: Amount of matter in an object.

    • Weight: Force due to gravity (changes with gravity but mass remains constant).

  • Free fall is when only gravity acts on an object; all free-falling objects accelerate at 9.8 m/s² regardless of mass.

Inertia and Relations with Forces

  • Inertia dictates that all objects resist changes in motion.

    • The more massive an object, the greater the resistance.

  • Newton's Second Law reflects the relationship as follows:

    • Force = Mass x Acceleration, or rearranged: Acceleration = Force/Mass.

    • Force and acceleration have a direct relationship, while mass and acceleration are inversely related.

  • All objects in the absence of air resistance fall at the same rate (9.8 m/s²).

    • This is also expressed as Weight = Mass x Gravity.