Science 9: Inertia and Newton's First Law of Motion Notes

Learning Objectives and Competencies

• Definition of Inertia: Identify inertia as the tendency for an object to stay at rest or in motion unless acted on by an unbalanced net force. It is defined as the inherent property of an object to resist changes in its motion, either remaining at rest or moving uniformly in a straight line unless acted upon by an external force.

• Learning Competencies:

  • Recognize friction as an external force.

  • Identify instances of inertia in objects already in motion and describe how these objects persist in their motion without changing velocity unless an external force is applied.

  • Apply understanding of inertia to analyze and explain everyday occurrences, including:

    • The use of seatbelts in vehicles.

    • Skidding on a slippery road.

    • Balancing on a bicycle.

    • Jerking wet clothes to remove water.

Review of Forces and Motion

• Effects of Forces: Forces are responsible for changing the state of motion of an object.

• Differentiating Forces:

  • Balanced Forces: Occur when the net force is zero and the object's state of motion does not change.

  • Unbalanced Forces: Occur when the net force is non-zero, leading to a change in the object's state of motion.

• Force Vector Scenarios: Identifying and drawing force vectors for specific scenarios:

  • A falling fruit: Gravitational force acting downward.

  • An accelerating car to the right: Applied force to the right, possibly opposing friction/air resistance.

  • A book on top of a table: Normal force acting upward and gravity acting downward.

  • A picture frame on the wall: Gravity acting downward and tension or static friction acting upward.

Activity #1: Identifying Forces and Their Effects on Motion

• Objective: Identify and draw forces acting on an object and determine the object's state of motion.

• Materials Required: Ruler and drawing pen.

• Procedure:

  1. Identify forces acting on illustrated objects.

  2. Draw a Free-Body Diagram (FBD) and label all forces involved.

  3. Determine the magnitude and direction of the net force ($F_{\text{net}}$).

• Analysis Scenarios:

  • Case A: A book on top of a table (Static equilibrium).

  • Case B: An airplane flying at a constant velocity (Dynamic equilibrium).

  • Case C: A jeepney speeding up (Acceleration).

  • Case D: A jeepney slowing down to pick up a passenger (Deceleration).

• Observations and Findings:

  • Net Force Zero: Occurs when an object is at rest or moving at a constant velocity (constant speed in a straight line). All individual forces (gravity, friction, applied force, etc.) perfectly cancel each other out, resulting in a state of equilibrium.

  • Non-Zero Net Force: Occurs whenever an object is accelerating, meaning its speed or direction is changing due to unbalanced forces.

  • Changing State of Motion: An object changes its state of motion (speed, direction, or both) whenever a non-zero net force acts upon it.

  • Maintaining State of Motion: No net force is required to maintain an object's state of motion; it naturally continues moving at a constant velocity unless an unbalanced force acts on it.

Characteristics and Determinants of Friction

• Definition of Friction: A force that opposes sliding between two surfaces.

• Frictional Force Influences:

  • Surface Type: Friction is influenced by the type of surface in contact. Rougher surfaces are more likely to have a larger frictional force.

  • Force Dependency: Frictional force is zero if there is no force trying to make the surfaces of the two objects slide.

• Activity #1.2: Friction Experiment:

  • Materials:

    • Wooden flank ($20 \times 50\,\text{cm}$).

    • Sandpaper Grit #60-80 (Extremely coarse).

    • Sandpaper Grit #100-150 (Medium grit).

    • Sandpaper Grit #180-220 (Fine grit).

    • Plastic book cover.

    • Set of identical books.

    • A small block of wood.

  • Experimental Procedure:

    1. Set up a ramp using books to create height.

    2. Cover the flank with different materials (sandpapers or plastic cover).

    3. Determine the ramp height (number of books) needed for the wood block to start sliding on Grit #100-150.

    4. Measure the time taken for the block to reach the bottom for each surface over three trials and calculate the average time.

• Experiment Discussion:

  • Fastest Descent: Occurs in setups with the steepest incline and least friction, as these produce the greatest net force and acceleration.

  • Lack of Movement: Occurs on low inclines or high-friction surfaces where the component of gravity is insufficient to overcome static friction.

  • Variation in Time: Time differences are due to changes in net force caused by varying friction levels or incline angles.

  • Perfectly Smooth Surface: If the surface were perfectly smooth, frictional force would be eliminated, causing maximum acceleration due to the unopposed component of gravity.

Historical Perspectives on Motion

• Aristotle's Theory (Over 2,000 years ago): Claimed a constant force is required to keep an object moving at a steady speed and that extra force is necessary for acceleration.

• Galileo's Challenge (End of the 16th Century): Argued that in the absence of friction, an object would continue to move at a constant speed in a straight line indefinitely.

• Galileo's Thought Experiment: Involves observing object behavior on ramps to demonstrate that objects do not require a force to maintain motion once started, provided friction is absent.

The Law of Inertia (Newton's First Law)

• Core Definition: Inertia is the tendency of an object to resist any change in its motion.

• Fundamental Principles:

  • An object will stay at rest or keep moving unless a force acts on it.

  • Objects do not change motion on their own.

  • A force is required only to start motion, stop motion, or change the direction of motion.

• Real-World Examples:

  • Sudden Bus Stops: When a bus brakes suddenly, the passengers' bodies continue in their forward motion due to inertia.

  • Coin and Card Experiment: If a card placed over a glass with a coin on top is pulled quickly, the coin falls into the glass because its inertia keeps it in place until gravity acts on it.

The Relationship Between Inertia and Mass

• Proportionality: $More\,mass = more\,inertia$.

• Mass Persistence: Heavier objects are harder to move (from rest) or stop (from motion) compared to lighter objects.

• Example: It is significantly easier to push a small cart than it is to push a full-sized car because the car has much more mass and, therefore, more inertia.

Forces and Motion Virtual Simulation (PhET)

• Objectives: Relate net force to the state of motion and identify friction as the force responsible for slowing objects down.

• Simulation Elements:

  • Net Force Sim: Demonstrates that an object remains at rest if the net force is zero.

  • Motion Sim: Demonstrates that an object accelerates when a net force is applied. If the net force is removed ($F_{\text{applied}} = 0\,\text{N}$), the object continues moving at the speed it reached.

• Friction Simulation Steps:

  1. Apply a force (e.g., $125\,\text{N}$) until the box moves.

  2. Adding a small increment (e.g., to $126\,\text{N}$) shows resulting acceleration.

  3. If the pushing person stops, friction ($F_{\text{friction}}$) acts in the opposite direction, causing the speed to decrease.

  4. Observations: In a frictionless environment (Step 4), speed remains constant once the force is removed. In an environment with friction, speed decreases to zero once the active pushing force stops.

• Speed Threshold: In simulations, if a box reaches high speeds (e.g., $40\,\text{m/s}$), external actors (like the person pushing) may no longer be able to maintain the pace, highlighting the limits of applied force vs. the persistence of inertia.