Year 10 Physics: Motion, Forces, Energy and Work

Fundamental Motion Concepts

• Speed ($v$): A measurement of the rate at which an object covers distance. It is often used interchangeably with velocity in introductory contexts.
• Average Speed Formula: Calculated as total distance divided by total time:

$v = \frac{d}{t}$

• Units: Standard units include kilometers per hour ($km/h$), meters per second ($m/s$), and centimeters per minute ($cm/min$).
• Conversions:
  • To convert from $km/h$ to $m/s$, divide by $3.6$.
  • To convert from $m/s$ to $km/h$, multiply by $3.6$.
  • $1\,kilometer = 1000\,meters$; $1\,hour = 3600\,seconds$.

Scalars and Vectors

• Physical Quantities: Characteristics that can be measured.
• Scalar Quantities: Possess magnitude only. Examples include distance ($m$, $km$), speed ($m/s$), time ($s$), mass ($kg$), temperature ($K$), and energy ($J$).
• Vector Quantities: Possess both magnitude and direction. Examples include displacement, velocity, acceleration, weight ($N$), and force ($N$).

Motion Graphs

• Distance-Time Graphs:
  • Gradient: The slope of the line equals the speed of the object.
  • Positive Gradient: Increasing distance from the reference point.
  • Negative Gradient: Decreasing distance from the reference point.
  • Horizontal Line: The object is stationary (velocity is zero).
• Velocity-Time Graphs:
  • Gradient: The slope of the line represents acceleration ($a$).
  • Area Under Curve: Represents the total distance traveled by the object.

Newton’s Laws of Motion

• First Law (Inertia): An object will remain at rest or in uniform motion in a straight line unless acted upon by an external unbalanced force. Inertia is the resistance to change in motion and is directly proportional to mass.
• Second Law ($F = ma$): The acceleration of an object depends on the mass of the object and the amount of force applied. Force ($F$) is measured in Newtons ($N$).
• Third Law (Action and Reaction): Whenever two bodies interact, they exert forces on each other that are equal in magnitude and opposite in direction ($F_{A\rightarrow B} = -F_{B\rightarrow A}$).

Forces and Resultant Force

• Force: A push or pull acting on an object due to interaction. Types include gravitational, electrostatic, thrust, upthrust, air resistance (drag), compression, and tension.
• Resultant (Net) Force: The single force representing the vector sum of all forces acting on a body.
  • Balanced Forces: Resultant force is zero; motion remains constant.
  • Unbalanced Forces: Results in acceleration or deceleration.

Energy, Work, and Power

• Work Done ($W$): Energy transferred when a force moves an object over a distance ($d$) in the direction of the force:

$W = F \times d$

• Gravitational Potential Energy (GPE): Stored energy due to position in a gravitational field:

$GPE = m \times g \times h$

*   Earth's gravitational field strength ($g$) is approximately $9.8\,N/kg$.

• Kinetic Energy (KE): Energy of a moving object:

$KE = \frac{1}{2}mv^2$

Momentum and Collisions

• Momentum ($p$): Defined as mass in motion:

$p = m \times v$

• Law of Conservation of Momentum: The total momentum of a closed system before a collision is equal to the total momentum after the collision.
• Car Safety: Features like crumple zones, seatbelts, and airbags reduce the force of impact by increasing the time over which the change in momentum (impulse) occurs.

Questions & Discussion

• Q: How do we convert between $m/s$ and $km/h$?
• A: Use the factor of $3.6$ ($m/s \times 3.6 = km/h$). Using $0.2777$ is valid but less convenient.
• Q: Why did Usain Bolt not achieve maximum velocity immediately?
• A: Initial motion requires overcoming inertia; acceleration takes time based on the force generated against mass.
• Q: Explain terminal velocity.
• A: It occurs when the upward force of air resistance equals the downward force of weight, resulting in zero acceleration and constant velocity.