The Foundation of Physics. Master this, and Topic 5 becomes easy.
Scalar: A quantity that only has magnitude (size) and no direction.
(e.g., Temperature, Speed, Mass)
Vector: A quantity that has both magnitude AND direction.
(e.g., Velocity, Force, Displacement)
Q1: A runner runs 400m around a track and finishes exactly where they started. What is their Displacement?
0 meters. (Even though distance is 400m, their position relative to the start hasn't changed.)
Q2: Which of the following is a Scalar quantity: Velocity, Force, or Speed?
Speed. (Velocity and Force both have direction, making them Vectors).
Velocity is speed in a specific direction. If you turn a corner at constant speed, your velocity changes because direction changes.
$$ a = \frac{v - u}{t} $$
$v$ = final velocity, $u$ = initial velocity, $t$ = time.
Q1: A car accelerates from 10 m/s to 30 m/s in 5 seconds. Calculate acceleration.
$$ a = \frac{30 - 10}{5} = 4 \, m/s^2 $$
Q2: A car slows down from 20 m/s to 0 m/s in 4 seconds. What is the acceleration?
$$ a = \frac{0 - 20}{4} = -5 \, m/s^2 $$ (The negative sign means deceleration).
Distance-Time
Velocity-Time
Gradient (Slope) = Speed
Gradient (Slope) = Acceleration
Area under line = Distance Travelled
Q1: On a velocity-time graph, what does a horizontal line (above zero) represent?
Constant Velocity. (The speed isn't changing, but the object is moving).
Q2: How do you calculate the distance travelled from a Velocity-Time graph?
Calculate the total Area under the line (Area of rectangle + Area of triangle).
1. First Law (Inertia): An object remains at rest or moves at constant velocity unless acted on by a resultant force.
Exam Tip: Use the phrase "resultant force is zero" instead of "no force acting".
2. Second Law (F=ma): Acceleration is proportional to Force and inversely proportional to Mass.
$$ F = m \times a $$
3. Third Law: Whenever two objects interact, the forces they exert on each other are equal and opposite.
Q1: A 1000kg car accelerates at 3 $m/s^2$. Calculate the resultant force.
$$ F = 1000 \times 3 = 3000 \, N $$
Q2: If you push a wall with 50N, the wall pushes back with 50N. Why don't the forces cancel out?
Because the forces act on different objects. You push the wall; the wall pushes you.
🔗 Connecting Concepts:
Mastering Newton's Laws now is crucial. They are directly applied in Topic 5 (Energy), Momentum, and Forces in Materials later in the course.
Stopping Distance = Thinking Distance + Braking Distance
| Thinking Factors (Driver) | Braking Factors (Car/Road) |
|---|---|
| Tiredness, Drugs, Alcohol, Distractions | Worn brakes, Icy/Wet roads, Mass of car |
Q1: Name one factor that specifically increases Thinking Distance.
Tiredness, Alcohol, Drugs, or Distractions. (Wet roads affect Braking, not Thinking!)
Q2: An astronaut goes to the Moon. What happens to his Mass and Weight?
Mass: Stays the same.
Weight: Decreases (because gravity is weaker).
Total momentum before collision = Total momentum after collision.
$$ p = m \times v $$
$$ F = \frac{\Delta p}{t} $$
(Force is the rate of change of momentum).
Q1: Why do cars have crumple zones? (Use momentum ideas)
Crumple zones increase the time taken to stop. This decreases the rate of change of momentum, reducing the Force felt by passengers.
Q2: Two trolleys collide and stick together. If the total momentum before is 10 kg m/s, what is the momentum after?
10 kg m/s. (Momentum is always conserved in a closed system).