Science Y10: Motion and Energy

Scalar Quanities

• Have magnitude (size) only.

• Examples: distance, speed, mass, time, energy, temperature.

• e.g Speed = 60 km/h (scalar)

Vector Quantities

• Have magnitude and direction. (vector ref.)

• Examples: displacement, velocity, acceleration, force.

• Velocity = 60 km/h north (vector)

Distance

Distance:

• The total path length an object travels, regardless of direction.

• Scalar quantity → only has magnitude (size), no direction.

• Example: If you walk 3 m east and 4 m west, your total distance is 7 m.

Displacement

• The straight-line distance from the starting point to the final position.

• Vector quantity → has both magnitude and direction.

• Example: If you walk 3 m east then 4 m west, your displacement is 1 m west.

Speed

Speed:

• How fast something moves.

• Formula: Speed=DistanceTime\text{Speed} = \frac{\text{Distance}}{\text{Time}}Speed=TimeDistance​

• Units: m/s or km/h

• Average speed: total distance ÷ total time.

• Instantaneous speed: speed at a specific moment.

• Scalar quantity.

Velocity

Velocity:

• The rate of change of displacement (includes direction).

• Formula: Velocity=DisplacementTime\text{Velocity} = \frac{\text{Displacement}}{\text{Time}}Velocity=TimeDisplacement​

• Vector quantity.

• Example: 20 m/s north.

DST Triangle

DST Triangle Reminder:

• Speed = Distance ÷ Time

• Distance = Speed × Time

• Time = Distance ÷ Speed

Distance vs Time Graph

Distance vs Time Graph

• Shows how far an object travels over time.

• Steeper line = faster speed.

• Horizontal line = stationary object.

• Curved line = changing speed (acceleration or deceleration).

Displacement vs Time Graph

Displacement vs Time Graph

• Shows how position changes compared to starting point.

• Positive slope: moving away from start.

• Negative slope: returning toward start.

• Flat line: stationary.

Speed vs Time

Speed vs Time Graph

• Shows how speed changes over time.

• Horizontal line: constant speed.

• Upward slope: acceleration.

• Downward slope: deceleration.

• Area under graph = total distance travelled.

Acceleration

Acceleration: the rate at which velocity changes over time.

Formula: a = Vf - Vi /t

• Positive acceleration: speeding up.

• Negative acceleration (deceleration): slowing down.

• Example: A car increasing speed from 10 m/s to 20 m/s in 2 s →
  a=(20−10)/2=5 m/s2a = (20 - 10)/2 = 5 \, \text{m/s}^2a=(20−10)/2=5m/s2

• Gravity example: Objects in free fall accelerate at approximately 9.8 m/s² downwards

Newtons 1st Law - Inertia

First Law – Law of Inertia

• An object stays at rest or in uniform motion (constant speed in a straight line) unless acted upon by an unbalanced force.

• Inertia: an object’s resistance to change its motion.

• Example: When a car stops suddenly, your body continues moving forward due to inertia.

Newtons 2nd Law - Force, Mass, and Acceleration

Second Law – Force, Mass, and Acceleration

• The acceleration of an object depends on the net force and its mass.

• Formula:

  F=m×a

  • F = force (N)

  • m = mass (kg)

  • a = acceleration (m/s²)

• Meaning:

  • Larger force → greater acceleration.

  • Larger mass → smaller acceleration (for same force).

• Example: 2 kg object accelerating at 3 m/s² →
  F= 2×3 = 6N (always put answer in N)

Newtons 3rd Law - Action and Reaction

Third Law – Action and Reaction

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

• Forces occur in pairs, equal in size but opposite in direction.

• Example: When a bat hits a ball:

  • Bat exerts a force on ball (action).

  • Ball exerts an equal and opposite force on bat (reaction).

  • This is why the batter feels the impact.

Force and Net force

Force and Net Force

• Force: a push or pull that changes an object’s motion.

  • Measured in newtons (N).

• Net Force:

  • The overall force acting on an object when all individual forces are combined.

  • If forces are balanced → net force = 0 N → no acceleration.

  • If unbalanced → object accelerates in the direction of the net force.

Examples:

• 10 N right and 6 N left → Net force = 4 N right.

• 5 N right and 5 N left → Net force = 0 N (balanced).

Friction

Friction

• Definition: A force that opposes motion between two surfaces in contact.

• Always acts in the opposite direction to movement.

• Types:

  • Static friction: prevents motion when still.

  • Kinetic (sliding) friction: acts when surfaces slide past each other.

Examples:

• Friction between tyres and road allows cars to move and stop safely.

• Rubbing hands together produces heat due to friction.

Note:

• Friction can be useful (grip, traction) or wasteful (energy lost as heat).