Kinematics and mechanics: motion, forces, pressure, work and energy
What this domain tests
Kinematics describes motion with speed, acceleration and graphs; mechanics explains it with forces, pressure, work and energy. The Estonian basic-school exam asks you to calculate in SI units, read motion graphs, draw and use force arrows, apply Archimedes' principle and the lever rule, and decide whether mechanical energy is conserved or dissipates through friction.
Core rules
- Speed: v = s/t. Average speed uses total distance over total time; instantaneous speed is what the speedometer shows.
- Acceleration: a = Δv/t, in m/s². Uniform acceleration means equal velocity changes in equal times.
- Free fall and weight: g ≈ 9.8 m/s² (often rounded to 10 for estimates); weight G = mg, measured in newtons.
- Forces add as vectors. A body stays at rest or moves uniformly only when the resultant force is zero (Newton's first law).
- Pressure: p = F/S in pascals; in a liquid p = ρgh, so it depends on depth and density, not on the shape of the vessel.
- Buoyancy: F = ρ_fluid · g · V_displaced.
- Work and power: A = F·s only when the force acts along the displacement; P = A/t.
- Energy: E_k = mv²/2, E_p = mgh. In a closed system the total stays constant; friction converts mechanical energy into heat.
- Lever: F₁l₁ = F₂l₂, with lever arms measured perpendicular to the forces.
Worked example
A car speeds up uniformly from 10 m/s to 25 m/s in 6.0 s. Find its acceleration and the distance covered.
a = (25 − 10)/6.0 = 2.5 m/s². For uniform acceleration the average speed is (10 + 25)/2 = 17.5 m/s, so s = 17.5 × 6.0 = 105 m, about 110 m to two significant figures.
Traps that cost marks
- Converting km/h wrongly: divide by 3.6, so 72 km/h = 20 m/s.
- Confusing mass in kilograms with weight in newtons.
- Using p = F/S but inserting the area in cm² - convert first, since 1 cm² = 10⁻⁴ m².
- Claiming work is done whenever a force acts: carrying a bag horizontally does no work against gravity.
- Reading a distance–time graph gradient as acceleration, or forgetting that the area under a velocity–time graph is a distance.
- Using the whole stick as the lever arm instead of the perpendicular distance to the pivot.