Motion, forces and energy flashcards

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What is the difference between a scalar and a vector? Give an example of each.
A scalar has magnitude only (e.g. speed, mass, energy). A vector has magnitude and direction (e.g. velocity, force, acceleration, momentum).
What is the difference between speed and velocity?
Speed is distance travelled per unit time (a scalar). Velocity is speed in a stated direction (a vector), so a car going round a bend at constant speed is still changing velocity.
State the equation for average speed and what each symbol means.
speed = distance ÷ time, v = d/t. v is in m/s, d in metres (m), t in seconds (s).
A runner covers 150 m in 20 s. What is the average speed?
v = d/t = 150 ÷ 20 = 7.5 m/s.
How do you find distance travelled from a speed–time graph?
Find the area under the graph. For a straight-sloped section use area of a trapezium or triangle; for a rectangle use speed × time.
What does the gradient of a distance–time graph tell you? What does a flat section mean?
The gradient is the speed. A flat (horizontal) section means the object is stationary. A steeper line means a greater speed.
What does the gradient of a speed–time graph tell you? What do a flat line and a curve mean?
The gradient is the acceleration. A flat line means constant speed. A curve means the acceleration is changing. A line sloping down means deceleration.
Define acceleration and give its equation.
Acceleration is the rate of change of velocity: a = (v − u) ÷ t, with a in m/s², v final speed, u initial speed, t time taken.
A car speeds up from rest to 24 m/s in 8 s. What is its acceleration?
a = (v − u)/t = (24 − 0) ÷ 8 = 3 m/s².
What is free fall and what is the acceleration of free fall near Earth's surface?
Free fall is motion under gravity alone, ignoring air resistance. Near Earth's surface g ≈ 9.8 m/s² (often taken as 10 m/s²), the same for all masses.
Why does a falling object reach terminal velocity?
As speed rises, air resistance grows until it equals the weight. The resultant force is then zero, so acceleration is zero and the object falls at constant speed.
What is the difference between mass and weight?
Mass is the amount of matter in an object (kg) and does not change with place. Weight is the gravitational force on it (N): W = mg.
What is gravitational field strength, and what are typical values for Earth and the Moon?
g is the force per unit mass, in N/kg. Earth about 10 N/kg (9.8). The Moon's value is about 1.6 N/kg, so objects weigh less there but have the same mass.
Find the weight of a 6.0 kg object where g = 10 N/kg.
W = mg = 6.0 × 10 = 60 N.
State the equation for density and describe how to find the density of an irregular solid.
ρ = m/V. Measure mass with a balance. Find volume by displacement: submerge it in a measuring cylinder of water and read the rise in water level. Then divide m by V.
A block has mass 540 g and volume 200 cm³. What is its density?
ρ = m/V = 540 ÷ 200 = 2.7 g/cm³ (that is 2700 kg/m³).
What is Newton's first law of motion?
An object stays at rest or moves at constant velocity unless a resultant force acts on it. So zero resultant force means no change in speed or direction.
State Newton's second law in equation form with units.
F = ma. The resultant force F (N) equals mass m (kg) times acceleration a (m/s²). The acceleration is in the direction of the resultant force.
A 1200 kg car has a resultant force of 3000 N. What is its acceleration?
a = F/m = 3000 ÷ 1200 = 2.5 m/s².
What does Newton's third law say?
If body A exerts a force on body B, then B exerts an equal and opposite force on A. The two forces act on different bodies and are of the same type.
How does a spring obey Hooke's law, and what is the limit of proportionality?
Extension is proportional to the load: F = kx, where k is the spring constant (N/m). Beyond the limit of proportionality the extension grows faster than the load and the law fails.
What is the moment of a force and when is an object in equilibrium?
Moment = force × perpendicular distance from the pivot (N m). In equilibrium the resultant force is zero and the sum of clockwise moments equals the sum of anticlockwise moments.
(Extended) Define momentum and give the equation.
Momentum is mass × velocity: p = mv, measured in kg m/s. It is a vector, so direction matters.
(Extended) State the principle of conservation of momentum.
In a closed system with no external force, the total momentum before a collision equals the total momentum after it, in any given direction.
(Extended) Define impulse and relate it to force and momentum.
Impulse = force × time for which it acts = change in momentum: FΔt = Δ(mv). A longer collision time means a smaller force for the same momentum change, as with crumple zones.
What is the principle of conservation of energy, and name the main forms of energy store.
Energy cannot be created or destroyed, only transferred between stores. Stores include kinetic, gravitational potential, chemical, elastic, thermal, nuclear, electrical and internal.
Give the equations for kinetic energy and gravitational potential energy change.
KE = ½mv² and ΔGPE = mgΔh. Mass in kg, speed in m/s, g in N/kg, height in m, energy in joules (J).
How much kinetic energy has a 2.0 kg trolley moving at 5.0 m/s?
KE = ½mv² = 0.5 × 2.0 × 5.0² = 25 J.
Define work done and power, with equations.
Work done = force × distance moved in the direction of the force (W = Fd), equal to energy transferred. Power = work done ÷ time (P = W/t), in watts (W).
Define efficiency and give an example calculation.
Efficiency = useful energy output ÷ total energy input (× 100%). A motor giving 40 J useful out of 160 J in is 40 ÷ 160 = 25% efficient.
Give the equation for pressure. What is the pressure of 200 N spread over 0.50 m²?
p = F/A, in pascals (Pa = N/m²). p = 200 ÷ 0.50 = 400 Pa.
(Extended) How does pressure in a liquid depend on depth and density? Give the equation and a value for 5.0 m of water.
Δp = ρgΔh. Pressure rises with depth and density. For water (ρ = 1000 kg/m³, g = 10 N/kg): 1000 × 10 × 5.0 = 50 000 Pa (50 kPa).

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