Flashcards · 5054 · Topic 1

Motion, forces and energy flashcards

66 cards on Topic 1, subtopics 1.1 to 1.8. Pick a subtopic or study them all.

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1.1 Physical quantities and measurement techniques

Resolution of a metre rule and of a micrometer screw gauge: Metre rule: 1 mm. Micrometer screw gauge: 0.01 mm.
How do you read a micrometer screw gauge?: Add the main scale reading (whole and half millimetres) to the thimble scale reading (hundredths of a millimetre) at the line level with the datum. Both scales are needed.
Parallax error, and how to avoid it: An error from reading a scale at an angle. Look perpendicular to the scale (eye level with a meniscus, read at its bottom).
A pendulum takes 18.4 s for 20 oscillations. Period? Why time 20?: Period = 18.4 ÷ 20 = 0.92 s. Human reaction time (about 0.2 s) is a fixed error; spreading it over 20 swings cuts its effect on each period by a factor of 20.
Scalars and vectors: classify distance, displacement, speed, velocity, acceleration: Scalars (size only): distance, speed. Vectors (size and direction): displacement, velocity, acceleration.
A runner completes one lap of a 400 m track. Distance travelled and displacement?: Distance = 400 m (total path length). Displacement = 0 m (back at the start).

1.2 Motion

Acceleration: definition and equation: Rate of change of velocity: a = (v − u)/t, in m/s². u = initial velocity, v = final velocity. A deceleration is a negative acceleration.
A cyclist speeds up from 2 m/s to 8 m/s in 4 s. Acceleration?: a = (8 − 2) ÷ 4 = 1.5 m/s².
Gradients of distance–time and speed–time graphs: Distance–time: gradient = speed. Speed–time: gradient = acceleration.
Uniform acceleration from rest to 10 m/s in 5 s, then 10 m/s for 8 s. Total distance?: Area under the speed–time graph: ½ × 5 × 10 = 25 m, plus 8 × 10 = 80 m. Total 105 m.
How do objects fall without air resistance?: All fall with the same acceleration of free fall, g = 9.8 m/s², whatever their mass.
Explain terminal velocity: As the object speeds up, drag increases until drag = weight. The resultant force is then zero, so the acceleration is zero and it falls at constant (terminal) velocity. It does not slow down.
Is an object moving in a circle at constant speed accelerating?: Yes. Velocity is a vector and its direction keeps changing, so the velocity changes.
Convert km/h to m/s: Divide by 3.6. For example, 72 km/h = 20 m/s.

1.3 Mass and weight

Mass vs weight: Mass: amount of matter, in kg; does not change with location; measured with a balance. Weight: the force due to gravity, in N; changes with location; measured with a newtonmeter.
Weight of a 60 kg astronaut on Earth (g = 9.8 N/kg) and on the Moon (g = 1.6 N/kg)?: Earth: 60 × 9.8 = 588 N. Moon: 60 × 1.6 = 96 N. The mass is 60 kg in both places.
Gravitational field strength: Force per unit mass: g = W/m, in N/kg. About 9.8 N/kg on Earth. It is numerically equal to the acceleration of free fall in m/s².
Gravitational field: The region around a mass in which another mass feels a force due to gravitational attraction.
Inertia: The resistance of an object to a change in its state of rest or motion. It depends on mass only, not weight: a weightless object in space still has inertia.

1.4 Density

Density: equation and units: ρ = m/V, in kg/m³ or g/cm³. 1 g/cm³ = 1000 kg/m³. Water: 1 g/cm³.
Density of a regular solid: Measure dimensions, calculate volume, weigh.
Density of an irregular solid: Weigh, then use a displacement can or measuring cylinder; volume = water displaced.
Density of a liquid: Weigh empty cylinder, add liquid, reweigh, subtract; read volume directly.
A 2 cm × 3 cm × 4 cm block has mass 96 g. Density? Does it float in water?: V = 24 cm³, ρ = 96 ÷ 24 = 4 g/cm³. It sinks: an object floats only if its density is less than the fluid's.
Convert a volume from cm³ to m³: Divide by 1 000 000 (1 m³ = 10⁶ cm³).

1.5 Forces

Newton's first law: An object stays at rest or at constant velocity unless acted on by a resultant force. Constant velocity means zero resultant force.
Newton's second law: A resultant force produces an acceleration in its direction: F = ma.
Newton's third law: If object A exerts a force on object B, then object B exerts an equal and opposite force on object A ("action and reaction"). The pair acts on two different objects.
A book rests on a table. Are its weight and the table's push on it a third-law pair?: No. Both act on the book (and are different types of force), so they are a first-law balance. A third-law pair is the same type of force acting on two different bodies.
A car has a driving force of 4000 N and drag of 1500 N. Resultant force?: 4000 − 1500 = 2500 N forward. The driving force is not the resultant.
A 1500 kg car accelerates at 2 m/s². Resultant force?: F = ma = 1500 × 2 = 3000 N.
Hooke's law: F = kx: extension x is proportional to load up to the limit of proportionality. k = spring constant (N/m), the gradient of a load–extension graph. Use extension, not total length.
A spring of natural length 12 cm stretches to 18 cm under 3 N. Find k.: Extension = 0.06 m. k = F/x = 3 ÷ 0.06 = 50 N/m.
Moment of a force: Moment = force × perpendicular distance from the pivot, in N m. It is zero if the force acts through the pivot.
Principle of moments. 30 N acts 0.8 m left of a pivot; what force 0.5 m right balances it?: In equilibrium, sum of clockwise moments = sum of anticlockwise moments. F × 0.5 = 30 × 0.8 = 24 N m, so F = 48 N.
Centre of gravity, and finding it for an irregular lamina: The point where the whole weight appears to act. Suspend the lamina freely, mark the vertical with a plumb line; repeat from a second point; the lines cross at the centre of gravity.
What makes an object stable, and when does it topple?: A low centre of gravity and a wide base make it stable. It topples when the vertical line through its centre of gravity falls outside its base.
Circular motion: how does the force needed change with speed, radius and mass?: A bigger force is needed for a higher speed, a smaller radius, or a larger mass. (Qualitative only: F = mv²/r is not required.)
Stopping distance, and the effect of doubling speed: Stopping distance = thinking distance + braking distance. Doubling the speed doubles the thinking distance and quadruples the braking distance.

1.6 Momentum

Momentum and impulse: Momentum p = mv (kg m/s), a vector. Impulse = Ft = mv − mu, the change in momentum (N s).
Principle of conservation of momentum: In a closed system with no external forces, total momentum before = total momentum after. It applies to all collisions and explosions.
Elastic vs inelastic collisions: Momentum is conserved in both. Elastic: kinetic energy is also conserved. Inelastic: kinetic energy is not conserved (objects often stick together).
A 2 kg trolley at 3 m/s hits a stationary 4 kg trolley and they move off together. Velocity?: Before: 2 × 3 = 6 kg m/s. After: 6v. So v = 1 m/s.
A 5 kg object at rest explodes; a 2 kg piece moves at 6 m/s. Velocity of the 3 kg piece?: 0 = (2 × 6) + 3v, so v = −4 m/s: 4 m/s in the opposite direction.
A 0.15 kg ball hits a wall at 12 m/s and rebounds at 8 m/s. Change in momentum?: Towards the wall positive: 0.15 × (−8) − 0.15 × 12 = −1.2 − 1.8 = −3.0 kg m/s (magnitude 3.0 N s). Not 0.6.
How do airbags, crumple zones and seatbelts reduce injury?: They increase the time over which momentum changes. Since F = Δp/t, a longer time means a smaller force.

1.7 Energy, work and power

Energy stores: Kinetic, gravitational potential, chemical, elastic (strain), nuclear, electrostatic, internal (thermal).
Ways energy is transferred between stores: By forces (mechanical work), by electrical currents (electrical work), by heating, and by electromagnetic, sound and other waves.
Kinetic energy of a 1500 kg car at 20 m/s: Ek = ½mv² = ½ × 1500 × 20² = 300 000 J.
Change in gravitational potential energy when 5 kg is lifted 3 m (g = 9.8 N/kg): ΔEp = mgΔh = 5 × 9.8 × 3 = 147 J.
Work done: equation and condition: W = F × d, where d is the distance moved in the direction of the force. Work done = energy transferred (J).
Does a waiter carrying a tray horizontally at constant speed do work against gravity?: No. The force on the tray is upward, perpendicular to the horizontal motion.
Power. A motor does 6000 J of work in 15 s: power?: P = W/t = ΔE/t (watts, W = J/s). 6000 ÷ 15 = 400 W.
A 2 kg ball is dropped 5 m (no air resistance, g = 9.8 m/s²). Landing speed?: mgh = ½mv², so v = √(2gh) = √(2 × 9.8 × 5) = √98 ≈ 9.9 m/s. Mass cancels.
Efficiency. A motor uses 500 J and does 350 J of useful work: efficiency?: Efficiency = (useful energy out ÷ total energy in) × 100% (or the same with power). 350 ÷ 500 × 100 = 70%. The other 150 J is dissipated, usually as thermal energy.
Renewable and non-renewable energy resources: Non-renewable (finite): coal, oil, natural gas, nuclear fuel. Renewable: solar, wind, hydroelectric, tidal, wave, geothermal, biofuels.
How do most power stations (including biofuel) generate electricity?: Burning fuel (or nuclear fission) heats water in a boiler to steam; the steam turns a turbine, which drives a generator. Wind and hydroelectric turn the turbine directly.
Which three energy resources do not come from the Sun?: Nuclear (mined fuel), geothermal (heat from radioactive decay inside the Earth) and tidal (mainly the Moon's gravity).
The three factors the syllabus uses to compare energy resources: Whether it is renewable, when or whether it is available, and its environmental impact. (Not cost, reliability or public opinion.)

1.8 Pressure

Pressure from a force: p = F/A, in pascals: 1 Pa = 1 N/m². The same force on a smaller area gives a greater pressure (knives, drawing pins); a large area gives a low pressure (skis, tractor tyres).
A 600 N person stands on one foot of area 0.015 m². Pressure?: p = 600 ÷ 0.015 = 40 000 Pa.
Pressure in a liquid: p = ρgh, where h is the depth. It increases with depth and density, acts equally in all directions, and does not depend on the container's shape or width.
Water is 2.5 m deep (ρ = 1000 kg/m³, g = 9.8 N/kg). Pressure due to the water at the bottom?: p = 1000 × 9.8 × 2.5 = 24 500 Pa. Adding atmospheric pressure (about 100 000 Pa) gives the total, 124 500 Pa.
Why are dam walls thicker at the base?: Pressure increases with depth, so the force on the wall is greatest at the bottom.
Atmospheric pressure and the mercury barometer: Caused by the weight of air above; about 1 × 10⁵ Pa at sea level, less at altitude. It supports a mercury column of about 760 mm. Mercury is used because its high density keeps the column short (water would need over 10 m).
What does a manometer measure?: The pressure of a gas supply: the difference in the two liquid levels of the U-tube gives the pressure above atmospheric.