Flashcards · 0625 · Topic 1

Motion, forces and energy flashcards

63 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

How do you read the volume in a measuring cylinder correctly?: Stand the cylinder on a level bench and read the bottom of the meniscus with your eye level with it. 1 cm³ = 1 ml.
How do you measure a length with a ruler without parallax error?: Measure from the zero mark (not the worn end of the ruler), with your eye directly above the reading.
Why measure multiples, for example timing 20 swings of a pendulum?: A single short time or small length is hard to measure accurately, so measure many and divide. 20 swings in 28.4 s → period = 28.4 ÷ 20 = 1.42 s.
Why use light gates and a digital timer, rather than a hand-held stopwatch, for a short time interval?: Starting and stopping a stopwatch by hand adds your reaction time (a few tenths of a second) to each reading.
Scalar and vector quantitiesExtended: Scalar: magnitude only (distance, speed, time, mass, energy, temperature). Vector: magnitude and direction (force, weight, velocity, acceleration, momentum, electric and gravitational field strength).
Find the resultant of 3.0 N east and 4.0 N north.Extended: √(3.0² + 4.0²) = 5.0 N, at tan⁻¹(4.0 ÷ 3.0) = 53° north of east. (Or use a scale drawing.)

1.2 Motion

Speed and velocity: Speed = distance ÷ time, v = s/t (a scalar). Velocity is speed in a given direction (a vector).
Average speed: Average speed = total distance travelled ÷ total time taken.
What does the gradient of a distance–time graph show?: The speed. A flat line means stationary; a steepening curve means the speed is increasing.
What does the area under a speed–time graph show?: The distance travelled.
A speed–time graph is a straight line from 0 m/s at 0 s to 20 m/s at 10 s. How far does the object travel?: Area of the triangle = ½ × 10 × 20 = 100 m.
Approximate acceleration of free fall near the Earth: About 9.8 m/s².
Acceleration: definition and how to find it from a speed–time graphExtended: a = Δv/Δt, the change in velocity per unit time, in m/s². It is the gradient of a speed–time graph. A deceleration is a negative acceleration.
Explain terminal velocity for an object falling through airExtended: It accelerates at first. As it speeds up, air resistance (drag) increases until it balances the weight. The resultant force is then zero, so it falls at a constant (terminal) velocity.
An object moves in a circle at constant speed. Is it accelerating?Extended: Yes. Its direction keeps changing, so its velocity changes. The acceleration, and the force causing it, point towards the centre of the circle.

1.3 Mass and weight

Mass: The quantity of matter in an object, measured in kilograms (kg). It is the same everywhere.
Weight: The gravitational force on an object that has mass, measured in newtons (N). It changes if the gravitational field strength changes.
Gravitational field strength: equation and value at the Earth's surface: g = W/m (weight per unit mass), so W = mg. At the Earth's surface g ≈ 9.8 N/kg.
Weight of a 5.0 kg mass on Earth, and roughly on the Moon (g about a sixth as large)?: Earth: 5.0 × 9.8 = 49 N. Moon: about 8.2 N. The mass is still 5.0 kg.
How can two masses (or weights) be compared?: Using a balance.
Weight, described in terms of a gravitational fieldExtended: Weight is the effect of a gravitational field on a mass.

1.4 Density

Density: definition, equation and units: Mass per unit volume: ρ = m/V. Units g/cm³ or kg/m³ (1 g/cm³ = 1000 kg/m³).
How do you find the density of a liquid?: Find the mass of an empty measuring cylinder, pour in the liquid and read its volume, then find the mass again. Mass of liquid = difference. ρ = m/V.
How do you find the volume of an irregular solid that sinks?: Displacement: part-fill a measuring cylinder with water, read the level, lower the object in fully on a thread and read again. Volume = rise in level.
A 54 g stone raises the water level from 40.0 cm³ to 60.0 cm³. Find its density.: V = 20.0 cm³, so ρ = 54 ÷ 20.0 = 2.7 g/cm³ (2700 kg/m³).
When does an object float in a liquid?: When its density is less than the liquid's density. It sinks if its density is greater. Water: 1.0 g/cm³.
Two liquids that do not mix are poured together. Which ends up on top?Extended: The less dense liquid floats on top, e.g. oil (0.92 g/cm³) on water (1.0 g/cm³).

1.5 Forces

Newton's first law: An object stays at rest, or keeps moving at constant velocity in a straight line, unless a resultant force acts on it. No resultant force does not mean "at rest".
Resultant force: The single force that has the same effect as all the forces acting together. If the forces are balanced, the resultant is zero.
Friction and drag: Solid friction opposes relative motion between two touching surfaces. Drag (fluid friction) opposes motion through a liquid or gas, and increases with speed.
Load–extension experiment for a spring: method: Hang the spring from a clamp beside a vertical ruler, with a pointer. Add equal loads one at a time, reading the pointer at eye level. Extension = new length − original length. Plot load against extension.
Moment of a force: Moment = force × perpendicular distance from the pivot (N m). A long spanner or a door handle far from the hinge gives a bigger moment for the same force.
Principle of moments: 2.0 N hangs 0.30 m left of the pivot. What weight 0.20 m to the right balances it?: For equilibrium, clockwise moments = anticlockwise moments. 2.0 × 0.30 = W × 0.20, so W = 3.0 N.
Conditions for an object to be in equilibrium: No resultant force AND no resultant moment. Balanced moments alone are not enough.
Centre of gravity: The single point where an object's whole weight can be considered to act. A wider base and lower centre of gravity make an object more stable.
How do you find the centre of gravity of an irregular flat card?: Hang it from a pin so it swings freely, hang a plumb line from the same pin and mark its line. Repeat from a second hole. The centre of gravity is where the lines cross.
Hooke's lawExtended: F = kx: the extension is directly proportional to the force, up to the limit of proportionality. k is the spring constant (N/m). Beyond the limit the graph is no longer a straight line.
A spring with k = 40 N/m has a 6.0 N load (within the limit of proportionality). Find the extension.Extended: x = F/k = 6.0 ÷ 40 = 0.15 m.
Newton's second law as an equationExtended: F = ma: resultant force (N) = mass (kg) × acceleration (m/s²). The acceleration is in the direction of the resultant force.

1.6 Momentum

MomentumExtended: p = mv (kg m/s). It is a vector, so velocities in opposite directions take opposite signs.
ImpulseExtended: Impulse = FΔt = Δp, the change in momentum (N s).
Resultant force in terms of momentumExtended: F = Δp/Δt: resultant force = rate of change of momentum.
Principle of conservation of momentumExtended: In a closed system with no external forces, the total momentum before a collision or explosion equals the total momentum after.
A 2 kg trolley at 3 m/s hits a stationary 1 kg trolley and they stick together. Find their velocity.Extended: (2 × 3) + (1 × 0) = (2 + 1) × v, so v = 2 m/s.
Two trolleys at rest are pushed apart by a spring. What can you say about their momenta afterwards?Extended: Total momentum was zero before, so it is zero after: the two momenta are equal in size and opposite in direction.

1.7 Energy, work and power

Name the energy stores: Kinetic, gravitational potential, chemical, elastic (strain), nuclear, electrostatic and internal (thermal).
Four ways energy is transferred between stores: By forces (mechanical work), by electrical currents (electrical work), by heating, and by waves (electromagnetic, sound).
Principle of conservation of energy: Energy cannot be created or destroyed, only transferred from one store to another.
Work done, W = Fd = ΔE: Work done equals the energy transferred; the force must act in the direction of motion; only that component does work.
A 250 N push moves a box 4.0 m. How much work is done?: W = Fd = 250 × 4.0 = 1000 J, so 1000 J of energy is transferred.
Power, P = W/t = ΔE/t: The rate of doing work, or the rate of energy transfer. Units: watts (W), 1 W = 1 J/s.
Kinetic energy and gravitational potential energy equationsExtended: KE = ½mv². Change in GPE = mgΔh.
EfficiencyExtended: Efficiency = useful energy (or power) output ÷ total energy (or power) input, as a fraction or percentage. It can never exceed 100%, because some energy is always wasted.
A power station: 100 J in; 10 J, 50 J and 5 J wasted at boiler, turbine and generator. Electrical output and efficiency?Extended: Output = 100 − 10 − 50 − 5 = 35 J. Efficiency = 35 ÷ 100 = 35%. On a Sankey diagram, arrow width is in proportion to the energy.
How do fossil-fuel and nuclear power stations generate electricity?: The energy heats water in a boiler to make steam, which turns a turbine that drives a generator.
Renewable and non-renewable energy resources: Renewable: biofuels, hydroelectric, tides, waves, geothermal, solar, wind. Non-renewable: fossil fuels and nuclear fuel.
Solar cells vs solar panels: Solar cells transfer light directly to electrical energy. Solar panels use the Sun's radiation (infrared) to heat water.
Which energy resources do not get their energy mainly from the Sun? How does the Sun release energy?Extended: Geothermal, nuclear and tidal. The Sun releases energy by nuclear fusion.

1.8 Pressure

p = F/A: For the same force, a smaller area gives a larger pressure. This is the idea behind sharp blades (small contact area) and snowshoes (large contact area).
Unit of pressure: The pascal (Pa). 1 Pa = 1 N/m².
Why does a drawing pin have a sharp point?: The small contact area at the point gives a much larger pressure for the same force (p = F/A), so it goes into the surface.
Pressure change with depth in a liquidExtended: Δp = ρgΔh. Pressure increases with depth and with the liquid's density. It does not depend on the shape or width of the container.
Pressure change 2.0 m below the surface of water (ρ = 1000 kg/m³, g = 9.8 N/kg)?Extended: Δp = 1000 × 9.8 × 2.0 = 19 600 Pa.