Flashcards · 8463 · Topic 5

Forces flashcards

48 cards on Topic 5, subtopics 4.5.1 to 4.5.7. Pick a subtopic or study them all.

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4.5.1 Forces and their interactions

Scalar vs vector: Scalar: magnitude only. Vector: magnitude and direction, shown by an arrow whose length represents the size.
Contact forces: examples: Friction, air resistance, tension and normal contact force.
Non-contact forces: examples: Gravitational, electrostatic and magnetic forces.
Weight equation, and where weight acts: W = mg (W in N, m in kg, g in N/kg). Weight acts at the object’s centre of mass and is measured with a newtonmeter.
Mass vs weight: Mass: in kg, the same everywhere. Weight: a force in N, which depends on the gravitational field strength.
Find the resultant of 45 N to the right and 60 N to the left.: 60 − 45 = 15 N to the left
Resolving forces and scale diagramsHigher: A single force can be resolved into two components at right angles. If an object is in equilibrium, the force arrows drawn tip-to-tail make a closed triangle.

4.5.2 Work done and energy transfer

Work done equation, with units: W = Fs W in J, F in N, s (distance moved along the line of action of the force) in m.
How many newton-metres is one joule?: 1 J = 1 N m: one joule of work is done when a force of one newton causes a displacement of one metre.
What happens when work is done against friction?: Energy is transferred to the thermal store, so the temperature of the object rises.
A 25 N force moves an object 4.0 m along its line of action. Work done?: W = 25 × 4.0 = 100 J

4.5.3 Forces and elasticity

Why does stretching, bending or compressing an object need more than one force?: With only one force acting, the object would simply accelerate instead of changing shape.
Elastic vs inelastic deformation: Elastic: the object returns to its original shape when the forces are removed. Inelastic: it does not.
Force–extension equation, and when it applies: F = ke (F in N, k in N/m, e in m). It applies up to the limit of proportionality, where extension is directly proportional to force.
Required practical 6: how do you find a spring constant from the results?: Extension = stretched length − original length (in m). In the linear region, k = F ÷ e, or the gradient of the force–extension graph.
A spring (k = 25 N/m) is stretched 0.20 m. Force and energy stored?: F = 25 × 0.20 = 5.0 N Ee = ½ × 25 × 0.20² = 0.50 J
Work done stretching a spring: Equals the elastic potential energy stored, provided the spring is not inelastically deformed.

4.5.4 Moments, levers and gears

Moment of a force: equation: M = Fd M in N m, F in N, d = perpendicular distance from the pivot to the line of action of the force, in m.
Condition for a balanced object (principle of moments): Total clockwise moment about a pivot = total anticlockwise moment about that pivot.
Calculate the moment of a 30 N force acting 0.40 m from a pivot.: M = 30 × 0.40 = 12 N m
What do levers and gears do?: They transmit the rotational effects of forces. A small gear driving a larger gear gives a bigger moment, but the larger gear turns more slowly.

4.5.5 Pressure and pressure differences in fluids

Pressure at a surface: equation and units: p = F / A p in Pa, F (force normal to the surface) in N, A in m².
In which direction does the pressure in a fluid act on a surface?: At right angles (normal) to the surface.
Why does atmospheric pressure decrease with height?: The atmosphere gets less dense with height, and there is less air above a surface, so the weight of air above it is smaller.
Pressure due to a column of liquidHigher: p = hρg (h height of the column in m, ρ density in kg/m³, g in N/kg). Pressure increases with depth and with the density of the liquid.
Pressure due to 5.0 m of water? (ρ = 1000 kg/m³, g = 9.8 N/kg)Higher: p = 5.0 × 1000 × 9.8 = 49 000 Pa
What causes upthrust, and when does an object float?Higher: The pressure on the bottom surface is greater than on the top, giving a resultant upward force. An object floats if the upthrust equals its weight before it is fully submerged (its density is less than the fluid’s).

4.5.6 Forces and motion

Distance vs displacement; speed vs velocity: Distance and speed are scalars. Displacement and velocity are vectors: they include a direction.
Typical speeds: walking, running, cycling, sound in air: Walking ~1.5 m/s, running ~3 m/s, cycling ~6 m/s, sound in air ~330 m/s.
Acceleration equation, and the equation-sheet equation without time: a = Δv / t (m/s²) v² − u² = 2as
A car accelerates from rest at 2.5 m/s² over 80 m. Final velocity?: v² = 0 + 2 × 2.5 × 80 = 400, so v = 20 m/s
What does the gradient of a distance–time graph and of a velocity–time graph give?: Distance–time: speed. Velocity–time: acceleration.
What does the area under a velocity–time graph give?Higher: The distance travelled (displacement).
Terminal velocity: The steady velocity reached when the resistive force (drag) equals the weight, so the resultant force is zero.
Newton’s first law: If the resultant force on an object is zero, its velocity does not change: it stays at rest or keeps moving at the same speed in the same direction.
Newton’s second law: Acceleration is proportional to the resultant force and inversely proportional to mass: F = ma.
Newton’s third law pair vs balanced forces: A third-law pair acts on two different objects and is the same type of force. Balanced forces act on one object.
Inertia and inertial massHigher: Inertia: the tendency of an object to stay at rest or in uniform motion. Inertial mass = force ÷ acceleration: a measure of how hard it is to change its velocity.
Stopping distance: Stopping distance = thinking distance + braking distance. Typical reaction times are 0.2–0.9 s.
Thinking vs braking distance: what affects each?: Thinking: the driver’s reaction time (tiredness, drugs, alcohol, distractions) and speed. Braking: speed, wet or icy roads, worn brakes or tyres.
Speed doubles (same reaction time and deceleration). Effect on thinking and braking distance?: Thinking distance doubles; braking distance quadruples.
Why does an object moving in a circle at constant speed have a changing velocity?Higher: Its direction is always changing, and velocity is a vector, so the velocity changes (it is accelerating).

4.5.7 Momentum

Momentum equation, with unitsHigher: p = mv p in kg m/s, m in kg, v in m/s.
Conservation of momentumHigher: In a closed system, the total momentum before an event is equal to the total momentum after it.
Calculate the momentum of a 0.060 kg ball at 25 m/s.Higher: p = 0.060 × 25 = 1.5 kg m/s
A 2.0 kg trolley at 3.0 m/s hits a stationary 1.0 kg trolley and they move off together. Their velocity?Higher: Momentum before = 2.0 × 3.0 = 6.0 kg m/s. After: 3.0 kg × v = 6.0, so v = 2.0 m/s.
Force and change in momentumHigher: F = mΔv / Δt: force equals the rate of change of momentum.
How do air bags, seat belts and crash mats reduce injury?Higher: They increase the time taken for the change in momentum, so the rate of change of momentum, and so the force, is smaller.