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Flashcards · 4PH1 · Topic 1
Forces and motion flashcards
23 cards on Topic 1, subtopics 1a to 1c. Pick a subtopic or study them all.
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1a Units
- SI units of speed, acceleration, force and gravitational field strength
- Speed: metre/second (m/s). Acceleration: metre/second² (m/s²). Force: newton (N). Gravitational field strength: newton/kilogram (N/kg).
- Unit of the moment of a force
- Newton metre (N m). (Paper 2 only)
- Unit of momentum
- Kilogram metre/second (kg m/s). (Paper 2 only)
1b Movement and position
- Average speed
- Average speed = distance moved ÷ time taken.
- What does a distance–time graph show?
- Gradient = speed. A horizontal line means stationary; a curve means the speed is changing.
- Acceleration equation
- Acceleration = change in velocity ÷ time taken: a = (v − u)/t, in m/s².
- What do the gradient and the area of a velocity–time graph show?
- Gradient = acceleration. Area under the graph = distance travelled. (Do not mix them up.)
- A car starts from rest and accelerates at 2.0 m/s² over 25 m. Final speed?
- v² = u² + 2as = 0 + 2 × 2.0 × 25 = 100, so v = 10 m/s.
1c Forces, movement, shape and momentum
- Scalars and vectors
- Scalars have magnitude only (distance, speed, mass). Vectors have magnitude and direction (displacement, velocity, force, momentum).
- Resultant force of forces along a line
- Add them, taking direction into account. 4000 N forward and 1500 N backward gives 2500 N forward.
- Force, mass and acceleration; weight
- F = m × a. Weight = mass × gravitational field strength, W = m × g.
- Newton's first law
- An object stays at rest or moves at constant velocity unless a resultant force acts on it.
- Stopping distance, and what increases each part
- Stopping distance = thinking distance + braking distance. Thinking: speed, tiredness, alcohol, drugs, distraction. Braking: speed, poor brakes, worn tyres, wet or icy roads.
- Why does braking distance rise so sharply with speed?
- Kinetic energy is proportional to v², so doubling the speed quadruples the braking distance.
- Explain terminal velocity in four steps
- Weight makes the object accelerate. As speed rises, air resistance increases. When air resistance equals weight, the resultant force is zero. Acceleration is zero, so it falls at constant velocity.
- A skydiver at terminal velocity opens her parachute. What happens?
- Air resistance suddenly increases above her weight, so the resultant force is upward and she decelerates. Air resistance falls until it equals weight again: a new, lower terminal velocity.
- Hooke's law and elastic behaviour
- F = kx: extension is proportional to force up to the limit of proportionality. Elastic behaviour: the object returns to its original shape when the force is removed. Beyond the elastic limit the deformation is permanent.
- Momentum
- Momentum = mass × velocity, p = mv. It is a vector: give opposite directions opposite signs. (Paper 2 only)
- A 1200 kg car slows from 25 m/s to 15 m/s in 5.0 s. Average force?
- Force = change in momentum ÷ time, F = (mv − mu)/t = 1200 × 10 ÷ 5.0 = 2400 N. (Paper 2 only)
- How do crumple zones, airbags and seatbelts reduce injury?
- They increase the time over which the momentum changes. Force is the rate of change of momentum, so the force on the passenger is smaller. (Paper 2 only)
- Conservation of momentum
- In a closed system, the total momentum before a collision or explosion equals the total momentum after. (Paper 2 only)
- Newton's third law
- For every force there is an equal and opposite force, acting on a different body. (Paper 2 only)
- Moments: a 2.5 N weight is 40 cm from the pivot. What weight 20 cm on the other side balances it?
- Moment = force × perpendicular distance from the pivot. In equilibrium, clockwise = anticlockwise: 2.5 × 40 = 100 N cm, so W = 100 ÷ 20 = 5.0 N. (Paper 2 only)