Flashcards · 9702 · AS Topic 3

Dynamics (AS Level) flashcards

17 cards on AS Topic 3, subtopics 3.1 to 3.3. Pick a subtopic or study them all.

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Newton’s first law: An object stays at rest or moves with constant velocity unless acted on by a resultant force.
Newton’s second law, stated properly: The resultant force is proportional to the rate of change of momentum, and acts in its direction: F = Δp ÷ Δt. F = ma follows only when the mass is constant.
Newton’s third law, and the conditions for a force pair: If body A exerts a force on body B, B exerts an equal and opposite force on A. The pair act on two different bodies and are the same type of force.
Linear momentum: definition and unit: p = mv, mass × velocity. A vector. Unit: kg m s⁻¹ (equivalent to N s).
Impulse: Impulse = FΔt = change in momentum. Unit N s. It is the area under a force–time graph.
How does a crumple zone or airbag reduce the force in a crash?: It increases the time over which the same momentum change happens. Since F = Δp ÷ Δt, the average force is smaller. (Not "absorbing the force".)

How do frictional and viscous (drag) forces such as air resistance behave?: They oppose the relative motion, and drag increases as the speed increases.
Describe the motion of an object falling from rest through air: It starts with acceleration g. As speed rises, drag rises, so the resultant force and the acceleration fall. Eventually drag equals weight and it falls at constant terminal velocity.
What is the resultant force on an object at terminal velocity?: Zero: the drag equals the weight, so the acceleration is zero and the velocity is constant.
With no air resistance, how does the acceleration of a falling object depend on its mass?: It does not: every object falls with the same uniform acceleration g.
A parachute opens during a fall at terminal velocity. What happens?: Drag suddenly exceeds weight, so there is an upward resultant force and the skydiver decelerates. Drag falls as speed falls until it equals weight again at a new, lower terminal velocity.

Principle of conservation of momentum: In a closed system (no external resultant force), the total momentum before an interaction equals the total momentum after it.
Elastic collision: is momentum conserved? Is kinetic energy conserved?: Momentum: conserved. Kinetic energy: conserved.
Inelastic collision: is momentum conserved? Is kinetic energy conserved?: Momentum: conserved. Kinetic energy: not conserved (some becomes thermal energy, sound and deformation).
In a perfectly elastic collision, how do the relative speeds compare?: The relative speed of approach equals the relative speed of separation.
A 2.0 kg trolley at 3.0 m s⁻¹ hits a stationary 1.0 kg trolley and they stick. Find the common speed and the KE lost.: 2.0 × 3.0 = 3.0v, so v = 2.0 m s⁻¹. KE before = 9.0 J, KE after = 6.0 J, so 3.0 J is transferred to heat and sound.
An object at rest explodes into two pieces. What can you say about their momenta?: Total momentum stays zero, so the pieces have equal and opposite momenta. Kinetic energy increases, supplied from chemical or elastic potential energy.