Flashcards · 7357 · Topic 19

R: Forces and Newton's laws flashcards

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Newton’s first law: A body remains at rest or moves with constant velocity unless acted on by a resultant force.
Newton’s second law: Resultant force = mass × acceleration, F = ma, in the direction of the acceleration (also in vector form).
Newton’s third law: If body A exerts a force on body B, then B exerts an equal and opposite force on A.
Resolving a force F acting at θ to the horizontal: Horizontal component F cos θ; vertical component F sin θ. On a slope at α, the weight has components mg sin α down the slope and mg cos α into it.
Particles of 3 kg and 2 kg hang either side of a smooth pulley (g = 9.8). Find the acceleration and the tension.: System: 3g − 2g = 5a, so a = 9.8/5 = 1.96 m s⁻². 2 kg particle: T − 2g = 2a, so T = 2(9.8 + 1.96) = 23.52 N.
Friction: the inequality and limiting friction: F ≤ μR, where R is the normal reaction and μ the coefficient of friction. F = μR when on the point of slipping or sliding.
A particle rests on a rough plane at angle α, on the point of slipping, with only weight, R and friction acting. Find μ.: R = mg cos α and μR = mg sin α, so μ = tan α.
Condition for a particle to be in equilibrium: The resultant force is zero: the forces balance when resolved in any two perpendicular directions.
Normal reaction on a person of mass m in a lift accelerating upwards at a: R − mg = ma, so R = m(g + a). When accelerating downwards, R = m(g − a).