Physics: Mechanics and Kinematics Flashcards

A ball is thrown horizontally from a cliff of height 80 m with initial speed 20 m/s. How far from the base of the cliff does it land? (g = 10 m/s²)

Answer: 80 m

Time to fall: h = ½gt² → 80 = ½(10)t² → t = 4 s. Horizontal distance: x = v₀t = 20 × 4 = 80 m.

Two objects collide in a perfectly elastic collision on a frictionless surface. What is conserved?

Answer: Both momentum and kinetic energy

In a perfectly elastic collision: (1) momentum is conserved (all collisions), and (2) kinetic energy is also conserved (elastic only). In inelastic collisions, momentum is conserved but kinetic energy is not.

A satellite orbits Earth at radius R with period T. If it moves to orbit at radius 4R, what is its new period?

Answer: 8T

Kepler's Third Law: T² ∝ R³. If R increases by factor 4, then T² increases by factor 4³ = 64, so T increases by factor 8. New period = 8T.

An object of mass 5 kg is pulled along a horizontal surface at constant velocity by a force of 20 N applied at an angle of 30° above horizontal. What is the coefficient of kinetic friction? (g = 10 m/s²)

Answer: ≈ 0.40

At constant velocity, net force = 0. Horizontal: Fcos30° = f = μN. Vertical: N = mg - Fsin30° = 50-10 = 40 N. μ = Fcos30°/N = 20×(√3/2)/40 = 10√3/40 ≈ 0.433 ≈ 0.43.

In simple harmonic motion (简谐运动), when is the acceleration of the oscillating object at maximum?

Answer: At the maximum displacement (amplitude position, turning points)

In SHM, acceleration a = -ω²x. Acceleration is maximum when displacement x is maximum (at the turning points/amplitude). At equilibrium (x=0), acceleration = 0.

A 10 kg block falls freely from rest. After 3 seconds, what is its kinetic energy? (g = 10 m/s²)

Answer: 4500 J

Velocity after 3 s: v = gt = 30 m/s. KE = ½mv² = ½ × 10 × 900 = 4500 J.