Physics: Momentum, Impulse & Collisions Flashcards

In a two-dimensional collision with no external forces, which components of momentum are conserved?

Answer: Both the x and y components independently

Conservation of momentum applies independently to each spatial component, so both p_x and p_y are separately conserved.

A rocket accelerates through space by expelling exhaust gas. This is best explained by:

Answer: Conservation of momentum: the exhaust's backward momentum gives the rocket forward momentum

With no external forces, the total momentum of rocket + exhaust is conserved; expelling mass backward gives the rocket an equal and opposite forward momentum.

What is the SI unit of impulse?

Answer: N·s (equivalent to kg·m/s)

Impulse J = FΔt has units of N·s, which is dimensionally equivalent to kg·m/s (the same as momentum).

Two skaters initially at rest push off each other. Skater A (mass 60 kg) moves right at 2 m/s. If skater B has mass 40 kg, what is B's velocity?

Answer: 3 m/s to the left

Initial total momentum = 0; 60(2) + 40(v_B) = 0 → v_B = −120/40 = −3 m/s (3 m/s to the left).

A 0.2 kg ball strikes a wall at 10 m/s and bounces back at 8 m/s. What is the magnitude of the change in momentum?

Answer: 3.6 kg·m/s

|Δp| = m|v_f − v_i| = 0.2 × |−8 − 10| = 0.2 × 18 = 3.6 kg·m/s.

The center of mass of a system of particles moves at constant velocity when:

Answer: The net external force on the system is zero

By Newton's second law for a system, F_net_external = M a_cm; if F_net_external = 0, then a_cm = 0 and v_cm is constant.

Compared to other types of collisions between the same objects with the same initial velocities, a perfectly inelastic collision:

Answer: Loses the maximum amount of kinetic energy

In a perfectly inelastic collision, objects move together with a common velocity, which represents the greatest possible loss of kinetic energy while still conserving momentum.