AS Daily A Level Physics question
On a low-friction air track, a 0.40 kg glider moves to the right at 5.0 m/s and collides head-on with a 0.80 kg glider moving to the left at 2.0 m/s. The gliders stick together on impact. Which statement must be true about the velocity of the joined gliders immediately after the collision?
Answer
The correct answer is D.
Correct: D — 0.33 m/s to the right. Using signed momentum, initial p = (0.40×5.0) + (0.80×−2.0) = 2.0 − 1.6 = +0.40 kg m/s; total mass = 1.20 kg, so v = 0.40/1.20 ≈ 0.33 m/s to the right. A uses the sum of speeds (magnitudes) divided by total mass and chooses the heavier cart’s direction, ignoring opposite signs. B gets the correct magnitude 0.33 but assigns the wrong direction, assuming the heavier cart must set the sign. C divides the net momentum by only the heavier mass (0.80 kg) to get 0.50 m/s, not by the combined mass. D follows the signed-momentum approach and divides by the total mass, matching conservation of momentum in 1D.