A stone is dropped from the top of a cliff of height 30m. At the same instant another stone is projected vertically upwards from the ground with a speed of 30 m/s. The two stones will cross each other after a time:
Correct answer: A. 1s
- A. 1s
- B. 2s
- C. 3s
- D. 4s
Explanation
Let t be crossing time. Downward displacement from top = 1/2 g t², upward stone displacement = v t − 1/2 g t². Sum equals 30 m ⇒ 1/2 g t² + (v t − 1/2 g t²) = v t = 30 ⇒ t = 30/30=1 s.
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Displacement, velocity and acceleration describe motion, including the horizontal and vertical components of projectile motion. The chapter connects these quantities with Newton's laws, momentum and impulse, then applies conservation of momentum to collisions, distinguishing elastic collisions from inelastic ones.
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