Mechanics and gravitation

Written by Dr. Hamza Farooq (Physics)

NET · Physics · 7 questions

Name: ____________________ Date: ____________

A car travelling at 72 km/h brakes with a constant deceleration of 5 m/s². What distance does it cover before stopping?
- A 72 m
- B 40 m
- C 20 m
- D 80 m
A stone is launched at 20 m/s at 30 degrees above the horizontal. What is its time of flight? (g = 10 m/s²)
- A 4 s
- B 3.5 s
- C 2 s
- D 1 s
If the length of a simple pendulum is made four times as large, its period becomes:
- A Four times
- B Double
- C Unchanged
- D Half
What power is needed to lift a 100 kg load at a steady 2 m/s? (g = 10 m/s²)
- A 2000 W
- B 200 W
- C 20 W
- D 1000 W
What is the moment of inertia of a thin ring of mass M and radius R about its central axis?
- A MR²
- B 2MR² / 5
- C MR² / 2
- D MR² / 12
A satellite moves in a circular orbit. If its orbital radius becomes four times as large, its orbital speed becomes:
- A Half
- B Double
- C A quarter
- D Four times
A 0.5 kg ball hits a wall at 10 m/s and rebounds at 10 m/s along the same line. What is the magnitude of the impulse on the ball?
- A 10 N s
- B 20 N s
- C 0
- D 5 N s

Answer key

1. B, 2. C, 3. B, 4. A, 5. A, 6. A, 7. A

72 km/h is 20 m/s. Using v² = 2as gives s = 400 / 10 = 40 m.
Time of flight = 2 u sin(theta) / g = 2 x 20 x 0.5 / 10 = 2 s.
The period is 2 pi sqrt(L / g), so it is proportional to the square root of L. Four times the length gives twice the period.
Power = force x velocity = mg x v = 1000 x 2 = 2000 W.
All the mass of a thin ring is at distance R from the axis, so I = MR².
Orbital speed is proportional to 1 / sqrt(r), so quadrupling r halves the speed.
The momentum changes from +5 to -5 kg m/s, a change of 10 kg m/s, so the impulse is 10 N s.