Quiz: momentum and force
Ten multiple-choice questions on Newton’s second and third laws and on momentum.
Ten questions. A wrong answer links back to the lesson that covers it, and you can try again as often as you like.
A trolley moves along a level track. A constant forward force F and a constant backward drag of 5.0 N act on it, and no other horizontal forces. Its momentum falls from 4.5 kg m s⁻¹ to 2.1 kg m s⁻¹ in 0.60 s. What is F?
A quadcopter of mass 0.80 kg hovers at a fixed point. Each of its four rotors pushes 0.25 kg of air vertically downwards every second. Take g = 9.81 m s⁻² and assume the air above is at rest. What is the speed of the air leaving each rotor?
The net external force on a trolley that starts from rest is slowly increased. Which describes the trolley’s momentum–time graph?
A hose delivers 180 kg of water per minute. The water leaves horizontally at 8.0 m s⁻¹. What is the minimum force needed to hold the hose still?
A 2.0 kg trolley moves forward at 9.0 m s⁻¹. A constant resultant force of 6.0 N acts backwards on it for 2.0 s. What is its velocity afterwards?
Two isolated trolleys, 3.0 kg and 1.5 kg, are pushed apart by a spring between them. During the push the 3.0 kg trolley has an average acceleration of 2.0 m s⁻². What is the average acceleration of the 1.5 kg trolley?
A 150 g tennis ball hits a wall at 24 m s⁻¹ and rebounds along the same line at 12 m s⁻¹. It is in contact with the wall for 0.030 s. What is the average force of the wall on the ball?
Which statement is the most general form of Newton’s second law?
A 250 g puck slides at 8.0 m s⁻¹ towards a rink board, rebounds at 3.0 m s⁻¹ in the opposite direction, and is in contact with the board for 0.20 s. What is the average force of the board on the puck?
A constant resultant force F acts on an object of mass m for a time t, starting from rest. What is the object’s change in velocity?
Key points
- • F_net = Δp / t; the gradient of a p–t graph is the net force.
- • Rate of change of momentum: (mass per second) × (velocity change).
- • A rebound reverses the direction: subtract velocities with signs.
Formulas
- F = Δp / t
- Ft = Δp
- Δp = m(vf − vi)
Key terms
- Impulse :
- Ft, equal to the change in momentum
Common errors
- • Subtracting speeds after a rebound.
- • Forgetting to convert grams to kilograms or minutes to seconds.