āš” Unit 2: Energy & Momentum ā€” Practice Quiz

Assessment AS Learning
šŸ”„ Not Graded
Purpose: Self-check on work, energy, momentum, and collisions.

šŸ“‹ Formulae

\( W = Fd\cos\theta \) | \( E_k = \tfrac{1}{2}mv^2 \) | \( E_p = mgh \) | \( W_{net} = \Delta E_k \)
\( p = mv \) | \( J = F\Delta t = \Delta p \) | \( E_{spring} = \tfrac{1}{2}kx^2 \) | \( P = W/t \)
Score: 0 / 8
Q1
A \(50\text{ N}\) force pushes a box \(8.0\text{ m}\) along a floor at \(30Ā°\) to horizontal. How much work is done?
Answer: J
Solution:
\( W = Fd\cos\theta = 50(8.0)\cos30Ā° = 400(0.866) = 346.4 \text{ J} \)
Q2
A \(2.0\text{ kg}\) ball moving at \(6.0\text{ m/s}\) hits a wall and bounces back at \(4.0\text{ m/s}\). What is the impulse?
Answer: NĀ·s
Solution:
Taking initial direction as +:
\( J = \Delta p = m(v_f - v_i) = 2.0(-4.0 - 6.0) = 2.0(-10) = -20 \text{ NĀ·s} \)
The negative sign means impulse is in the opposite direction (toward the wall).
Q3
A \(0.50\text{ kg}\) ball is dropped from \(12\text{ m}\). Using conservation of energy, what is its speed just before hitting the ground?
Answer: m/s
Solution:
\( mgh = \tfrac{1}{2}mv^2 \) (mass cancels)
\( v = \sqrt{2gh} = \sqrt{2(9.8)(12)} = \sqrt{235.2} = 15.3 \text{ m/s} \)
Q4
In a perfectly inelastic collision, which quantity is conserved?
Solution:
In ALL collisions, momentum is conserved. In a perfectly inelastic collision (objects stick together), kinetic energy is NOT conserved ā€” maximum KE is lost to heat, sound, and deformation.
Q5
A \(70\text{ kg}\) skater moving at \(5.0\text{ m/s}\) catches a \(50\text{ kg}\) stationary skater. What is their combined velocity?
Answer: m/s
Solution:
\( m_1v_1 = (m_1+m_2)v_f \)
\( 70(5.0) = (70+50)v_f \)
\( v_f = 350/120 = 2.92 \text{ m/s} \)
Q6
A spring with \(k = 200\text{ N/m}\) is compressed \(0.15\text{ m}\). How much elastic potential energy is stored?
Answer: J
Solution:
\( E_s = \tfrac{1}{2}kx^2 = \tfrac{1}{2}(200)(0.15)^2 = 100(0.0225) = 2.25 \text{ J} \)
Q7
A \(1000\text{ kg}\) car brakes from \(20\text{ m/s}\) to rest. How much work does friction do?
Answer: J (enter negative)
Solution:
\( W = \Delta E_k = \tfrac{1}{2}mv_f^2 - \tfrac{1}{2}mv_i^2 = 0 - \tfrac{1}{2}(1000)(20)^2 = -200{,}000 \text{ J} \)
Negative because friction removes energy from the system.
Q8
A crane lifts a \(500\text{ kg}\) beam \(30\text{ m}\) in \(2.0\text{ minutes}\). What power does the crane develop?
Answer: W
Solution:
\( W = mgh = 500(9.8)(30) = 147{,}000 \text{ J} \)
\( P = W/t = 147{,}000/120 = 1225 \text{ W} \approx 1.2 \text{ kW} \)