Angular Momentum and Angular Impulse
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AP Physics 1 › Angular Momentum and Angular Impulse
A wheel experiences a constant torque of $0.40 \text{N·m}$ for $0.75 \text{s}$. What is the magnitude of $\Delta L$?
$1.15 \text{ kg·m}^2/\text{s}$
$0.30 \text{ kg·m}^2/\text{s}$
$0.53 \text{ kg·m}^2/\text{s}$
$0.40 \text{ kg·m}^2/\text{s}$
Explanation
This problem assesses computing change in angular momentum from torque and time. For constant torque, $\Delta L$ equals $\tau$ times $\Delta t$, embodying the angular impulse. This ties into how sustained torque accumulates change in rotational momentum over time. The wheel experiences this direct proportionality. Choice C ($1.15 \text{ kg·m}^2/\text{s}$) could be selected by adding torque and time instead of multiplying, revealing a misconception about the multiplicative nature of impulse. A key strategy is to memorize the rotational equivalents: force $\to$ torque, momentum $\to$ angular momentum, impulse $\to$ angular impulse.
A wheel’s angular momentum changes by $1.8\ \text{kg·m}^2/\text{s}$ when a constant torque acts. If the torque is $0.60\ \text{N·m}$, how long did it act?
$2.4\ \text{s}$
$3.0\ \text{s}$
$1.2\ \text{s}$
$0.33\ \text{s}$
Explanation
This scenario tests calculating time from change in angular momentum and torque. Rearranging ΔL = τ Δt gives Δt = ΔL / τ for constant torque. This reflects the duration needed for torque to effect the momentum change. The wheel's constant torque determines this time. Choice D (2.4 s) could result from dividing incorrectly, like 1.8 / 0.75, indicating a numerical misconception in division. A transferable approach is to check reasonability: larger ΔL or smaller τ should yield longer times, aiding error detection.
A spinning platform experiences a constant net torque of $2.5 , \text{N·m}$ for $4.0 , \text{s}$. What is the resulting change in angular momentum?
$6.5 , \text{kg·m}^2/\text{s}$
$0.63 , \text{kg·m}^2/\text{s}$
$10 , \text{kg·m}^2/\text{s}$
$2.5 , \text{kg·m}^2/\text{s}$
Explanation
This question involves determining change in angular momentum for a spinning platform. The net torque over time produces angular impulse, equaling $ΔL$. Qualitatively, this shows how external torques alter a system's rotational state. The constant net torque here results in a straightforward calculation. Distractor B ($6.5 , \text{kg·m}^2/\text{s}$) might arise from using 2.5 * 2.6 or a miscalculation, pointing to an arithmetic misconception rather than conceptual error. For wider application, use the formula $ΔL = τ Δt$ as a checkpoint in more complex rotational problems involving moments of inertia.
A wheel’s motor applies a constant torque of $4.0\ \text{N·m}$ for $0.50\ \text{s}$. What angular impulse is delivered to the wheel?
$0.50\ \text{kg·m}^2/\text{s}$
$2.0\ \text{kg·m}^2/\text{s}$
$4.0\ \text{kg·m}^2/\text{s}$
$8.0\ \text{kg·m}^2/\text{s}$
Explanation
This question evaluates knowledge of angular impulse in the context of rotational motion. Angular impulse results from a torque applied over a time interval and is calculated as τ Δt for constant torque. It represents the total change in angular momentum imparted to the system, similar to how force over time changes linear momentum. In this case, the motor delivers this impulse directly to the wheel. A common distractor like choice C (4.0 kg·m²/s) could arise from forgetting to multiply by time and just using the torque value, indicating a misconception of impulse as instantaneous rather than time-integrated. To approach such problems effectively, always verify units: angular impulse has units of kg·m²/s, matching angular momentum.
A flywheel experiences a constant torque of $3.0 \text{N·m}$ for $0.10 \text{s}$. What angular impulse is delivered?
$0.30 \text{kg·m}^2/\text{s}$
$3.0 \text{kg·m}^2/\text{s}$
$30 \text{kg·m}^2/\text{s}$
$0.10 \text{kg·m}^2/\text{s}$
Explanation
This question evaluates angular impulse delivered to a flywheel. Angular impulse is $τ Δt$ for constant torque, matching the change in angular momentum. Qualitatively, it quantifies the rotational 'kick' from the torque over a short time. The flywheel receives this impulse directly. Choice C ($3.0 \text{kg·m}^2/\text{s}$) may be chosen by using torque without time, showing a misconception that impulse equals torque, not its time integral. Generally, reinforce understanding by comparing to linear impulse problems, noting the parallel structures in calculations.
A pulley experiences a net torque of $2.5,\text{N·m}$ for $1.2,\text{s}$. What is the change in angular momentum?
$1.2,\text{kg·m}^2/\text{s}$
$2.1,\text{kg·m}^2/\text{s}$
$2.5,\text{kg·m}^2/\text{s}$
$3.0,\text{kg·m}^2/\text{s}$
Explanation
This question requires calculating change in angular momentum from torque and time. Using the angular impulse-momentum theorem: $ \Delta L = \tau \Delta t = (2.5 , \text{N·m})(1.2 , \text{s}) = 3.0 , \text{kg·m}^2/\text{s} $. The change represents how much the angular momentum increases due to the applied torque. The calculation is straightforward multiplication of the two given values. Choice A (2.1) might result from calculation error or misreading the values, while C (2.5) incorrectly uses just the torque value. Always multiply torque by time to find the change in angular momentum.
A wheel experiences a constant net torque of $4.0,\text{N·m}$ for $0.50,\text{s}$. What is the angular impulse?
$2.0,\text{kg·m}^2/\text{s}$
$0.50,\text{kg·m}^2/\text{s}$
$4.0,\text{kg·m}^2/\text{s}$
$8.0,\text{kg·m}^2/\text{s}$
Explanation
This question requires calculating angular impulse from constant torque and time duration. Angular impulse $J = \tau \Delta t$, where $\tau$ is the net torque and $\Delta t$ is the time interval. Substituting the given values: $J = (4.0 , \text{N·m})(0.50 , \text{s}) = 2.0 , \text{N·m·s} = 2.0 , \text{kg·m}^2/\text{s}$. The angular impulse represents the total angular effect of the torque over the time period. Choice A (8.0) incorrectly multiplies 4.0 by 2 instead of 0.50, suggesting a calculation error or misreading of the time value. To find angular impulse, always multiply the constant torque by the time duration.
A flywheel’s net torque increases its angular momentum by $12,\text{kg·m}^2/\text{s}$ over $3.0,\text{s}$. What is the torque magnitude?
$36,\text{N·m}$
$9.0,\text{N·m}$
$12,\text{N·m}$
$4.0,\text{N·m}$
Explanation
This problem requires finding torque from given change in angular momentum and time. Using the angular impulse-momentum theorem: ΔL = τΔt, we can solve for torque: τ = ΔL/Δt = (12 kg·m²/s)/(3.0 s) = 4.0 N·m. The torque represents the rate of change of angular momentum. A larger torque would produce the same momentum change in less time. Choice A (36) incorrectly multiplies instead of dividing, showing confusion about rearranging the impulse equation. To find torque from momentum change and time, divide the momentum change by the time duration.
A spinning platform has $L_i=10,\text{kg·m}^2/\text{s}$. A constant torque of $-1.0,\text{N·m}$ acts for $6.0,\text{s}$. What is $\Delta L$?
$-1.0,\text{kg·m}^2/\text{s}$
$6.0,\text{kg·m}^2/\text{s}$
$-6.0,\text{kg·m}^2/\text{s}$
$-60,\text{kg·m}^2/\text{s}$
Explanation
This problem tests understanding of negative torque effects on angular momentum. The angular impulse J = τΔt = (-1.0 N·m)(6.0 s) = -6.0 kg·m²/s. Since angular impulse equals change in angular momentum, ΔL = -6.0 kg·m²/s. The negative value indicates the angular momentum decreases by this amount. Choice C (-60) incorrectly multiplies by an extra factor of 10, suggesting a decimal place error. When calculating change in angular momentum, multiply torque by time and preserve the sign to indicate direction.
A rotor experiences a constant opposing torque of magnitude $0.80\ \text{N·m}$ for $3.0\ \text{s}$. What is the magnitude of the angular impulse?
$3.0\ \text{kg·m}^2/\text{s}$
$2.4\ \text{kg·m}^2/\text{s}$
$0.27\ \text{kg·m}^2/\text{s}$
$0.80\ \text{kg·m}^2/\text{s}$
Explanation
This scenario examines angular impulse in an opposing torque context. The magnitude of angular impulse is τ Δt, regardless of direction, as it quantifies the total rotational impetus. Qualitatively, it equals the absolute change in angular momentum, opposing the rotor's motion in this case. The constant torque over time delivers this impulse. Choice C (0.80 kg·m²/s) may be picked by mistaking impulse for torque alone, reflecting a misconception that time is not a factor in impulse calculations. A broad strategy is to draw parallels between linear and angular dynamics, treating impulse as the 'push' over time in both realms.