Displacement, Velocity, and Acceleration
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AP Physics 1 › Displacement, Velocity, and Acceleration
A cart moves along a straight track; positive is to the right. Its position is recorded: at $t=0,\text{s}$, $x=0,\text{m}$; at $t=2,\text{s}$, $x=6,\text{m}$; at $t=5,\text{s}$, $x=3,\text{m}$. Assume the cart moves smoothly between these times.
What is the cart’s average velocity from $t=2,\text{s}$ to $t=5,\text{s}$?
$-3.0,\text{m/s}$
$+3.0,\text{m/s}$
$+1.0,\text{m/s}$
$-1.0,\text{m/s}$
Explanation
This question assesses the skill of calculating average velocity from position-time data in one-dimensional motion. Average velocity is defined as the change in position divided by the change in time, so for the interval from t=2 s to t=5 s, Δx = 3 m - 6 m = -3 m and Δt = 5 s - 2 s = 3 s. Thus, the average velocity v_avg = Δx / Δt = -3 m / 3 s = -1.0 m/s, which corresponds to choice B. The negative sign indicates the cart is moving to the left, toward the negative direction, even though it initially moved right. Choice A, +1.0 m/s, is incorrect because it ignores the direction of the displacement, treating it as positive when the cart actually moves leftward. To solve similar problems, always use the formula v_avg = (x_final - x_initial) / (t_final - t_initial) and pay attention to signs for direction.
A cart moves in 1D; positive is to the right. It travels from $x=0,\text{m}$ to $x=5,\text{m}$, then back to $x=2,\text{m}$.
What is the cart’s total distance traveled?
$3,\text{m}$
$8,\text{m}$
$7,\text{m}$
$2,\text{m}$
Explanation
This question tests distinguishing between displacement and total distance traveled in one-dimensional motion. Total distance is the sum of absolute path lengths, not the net change. The cart moves from 0 m to 5 m (distance 5 m), then back to 2 m (distance 3 m), totaling 5 m + 3 m = 8 m, which is choice D. This accounts for the actual ground covered in both directions. Choice C, 7 m, is wrong, perhaps from adding initial and final positions without considering the reversal. To solve path-dependent problems, break the motion into segments and sum the absolute distances, unlike net displacement which is just final minus initial position.
A robot moves along the $x$-axis; positive is to the right. Its velocity is $v(t)=-3,\text{m/s}$ for $0\le t\le 4,\text{s}$.
Which statement about the robot’s motion is correct?
The robot is at rest because velocity is negative.
The robot moves to the left at constant speed $3,\text{m/s}$.
The robot’s displacement is positive because speed is positive.
The robot’s speed is $-3,\text{m/s}$.
Explanation
This question tests understanding of velocity and speed in constant-velocity motion. Velocity is a vector with magnitude and direction, while speed is the magnitude alone. Given v(t) = -3 m/s, the robot moves left (negative direction) at a constant speed of 3 m/s, as stated in choice B. This describes steady motion without changing speed or direction during the interval. Choice A is incorrect because speed cannot be negative; it mistakenly equates speed with velocity. To analyze motion, distinguish between velocity (directional) and speed (scalar), using signs to determine direction in one-dimensional problems.
A car moves on a straight road; positive is north. At $t=0,\text{s}$ the car is at $x=5,\text{m}$, and at $t=8,\text{s}$ it is at $x=-3,\text{m}$.
What is the car’s displacement over the $8,\text{s}$ interval?
$2,\text{m}$
$-8,\text{m}$
$8,\text{m}$
$-2,\text{m}$
Explanation
This question evaluates calculating displacement in one-dimensional motion using position data. Displacement is the change in position, Δx = x_final - x_initial, independent of the path taken. Here, x_initial = 5 m at t=0 s and x_final = -3 m at t=8 s, so Δx = -3 m - 5 m = -8 m, matching choice B. The negative value indicates a net movement southward over the interval. Choice A, 8 m, is wrong because it represents the magnitude (distance) rather than the directed displacement. Always use Δx = x_f - x_i for displacement problems, remembering it includes direction unlike total distance traveled.
A robot moves along a straight line, with +x to the right. Its velocity changes from $v=+5,\text{m/s}$ at $t=1,\text{s}$ to $v=-1,\text{m/s}$ at $t=4,\text{s}$.
What is the robot’s average acceleration from $t=1,\text{s}$ to $t=4,\text{s}$?
$+2,\text{m/s}^2$
$+6,\text{m/s}^2$
$-6,\text{m/s}^2$
$-2,\text{m/s}^2$
Explanation
This question tests determining average acceleration from velocity changes in straight-line motion. Average acceleration is change in velocity divided by change in time. From v=+5 m/s at t=1 s to v=-1 m/s at t=4 s, Δv = -1 - 5 = -6 m/s, Δt=3 s, so a_avg = -6/3 = -2 m/s². The negative value indicates deceleration or direction change toward the left. Choice D, +6 m/s², is incorrect by using absolute Δv and ignoring signs. Always calculate a_avg as Δv/Δt, including signs to capture direction.
A cart moves along a straight track, with +x to the right. A motion sensor records the cart’s position as a function of time: $x=0,\text{m}$ at $t=0,\text{s}$, $x=4,\text{m}$ at $t=2,\text{s}$, and $x=4,\text{m}$ at $t=6,\text{s}$. The cart then moves to $x=1,\text{m}$ at $t=8,\text{s}$. Assume the motion between listed times is continuous.
What is the cart’s average velocity from $t=6,\text{s}$ to $t=8,\text{s}$?
$+3,\text{m/s}$
$+1.5,\text{m/s}$
$-1.5,\text{m/s}$
$-3,\text{m/s}$
Explanation
This question assesses the skill of calculating average velocity from position-time data in one-dimensional kinematics. Average velocity is defined as the change in position divided by the change in time over the specified interval. From t=6 s to t=8 s, the position changes from x=4 m to x=1 m, so Δx = 1 m - 4 m = -3 m and Δt = 8 s - 6 s = 2 s, yielding v_avg = -3 m / 2 s = -1.5 m/s. This negative value indicates the direction of the average velocity is to the left, consistent with the +x to the right convention. Choice C, +3 m/s, is incorrect because it uses the magnitude of the displacement without considering the direction, ignoring the sign of Δx. To find average velocity over any interval, always compute Δx/Δt, ensuring to account for the direction in the coordinate system.
A cart moves along a straight line, with +x to the right. It travels from $x=0,\text{m}$ to $x=5,\text{m}$, then back to $x=2,\text{m}$. All motion is along the same line.
What is the cart’s displacement for the entire trip?
$3,\text{m}$
$5,\text{m}$
$7,\text{m}$
$2,\text{m}$
Explanation
This question examines the distinction between displacement and distance in one-dimensional kinematics. Displacement is the net change in position, final minus initial, regardless of path. The cart goes from x=0 m to 5 m then back to 2 m, so Δx = 2 m - 0 m = +2 m. This positive value shows net motion to the right. Choice A, -4 m, might confuse total distance with displacement or misapply signs. Remember, displacement is vectorial; compute it as x_final - x_initial for transferable accuracy.