Velocity, acceleration and speed in space
Problem 9.391 · medium
A particle has position \( \displaystyle \mathbf r(t) = \left\langle \cos{\left(3 t \right)}, \sin{\left(3 t \right)}, t \right\rangle \). Find its velocity, acceleration and speed at \( \displaystyle t = \pi \).
- \[ \left[\begin{matrix}\frac{d}{d t} \cos{\left(3 t \right)}\\\frac{d}{d t} \sin{\left(3 t \right)}\\\frac{d}{d t} t\end{matrix}\right] = \left[\begin{matrix}- 3 \sin{\left(3 t \right)}\\3 \cos{\left(3 t \right)}\\1\end{matrix}\right] \]v = r′.✓ Proved
- \[ \left[\begin{matrix}\frac{d}{d t} \left(- 3 \sin{\left(3 t \right)}\right)\\\frac{d}{d t} 3 \cos{\left(3 t \right)}\\\frac{d}{d t} 1\end{matrix}\right] = \left[\begin{matrix}- 9 \cos{\left(3 t \right)}\\- 9 \sin{\left(3 t \right)}\\0\end{matrix}\right] \]a = v′.✓ Proved
- \[ \sqrt{10} \]Speed = ‖v(pi)‖.✓ Proved
Answer \( \mathbf v = \left\langle 0, -3, 1 \right\rangle,\ \mathbf a = \left\langle 9, 0, 0 \right\rangle,\ \text{speed} = \sqrt{10} \)
Every line of this solution was proved by the computer algebra system SymPy. The answer was also checked a second way, without looking at the solution. The reviewers disagree about how one step is explained; every verdict is in the receipt.
The full receipt
| Line | Status | Checked by | Detail |
|---|---|---|---|
| 1 | ✓ Proved | sympy 1.14.0 | simplify(lhs - rhs) reduced to 0 |
| 2 | ✓ Proved | sympy 1.14.0 | simplify(lhs - rhs) reduced to 0 |
| 3 | ✓ Proved | sympy 1.14.0 | simplify(lhs - rhs) reduced to 0 |
| answer, a second way | ✓ Checked independently | sympy 1.14.0 + mpmath 1.3.0 | difference quotients of the position |
Reviewers
gpt-oss:20b: passqwen3.6:27b-mlx: fail (error) — The solution fails to explicitly evaluate the velocity and acceleration vectors at t = \pi, which are required by the problem statement. It only computes the speed, omitting the specific vector answers \mathbf{v}(\pi) and \mathbf{a}(\pi).
Every verdict on record (4)
qwen3.6:27b-mlx: fail (error) 2026-10-08 — The solution fails to explicitly evaluate the velocity and acceleration vectors at t = \pi, which are required by the problem statement. It only computes the speed, omitting the specific vector answers \mathbf{v}(\pi) and \mathbf{a}(\pi).gpt-oss:20b: pass 2026-10-08gpt-oss:20b: pass 2026-10-08qwen3.6:27b-mlx: fail (style) 2026-10-08 — [domain objection, downgraded to style] The solution fails to explicitly evaluate the velocity and acceleration vectors at t = pi, which is required by the problem statement. It only computes the speed at t = pi, leaving the vector answers for v and a undefined in the text.
Proved: SymPy reduced the difference between the two sides to zero. Checked independently: a separate
method, named above, confirmed it. Checked numerically: the two sides agree at every sampled point, which is evidence,
not proof. Reviewed: a model or a person read it; that is all a sentence can have. Solution by
generator:structured/motion_in_space, checked 2026-10-08 with SymPy 1.14.0.