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Question:

A particle moves so that its position vector is given by 𝑟⃗=cosωt^x+sinωt^y. Where ω is a constant. Which of the following is true?

Velocity is perpendicular to 𝑟⃗ and acceleration is directed away from the origin.

Velocity and acceleration both are perpendicular to 𝑟⃗

Velocity and acceleration both are parallel to 𝑟⃗

Velocity is perpendicular to 𝑟⃗ and acceleration is directed towards the origin

Solution:

Step 1: Velocity and acceleration calculation
𝑟⃗=cosωt^x+sinωt^y (1)
Velocity is given by
𝑣⃗=d𝑟⃗/dt
𝑣⃗=−ωsinωt^x+ωcosωt^y m/s (2)
Acceleration is given by
a⃗=d𝑣⃗/dt
a⃗=−ω²cosωt^x−ω²sinωt^y m/s² (3)
Step 2: Analysing 𝑟⃗,𝑣⃗ and 𝑎⃗
If the dot product between two vectors is zero that means the vectors are perpendicular to each other.
𝑣⃗⋅𝑟⃗=−ωsinωtcosωt+ωsinωtcosωt=0
∴ 𝑣⃗ is perpendicular to 𝑟⃗
a⃗⋅𝑟⃗=−ω²cos²ωt−ω²sin²ωt≠0
∴ 𝑎⃗ is not perpendicular to 𝑟⃗
Negative sign in eqn(3) indicates acceleration is towards the origin (Opposite to 𝑟⃗)
Hence , 𝑣⃗ is perpendicular to 𝑟⃗ and the acceleration is directed towards the origin
∴Option D is the correct answer.