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JEE Advanced 2023 · Laws of Motion

A particle of mass mm is moving in the xyxy-plane such that its velocity at a point (x,y)(x, y) is given as v⃗=α(yx^+2xy^)\vec{v} = \alpha(y\hat{x} + 2x\hat{y}), where α\alpha is a non-zero constant. What is the force F⃗\vec{F} acting on the particle?

Answer: A. F⃗=2mα2(xx^+yy^)\vec{F} = 2m\alpha^{2}(x\hat{x} + y\hat{y})

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Written solution

Step 1: Identify the velocity components.

The velocity vector of the particle is given.

v⃗=α(yx^+2xy^)\vec{v} = \alpha(y\hat{x} + 2x\hat{y})

Extract the xx and yy components of the velocity.

vx=αyv_x = \alpha y vy=2αxv_y = 2\alpha x

Step 2: Calculate the acceleration components.

Differentiate the velocity components with respect to time tt.

ax=dvxdt=αdydt=αvya_x = \frac{dv_x}{dt} = \alpha \frac{dy}{dt} = \alpha v_y ay=dvydt=2αdxdt=2αvxa_y = \frac{dv_y}{dt} = 2\alpha \frac{dx}{dt} = 2\alpha v_x

Substitute the expressions for vxv_x and vyv_y.

ax=α(2αx)=2α2xa_x = \alpha(2\alpha x) = 2\alpha^2 x ay=2α(αy)=2α2ya_y = 2\alpha(\alpha y) = 2\alpha^2 y

Step 3: Determine the acceleration vector.

Combine the components to write the acceleration vector a⃗\vec{a}.

a⃗=axx^+ayy^\vec{a} = a_x\hat{x} + a_y\hat{y} a⃗=2α2(xx^+yy^)\vec{a} = 2\alpha^2(x\hat{x} + y\hat{y})

Step 4: Calculate the force acting on the particle.

Apply Newton's second law of motion.

F⃗=ma⃗\vec{F} = m\vec{a} F⃗=2mα2(xx^+yy^)\vec{F} = 2m\alpha^2(x\hat{x} + y\hat{y})

Answer: F⃗=2mα2(xx^+yy^)\vec{F} = 2m\alpha^2(x\hat{x} + y\hat{y})

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