arthham

JEE Advanced 1984 · Motion in a Plane

A projectile fired from the ground follows a parabolic path. The speed of the projectile is minimum at the top of its path.

Answer: True

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Written solution

Step 1: Define the initial parameters of the projectile.

Let a projectile be fired from the ground with an initial speed uu at an angle θ\theta to the horizontal.

u⃗=ucos⁡θ i^+usin⁡θ j^\vec{u} = u\cos\theta\, \hat{i} + u\sin\theta\, \hat{j}

Step 2: Determine the velocity components at any time tt.

The horizontal velocity remains constant due to the absence of horizontal forces, while the vertical velocity is affected by gravity.

vx=ucos⁡θv_x = u\cos\theta vy=usin⁡θ−gtv_y = u\sin\theta - gt

Step 3: Express the speed of the projectile as a function of time.

The speed v(t)v(t) is the magnitude of the velocity vector.

v(t)=vx2+vy2v(t) = \sqrt{v_x^{2} + v_y^{2}} v(t)=(ucos⁡θ)2+(usin⁡θ−gt)2v(t) = \sqrt{(u\cos\theta)^{2} + (u\sin\theta - gt)^{2}}

Step 4: Find the velocity components at the top of the path.

At the highest point of the parabolic path, the vertical component of velocity becomes zero.

vy=0v_y = 0 t=usin⁡θgt = \frac{u\sin\theta}{g}

Step 5: Calculate the minimum speed.

Since the squared term (usin⁡θ−gt)2≥0(u\sin\theta - gt)^{2} \ge 0, the speed is minimized when this term is zero.

vmin=(ucos⁡θ)2+0v_{\text{min}} = \sqrt{(u\cos\theta)^{2} + 0} vmin=ucos⁡θv_{\text{min}} = u\cos\theta

This minimum speed occurs at the top of the path.

Answer: The statement is TRUE. The speed of the projectile is minimum at the top of its path, with a value of ucos⁡θu\cos\theta.

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