arthham

JEE Advanced 1979 · Motion in a Straight Line

Answer the following giving reasons in brief: Is the time variation of position, shown in the figure observed in nature?

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Written solution

Step 1: Analyze the axes and features of the graph.

The graph plots position xx on the horizontal axis and time tt on the vertical axis. The curve forms an arch starting at x1x_1 and ending at x2x_2 on the position axis.

At both endpoints, the time is zero:

t=0atx=x1andx=x2t = 0 \quad \text{at} \quad x = x_1 \quad \text{and} \quad x = x_2

The peak of the arch represents the maximum time:

t=tmax⁡atx=xmt = t_{\max} \quad \text{at} \quad x = x_m

Step 2: Determine the number of positions at a given instant of time.

Drawing a horizontal line at any constant value of time tt (where 0<t<tmax⁡0 < t < t_{\max}) intersects the curve at two distinct positions.

x=xaandx=xb(where xa≠xb)x = x_a \quad \text{and} \quad x = x_b \quad (\text{where } x_a \neq x_b)

This indicates that at a single instant of time, the particle exists at two different locations simultaneously.

Step 3: Evaluate the physical possibility of the motion.

For any real physical motion, a particle can occupy exactly one position at any given instant. Thus, position must be a single-valued function of time:

x=x(t)x = x(t)

Since this graph yields two different positions for a single value of time, it represents a physical impossibility.

Answer: No, this time variation of position is not observed in nature because a particle cannot exist at two different positions at the same instant of time.

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