arthham

JEE Advanced 1985 · Motion in a Straight Line

Two identical trains are moving on rails along the equator on the earth in opposite directions with the same speed. They will exert the same pressure on the rails.

Answer: False

Watch this solved on the board The full walkthrough, written out a line at a time with narration explaining each step as it appears. Pause anywhere and ask why a line is there. Free to watch

Or read the worked solution below. Jump to the steps ↓

Written solution

Step 1: Define the parameters of the system.

Let the two identical trains each have mass mm, and let them move with speed vv relative to the Earth's surface along the equator in opposite directions.

Step 2: Determine the inertial speeds of the trains.

The Earth rotates about its axis with angular speed Ω\Omega. In an inertial reference frame, the speed of the Earth's surface at the equator is vE=ΩRv_E = \Omega R, where RR is the radius of the Earth.

The inertial speed of the train moving in the direction of the Earth's rotation is:

vwith=ΩR+vv_{\text{with}} = \Omega R + v

The inertial speed of the train moving against the direction of the Earth's rotation is:

vagainst=ΩR−vv_{\text{against}} = \Omega R - v

Step 3: Set up the equation of motion in the radial direction.

In the inertial frame, the net inward radial force provides the necessary centripetal acceleration for circular motion of radius RR.

mg0−N=mvI2Rm g_0 - N = \frac{m v_I^2}{R}

Here, g0=GMR2g_0 = \frac{GM}{R^2} is the acceleration due to gravity, NN is the normal force exerted by the rails on the train, and vIv_I is the inertial speed of the train.

Step 4: Calculate the normal force for each train.

Solve for the normal force NN, which is equal in magnitude to the force (and thus pressure) the train exerts on the rails.

For the train moving with the rotation:

Nwith=m(g0−(ΩR+v)2R)N_{\text{with}} = m\left(g_0 - \frac{(\Omega R + v)^2}{R}\right)

For the train moving against the rotation:

Nagainst=m(g0−(ΩR−v)2R)N_{\text{against}} = m\left(g_0 - \frac{(\Omega R - v)^2}{R}\right)

Step 5: Compare the normal forces.

Find the difference between the two normal forces.

Nagainst−Nwith=m(g0−(ΩR−v)2R)−m(g0−(ΩR+v)2R)N_{\text{against}} - N_{\text{with}} = m\left(g_0 - \frac{(\Omega R - v)^2}{R}\right) - m\left(g_0 - \frac{(\Omega R + v)^2}{R}\right) Nagainst−Nwith=mR[(ΩR+v)2−(ΩR−v)2]N_{\text{against}} - N_{\text{with}} = \frac{m}{R} \left[ (\Omega R + v)^2 - (\Omega R - v)^2 \right] Nagainst−Nwith=4mΩvN_{\text{against}} - N_{\text{with}} = 4 m \Omega v

Since 4mΩv>04 m \Omega v > 0, the train moving against the Earth's rotation exerts a greater force on the rails than the train moving with the rotation.

Answer: The statement is FALSE. The train moving against the Earth's rotation exerts a greater pressure on the rails than the train moving with the rotation, with a difference in normal force of 4mΩv4 m \Omega v.

More from Motion in a Straight Line

All 14 Motion in a Straight Line questions →