Question Details

Consider a car that overturns while taking a turn. Select the option that is experienced by the wheels of the car

Options

A

First, both inner, as well as the outer wheel, leave the ground

B

The inner wheel experiences no change

C

First, the inner wheel leaves the ground

D

None of the above

Correct Answer :

First, the inner wheel leaves the ground

Solution :

The correct answer is: First, the inner wheel leaves the ground.

To understand why the inner wheels leave the ground first when a car overturns while taking a turn, we need to analyze the forces and torques acting on the car during the turn.

Let us consider a car of mass M making a turn of radius R at a constant speed v. The center of gravity of the car is at a height h above the ground, and the distance between the inner and outer wheels (the track width) is 2a.

When the car takes a turn, two main forces create moments (torques) about the center of gravity or the contact points of the wheels:
1. Centrifugal Force: In the rotating frame of the car, an outward centrifugal force acts at the center of gravity, given by:
F c = M v 2 R
This force acts horizontally outwards and creates a clockwise overturning torque of magnitude Fc×h about the wheel contact points.
2. Gravity: The weight of the car W=Mg acts vertically downwards through the center of gravity, which provides a restoring torque (counter-clockwise) keeping the car stable.

Due to the overturning torque, vertical reaction forces from the ground on the inner wheels (Ri) and outer wheels (Ro) change. By taking moments about the outer wheel contact point, we find the reaction force on the inner wheels:
R i = M g 2 - M v 2 h 2 R a
Similarly, the reaction force on the outer wheels is:
R o = M g 2 + M v 2 h 2 R a

As the speed of the car v increases, the term representing the transfer of load due to the centrifugal force increases. Consequently, the normal reaction on the inner wheels Ri decreases, while the normal reaction on the outer wheels Ro increases.

At a critical speed, the normal reaction on the inner wheels becomes zero (Ri=0). At this exact moment, the inner wheels lose contact with the ground and lift off. Since Ro increases during this process, the outer wheels remain firmly pressed against the ground, meaning the inner wheel must leave the ground first.

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