UPSC Prelims 2008 Q127: Science & Technology – Physics
A person is sitting in a car which is at rest. The reaction from the road at each of the four
wheels of the car is R. When the car runs on a straight level road, how will the reaction at either
of the front wheels vary?
- (a) It will be greater than R
- (b) It will be less than R
- (c) It will be equal to R
- (d) It shall depend on the material of the road
Answer: (c) It will be equal to R
Explanation
How front-wheel reaction changes on a straight, level road
A car initially at rest has equal reaction force R at each wheel; we examine what happens when it moves at constant velocity on a straight, level road.
| Option | Verdict | Reason |
|---|---|---|
| (a) Greater than R | Incorrect | There's no additional load pushing down on the front wheels during constant-velocity, straight-line motion |
| (b) Less than R | Incorrect | There's also no reduction in load under these specific conditions |
| (c) Equal to R | Correct | On a straight, level road at constant velocity (no acceleration), there is no weight transfer, so the reaction at each wheel remains equal to R |
| (d) Depends on the road material | Incorrect | The road material does not affect the normal reaction force in this scenario |
Why the reaction stays equal to R:
| Point | Detail |
|---|---|
| No acceleration | Straight, level road, constant velocity |
| No weight transfer | Reaction force distribution remains unchanged |
Answer: (c) It will be equal to R.
Why each option is right or wrong
- (a) Incorrect. There's no additional downward force on the front wheels under these specific conditions.
- (b) Incorrect. There's also no reduction in the front wheels' load.
- (c) CORRECT. With no acceleration on a straight, level road, the reaction remains equal to R.
- (d) Incorrect. The road material doesn't affect this normal reaction force calculation.
Key point
On a straight, level road at constant velocity (no acceleration), there is no weight transfer, so the reaction force at the front wheels remains EQUAL TO R.
Remember
On a straight, level road at constant speed, the front-wheel reaction remains equal to R (no weight transfer).
Common mistakes
- Assuming any motion automatically changes the wheel-reaction forces, missing the 'constant velocity, straight, level' conditions.
- Confusing this scenario with acceleration/braking situations, which DO cause weight transfer.
- Attributing the reaction force to road material rather than the vehicle's dynamics.
Exam tip
Constant velocity on a straight, level road = no weight transfer = wheel reactions stay the same (R). Acceleration or braking would change this.
Sources
- NCERT Class XI Physics — Laws of Motion
- NCERT Class IX Science — Force and Laws of Motion