UPSC Prelims 2004 Q106: Science & Technology – Physics
A car is running on a road at a uniform speed of 60 km/hr. The net resultant force on the
car is
- (a) Driving force in the direction of car’s motion
- (b) Resistance force opposite to the direction of car’s motion
- (c) An inclined force
- (d) Equal to zero
Answer: (d) Equal to zero
Explanation
Net force on a car moving at uniform speed
| Condition | Net force |
|---|---|
| Uniform speed in a straight line | No acceleration, so net force is zero |
| Driving force | Balances the resistance (friction and air drag) |
| Resistance | Equal and opposite to the driving force |
By Newton's first law, a car running at a uniform speed on a road has no acceleration, so the net resultant force on the car is zero; the driving force just balances the resistance.
Answer: (d) Equal to zero.
Why each option is right or wrong
- (a) Incorrect. The driving force alone would accelerate the car.
- (b) Incorrect. A net resistance force would slow the car.
- (c) Incorrect. An inclined force is not required for uniform motion.
- (d) CORRECT. The net force is zero at uniform velocity.
Key point
Newton's first law: no net force means no change in velocity.
Remember
At uniform speed on a straight road the net force is zero.
Common mistakes
- Thinking a moving body always needs a net force.
- Confusing the driving force with the net force.
- Forgetting that friction balances the engine's push.
Exam tip
Uniform velocity means zero net force, not zero forces.
Sources
- NCERT Class XI Physics — Laws of Motion
- NCERT Class IX Science — Force and Laws of Motion