Physics · Motion In A Straight Line · NEET
No. This is the most tested point in NCERT. The sign of acceleration only tells its direction along your chosen axis. Whether the object slows down depends on whether acceleration is opposite to velocity. Example: a ball moving in the negative direction (v < 0) with negative acceleration (a < 0) is actually speeding up, because a and v point the same way. Negative acceleration means slowing down ONLY when velocity is positive.
Negative acceleration is about direction (acceleration points along the negative axis). Retardation is about speed (the object is slowing down, so its speed magnitude decreases). They match only when velocity is positive. Retardation is defined by the rule: acceleration is opposite in direction to velocity. So retardation can even have a positive sign if the object is moving in the negative direction.
Yes. If the object moves in the negative direction (velocity is negative) and acceleration is also negative, then a and v are parallel, so speed increases. A free-falling body with upward taken positive has a = -g (negative) but its downward speed keeps increasing. This is the trap NTA loves.
Yes, they mean the same thing: the object is slowing down. Both are defined by acceleration acting opposite to velocity, so the speed (magnitude of velocity) decreases with time. The word retardation is more common in Indian textbooks and NCERT.
If you take upward as positive, the acceleration is -g (about -9.8 m/s^2) during the WHOLE flight, going up and coming down. On the way up, velocity is positive and a is negative, so it is retardation (ball slows). At the top v = 0 but a is still -g. On the way down, velocity is negative and a is negative, so a and v are parallel and the ball speeds up. The acceleration never changes sign.
A bullet enters a wooden block with speed u and, after travelling 24 cm inside it, its speed reduces to u/3. Assuming uniform retardation, the further distance it travels before coming to rest is:
A toy car with charge q moves on a frictionless horizontal plane under a uniform electric field E. Due to the force qE its velocity increases from 0 to 6 m/s in one second. At that instant the field direction is reversed and the car moves for two more seconds. The average velocity and average speed between 0 and 3 s are respectively:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Acceleration is positive when its direction is along the positive axis you have chosen. It does not by itself mean speeding up; it means increasing velocity in the positive direction. If velocity is already positive, positive acceleration does speed the body up.
Retardation, also called deceleration, is when an object slows down. It occurs when the acceleration acts opposite to the velocity, so the magnitude of velocity (speed) decreases over time. Its numerical value can carry a positive or negative sign depending on the axis direction chosen.
Because speeding up or slowing down depends on whether acceleration and velocity point the same way or opposite ways, not on the sign alone. The sign only fixes the direction relative to your axis. You must compare a and v directions to know if the body speeds up or slows down.
First fix the positive direction. Then check the direction of velocity and acceleration. If they are opposite, it is retardation (slowing down). If they are the same, the object speeds up. This one rule handles every NEET question, including thrown-up balls and reversed fields.
