Key relationship
v = u + at; x − x₀ = ut + ½at²; v² = u² + 2a(x − x₀)
Physics · Class 11
Constant acceleration
Acceleration is the rate of change of velocity. When velocity and acceleration have opposite signs, speed decreases until velocity reaches zero. Continued acceleration then starts motion in the opposite direction. The displacement equations use signed displacement.
Open the interactive experiment →Acceleration changes velocity, including its sign.
முடுக்கம் திசைவேக மாற்ற வீதம். திசைவேகமும் முடுக்கமும் எதிர் குறிகளில் இருந்தால் வேகம் குறையும். பூஜ்ய திசைவேகத்திற்குப் பிறகும் அதே முடுக்கம் தொடர்ந்தால் இயக்கத் திசை மாறும்.
Key notes, relationships and model limits
Remember
Acceleration is the rate of change of velocity. When velocity and acceleration have opposite signs, speed decreases until velocity reaches zero. Continued acceleration then starts motion in the opposite direction. The displacement equations use signed displacement.
Common trap
Negative acceleration does not always mean slowing down; the velocity sign also matters.
Model limits
Constant signed acceleration for all 10 seconds. A braking preset continues to accelerate after reaching zero velocity and therefore reverses; it is not an automatic stop model.
What you will learn
Use all three constant-acceleration equations and distinguish deceleration, rest at an instant and reversal.
Try the experiment
Use the brake-and-reverse preset. Scrub before, at and after the zero-velocity time; compare distance with displacement.
Think of it this way
A ball thrown upward slows, reaches a turning point, then moves downward while gravity keeps the same direction.
Worked example
At 6 s, v = 8 − 12 = −4 m/s and displacement = 48 − 36 = 12 m. The turning point is at 16 m, so distance = 16 + 4 = 20 m.
Why this matters
A signed equation remains valid through a reversal if its assumptions remain valid.
Before you start
Signed numbers · coordinates · gradients · area under a graph
Check your understanding
For u = +8 m/s and a = −2 m/s², when does velocity first reach zero?
- 2 s
- 4 s
- 8 s
Read the explanation
0 = 8 − 2t gives t = 4 s. Continuing the same acceleration makes velocity negative afterward.
Course connection: CBSE XI Chapter 2; NEET and JEE Main Unit 2; JEE Advanced Mechanics. Check the selected syllabus for relative-velocity scope.