Acceleration is the rate of change of velocity: how much the velocity changes each second.
Acceleration The rate at which velocity changes, in m/s².
Step 1: Something to Notice
Watch first. The explanation comes later.
A ball thrown straight up has the same downward acceleration on the way up, at the top, and on the way down — including at the instant it is momentarily stationary.
How can something accelerate while it is not moving?
Step 2: Find Out
Isolate the signs of velocity and acceleration in each case and note when the object speeds up.
Step 1 — Predict
A car reverses and speeds up. What are the signs of its velocity and acceleration?
Choose what you think will happen. You cannot see the experiment until you do — guessing first is what makes it worth watching.
Step 3: So Here Is Why
Now the explanation, after you have seen it happen.
Signs together, or apart
Same sign as the velocity means speeding up. Opposite sign means slowing down.
Zero velocity is not zero acceleration
A ball at the top of its flight is momentarily still and accelerating downward the whole time.
The kinematic equations
For constant acceleration: v = u + at, and s = ut + ½at². Each relates a different set of quantities.
Choose the equation by what is missing
List what you know and what you want; pick the equation that omits the one quantity you cannot get.
A change in velocity, in any respect
Speeding up, slowing down and turning are all accelerations, because velocity includes direction. An object moving in a circle at constant speed is accelerating continuously.
Negative acceleration is not always slowing
A negative acceleration slows an object moving in the positive direction and speeds up one moving in the negative direction. The sign indicates direction, not deceleration.
Step 4: A Common Mistake
Lots of people think
“Negative acceleration always means slowing down.”
Step 5: Why Astronauts Float in a Falling Station
The same idea somewhere new.
The International Space Station is in free fall, accelerating toward Earth at about 8.7 m/s² — almost full gravity. It does not hit the ground because it is moving sideways fast enough that the surface curves away beneath it. Astronauts float because they are falling at the same rate as the station around them, not because gravity is absent.
Step 6: Think It Through
Practice makes it stick.
At the Top
Problem 1 of 2
A ball is momentarily stationary at the top of its flight. What is its acceleration?
Reversing Faster
Problem 2 of 2
A car reverses and speeds up. Is its acceleration positive or negative?
Changing Velocity
1 of 5
When does an object speed up?
2 of 5
A car starts at 24 m/s and decelerates at 3 m/s². How long to stop?
3 of 5
Can acceleration be non-zero when velocity is zero?
4 of 5
When are the kinematic equations valid?
5 of 5
Why do astronauts float in the space station?
Step 7: Quick Check
Show what you know.
Question 1 of 1
Does negative acceleration always mean slowing down?
Step 8: Explain It in Writing
Claim, evidence, then reasoning.
The question
Explain why a ball at the top of its flight has zero velocity and non-zero acceleration.
Fill in all three boxes. The reasoning box is the one that matters most.
What You Found Out
- Acceleration is the rate of change of velocity.
- Same sign as velocity means speeding up; opposite means slowing.
- Zero velocity does not imply zero acceleration.
- The kinematic equations hold only for constant acceleration.