6 mistakes learners make with speed and acceleration, each one named and explained.
Speed is distance for each second; acceleration is change in speed for each second.
The answer given is one of the values printed in the question, unchanged.
When a calculation will not come, the numbers on the page are the only thing to hold on to.
Question. 240 m in 12 s
A common answer. 12 m/s
The answer. 20 m/s
Why. The 12 was the time, so it cannot also be the speed.
Check the unit of your answer against the unit of the number you wrote down.
Every quantity in the question gets multiplied together, so 150 m in 10 s becomes 1500 m/s.
Multiplying is the operation you reach for first, and the numbers do combine, just not that way.
Question. A car goes 150 m in 10 s
A common answer. 150 x 10 = 1500 m/s
The answer. 150 / 10 = 15 m/s
Why. Ten seconds of travel cannot be faster than one second of travel.
Decide first whether the answer should be bigger or smaller than what you started with.
The division is set up correctly and the decimal point lands in the wrong place, so 150 divided by 10 comes out as 1.5.
Dividing by ten shifts the digits, and shifting them the wrong way looks identical on paper.
Question. 150 m in 10 s
A common answer. 1.5 m/s
The answer. 15 m/s
Why. Dividing by 10 makes 150 into 15, not into 1.5.
Estimate the answer roughly before dividing. Being out by ten is always visible against an estimate.
You divide the smaller number by the larger one, so a speed of 15 m/s comes out as 0.067.
A triangle mnemonic tells you three letters but not which one sits on top, and dividing the small number by the big one feels safer.
Question. A car goes 150 m in 10 s
A common answer. 10 / 150 = 0.067 m/s
The answer. 150 / 10 = 15 m/s
Why. Speed is metres for each second, so the metres go on top.
Ask what the unit says. Metres per second means metres divided by seconds, in that order.
Acceleration from 4 m/s to 16 m/s is worked out using 16 alone.
The final speed is the one the question ends on, so it feels like the number to use.
Question. 4 m/s to 16 m/s in 6 s
A common answer. 16 / 6 = 2.7 m/s^2
The answer. (16 - 4) / 6 = 2 m/s^2
Why. Acceleration is the change in speed, so the starting speed has to be taken off.
Acceleration always uses a difference. From rest is the special case where it looks like it does not.
A car travelling 150 m in 10 s is given a speed of 150 m/s, the distance copied out.
The distance is the number the question is about, so it feels like the answer.
Question. 150 m in 10 s
A common answer. 150 m/s
The answer. 15 m/s
Why. 150 m/s would be faster than a passenger jet.
A speed always has two units in it. If your answer used only one number, it is not a speed.
WAJD spots these patterns in your child's answers and names the one behind their wrong answers, instead of just marking them wrong.