4 mistakes learners make with forces, each one named and explained.
Balanced forces keep the motion as it is. Only a resultant force changes it.
12 V and 4 A are added to give 16 ohms, because the numbers were there and adding is easy.
When a formula will not come to mind, adding the given numbers at least uses them all.
Question. 12 V across a resistor carrying 4 A
A common answer. 12 + 4 = 16 ohms
The answer. 12 / 4 = 3 ohms
Why. Volts and amps are different quantities, and different quantities cannot be added.
Only quantities with the same unit can be added. Different units mean multiply or divide.
An object with all forces balanced is described as accelerating.
Acceleration is the answer most force questions want, so it becomes the default.
Question. All forces on an object are balanced
A common answer. it accelerates downwards
The answer. it stays still or moves at constant velocity
Why. Acceleration requires a resultant force, and the resultant here is zero.
Work out the resultant first. Everything else follows from whether it is zero.
Constant velocity is explained by gravity no longer acting.
If nothing is falling, it is easy to conclude nothing is pulling.
Question. A book resting on a table
A common answer. gravity is not acting on it
The answer. gravity pulls it down and the table pushes it up, equally
Why. Remove the table and it falls immediately, which shows gravity never stopped.
Draw both arrows. Balanced means equal and opposite, never missing.
An object at constant velocity is said to have an unbalanced force forwards, or no forces at all.
Everything you push does stop when you stop pushing, because friction is always there.
Question. A car moving at a steady 30 mph
A common answer. there must be a resultant force forwards
The answer. the driving force and the resistive forces are balanced
Why. A resultant force would make it speed up, and it is not speeding up.
Ask whether the speed is changing, not whether it is moving. Only change needs a force.
WAJD spots these patterns in your child's answers and names the one behind their wrong answers, instead of just marking them wrong.