Laws of Motion: common misconceptions

If your child says… “Moving things need a forward force.”

Ask together

After your hand leaves, what keeps pushing the spoon forward?

What’s true

A net force changes velocity; steady motion needs balanced forces.

If your child says… “The box moves right, so its acceleration must point right.”

Ask together

Which way does the stronger horizontal force pull?

What’s true

Acceleration follows the net force, which can oppose the current velocity.

If your child says… “The same speed means no impulse.”

Ask together

Which way does momentum point before and after the rebound?

What’s true

Reversing direction changes momentum even when speed stays unchanged.

If your child says… “Things pushing apart must have equal speeds.”

Ask together

If their momenta match in size, can unequal masses have equal speeds?

What’s true

Two unequal parts separating from rest have opposite, equal-sized momenta when external impulse is absent.

If your child says… “For the same tension, the upward part grows as the string gets closer to horizontal.”

Ask together

If the string were horizontal, how much of its pull would point upward?

What’s true

For the same tension, the upward part shrinks as the string approaches horizontal.

If your child says… “A surface always pushes with exactly the object’s weight.”

Ask together

Does the weight arrow point directly into the slope?

What’s true

On this slope, the normal force balances only the perpendicular part of weight.

If your child says… “Friction is always at its maximum when nothing slips.”

Ask together

On the level book, what sideways force would friction need to balance?

What’s true

Static friction supplies only what prevents slipping, up to its limit.

If your child says… “A separate turning force keeps things circling.”

Ask together

Which actual interaction supplies the inward push in this diagram?

What’s true

Centripetal force names the inward resultant; it adds no new interaction.