GED Science
Forces & Newton's Laws
A force is a push or pull. Newton's three laws say: things keep doing what they're doing unless a force intervenes; force equals mass times acceleration; and every push gets an equal push back. Nearly every question is one of the three laws wearing a costume.
The words first
- force
- a push or pull, measured in newtons (N)
- inertia
- matter's stubbornness — resistance to changing motion
- net force
- all forces combined; balanced forces net to zero
- friction
- the force resisting sliding, always opposing motion
The method
THE THREE LAWS, IN COSTUME
1st (inertia) "objects keep doing what they're doing"
costume: slam the brakes → passengers lurch FORWARD
(their bodies kept the old motion)
2nd (F = ma) force = mass × acceleration
costume: same push, heavier cart → less acceleration
a = F ÷ m F = m × a m = F ÷ a
3rd (pairs) every action has an equal, opposite reaction
costume: rocket pushes gas DOWN → gas pushes rocket UP
BALANCED vs UNBALANCED
tug-of-war, 500 N each way → net 0 → nobody moves
600 N vs 500 N → net 100 N → motion toward the stronger side
Worked all the way through
The problem
A 20 kg cart must accelerate at 3 m/s². How much force is needed — and why does the same force barely move a loaded 60 kg cart?
- Step 1 — Pick the law with the right variables.Mass, acceleration, force → the second law: F = ma.
- Step 2 — Multiply.F = 20 × 3 = 60 N.
- Step 3 — Flip the formula for the loaded cart.a = F ÷ m = 60 ÷ 60 = 1 m/s².
- Step 4 — Say the relationship in words.Triple the mass, same force → a third of the acceleration. Mass and acceleration trade off.
On the board
F = m × a
second law
F = 20 × 3 = 60 N
loaded cart: a = 60 ÷ 60
same F, bigger m
a = 1 m/s²
one third the acceleration
Answer
60 N — and on triple the mass that same force gives only 1 m/s²
A second one, in a different form
A book sits motionless on a table. Gravity pulls it down. Why doesn't it fall?
gravity pulls down on the book
the table pushes UP equally
support force
net force = 0
balanced
no net force → no change in motion
first law
Answer
The table's upward push balances gravity — net force zero, so the book stays put
The usual mistake
Adding opposing forces instead of subtracting them.
Check yourself
Name the law before answering: got mass and acceleration numbers? Second law. Something lurching or refusing to budge? First. Two objects pushing on each other? Third. The costume changes; the three laws never do.
Practice it
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