Forces · Topic 5 of 17

Newton's Laws of Motion

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Physics · forces

First law: no net force, no change in motion. Second law: F_net = ma. Third law: every push gets an equal, opposite push back.

01 · 1st lawNo net force → no change in motion

02 · 2nd lawF_net = m × a

03 · 3rd lawEqual and opposite force pairs

Net force = all forces added with direction; balanced → 0 must know

Inertia = resistance to a change in motion, measured by mass

Normal force = a surface's support push, perpendicular to it

Free-body diagram: one object, an arrow for each outside force

Push back on the wall, the wall pushes you forward (3rd law)

Second law: F_net = m × a (a points with the net force) must know

1 newton = 1 kg·m/s²

First law: no net force → no change in motion must know

Third law: A pushes B → B pushes A, equal size, opposite way must know

2× force → 2× acceleration; 2× mass → ½ acceleration

Level surface, nothing else vertical: normal force = weight = mg

30 N on 12 kg: a = 30 ÷ 12 = 2.5 m/s²

50 N right, 20 N left on 6 kg: net 30 N → a = 5 m/s² right

80 kg stretcher at 0.5 m/s²: F = 80 × 0.5 = 40 N

Commonly confused

	Balanced	Unbalanced
Net force	Zero	Not zero
Motion	At rest or constant velocity	Accelerates in the net-force direction
Law that applies	First law (inertia)	Second law (F_net = ma)

Action-reaction forces act on different objects — they never cancel

Net force 0 → at rest OR constant velocity, not necessarily stopped

Show 3 more facts

Normal force ≠ weight on a slope (N = mg cos θ) or when pushed down

N (normal force) is a quantity; N (newton) is a unit

Only external forces accelerate a system — internal pairs cancel

Pearl

Motion needs no force to keep going; only a change in motion needs a net force.

Practice this topic, then test the whole section.

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