IB Diploma Physics SL · Theme A.2 Forces and momentum
Mini-Lesson
Forces & Momentum
This mini-lesson covers Theme A.2 — Forces and momentum: Newton's three laws, drawing free-body diagrams, using F = ma, and the powerful ideas of impulse and conservation of momentum.
Work through each screen, answer the questions as you go (some are reasoning, some are calculations) and collect ⭐ stars. Watch for the HL flag on higher-level extensions. Press Start when you're ready.
A.2 · Newton I & II
Newton's first two laws
Forces are pushes and pulls (vectors, in newtons). The first two laws:
First law: an object stays at rest or moves at constant velocity unless a resultant (net) force acts. Constant velocity ⇒ zero resultant force (equilibrium).
Second law: the resultant force equals rate of change of momentum; for constant mass this is F = ma.
F = maresultant force (N) = mass (kg) × acceleration (m s⁻²)
Weight is the gravitational force on a mass: W = mg. A 6 kg bag weighs 6 × 9.81 = 58.9 N. Mass (kg) is not weight (N).
Quick check
Quick check
?A car drives along a straight, level road at a constant 25 m s⁻¹. What is the resultant force on it?
Calculate
Calculate
#A 1200 kg car accelerates at 2.5 m s⁻². Find the resultant force driving it.
N
Hint: F = ma = 1200 × 2.5.
Calculate
Calculate
#Find the weight of a 6.0 kg mass on Earth (g = 9.81 m s⁻²).
N
Hint: W = mg = 6.0 × 9.81.
A.2 · Newton III
Newton's third law
Third law: if body A exerts a force on body B, then B exerts an equal and opposite force on A. The pair acts on two different bodies and is the same type of force.
Classic trap: the weight of a book and the table's normal force on it are not a third-law pair — they act on the same body. The third-law partner of the book's weight is the book's gravitational pull on the Earth.
Quick check
Quick check
?A rifle fires a bullet forwards and recoils backwards. Which statement uses Newton's third law correctly?
A.2 · momentum & impulse
Momentum and impulse
Linear momentum p = mv is a vector (kg m s⁻¹). Newton's second law is really: resultant force = rate of change of momentum.
p = mv · impulse = FΔt = Δpimpulse (N s) equals the change in momentum
On a force–time graph, the area under the curve is the impulse — handy when a force varies during a short collision.
Worked example — a struck ball
A 0.058 kg tennis ball is served from rest to 50 m s⁻¹ during a 5.0 ms contact.
Δp = mΔv = 0.058 × 50 = 2.9 kg m s⁻¹
F = Δp ÷ Δt = 2.9 ÷ 0.005 = 580 N
Calculate
Calculate
#A 0.15 kg cricket ball travels at 40 m s⁻¹. Find its momentum.
kg m s⁻¹
Hint: p = mv = 0.15 × 40.
Calculate
Calculate
#A 0.058 kg ball is accelerated from rest to 50 m s⁻¹ in a contact time of 5.0 ms (0.005 s). Find the average force.
N
Hint: F = Δp ÷ Δt = (0.058 × 50) ÷ 0.005.
Sort it
Which of Newton's laws?
Tap an item, then tap the group it belongs to.
① First law
② Second law
③ Third law
A.2 · conservation
Conservation of momentum
When no external resultant force acts, total momentum is conserved: total p before = total p after. This governs collisions and explosions.
Elastic collision: kinetic energy is also conserved.
Inelastic collision: momentum is conserved but some KE becomes heat/sound; if the bodies stick together it is perfectly inelastic.
Worked example — a sticky collision
A 2.0 kg trolley at 3.0 m s⁻¹ hits a stationary 1.0 kg trolley and they couple.
p before = 2.0×3.0 + 0 = 6.0 kg m s⁻¹
v after = 6.0 ÷ (2.0+1.0) = 2.0 m s⁻¹
Calculate
Calculate
#A 2.0 kg trolley moving at 3.0 m s⁻¹ collides with and sticks to a stationary 1.0 kg trolley. Find their common velocity afterwards.
m s⁻¹
Hint: momentum conserved — v = (2.0×3.0) ÷ (2.0+1.0).
Match it
Match term to meaning
Tap a statement on the left, then its match on the right.
Statement
Answer
Recap
The big ideas to know
Newton I: no resultant force ⇒ rest or constant velocity (equilibrium)
Newton II: resultant F = ma = rate of change of momentum
Newton III: equal, opposite forces on two different bodies
Momentum & impulse: p = mv; impulse = FΔt = Δp = area under a force–time graph
Conservation: isolated system: total momentum before = after; KE extra-conserved only if elastic
That completes Forces & Momentum for IB Diploma Physics SL. Press Finish to see your score.
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