Momentum & Impulse
Cambridge IGCSE Physics Extended Tier
What You'll Master
- Momentum — how "hard to stop" an object is (mass × velocity)
- Direction matters — momentum is a vector, so sign is crucial
- Conservation of momentum — total momentum before a collision = total after
- Impulse — the change in momentum caused by a force over time
- Force & momentum link — force is the rate of change of momentum
Momentum: The Measure of "Hard to Stop"
Imagine pushing a stationary tennis ball versus pushing a stationary bowling ball. Same force, but the bowling ball barely moves because it's more "resistant" to acceleration. Now flip it: a tennis ball whizzing at 50 m/s is much harder to catch than a bowling ball moving at 2 m/s. Why? Because momentum combines both mass and speed. Momentum measures how hard an object is to stop.
Understanding the Equation
p is momentum, measured in kg m/s. If you double the mass, momentum doubles. If you double the velocity, momentum doubles. Double both, and momentum increases four-fold.
📌 Key Insight: Momentum is a Vector
Velocity is a vector (has direction), so momentum has direction too. If an object moves right (+), its momentum is positive. If it moves left (−), its momentum is negative. This matters enormously in collisions.
An object at rest (v = 0) has zero momentum. No matter how massive it is, if it's not moving, it exerts no momentum.
Why Momentum Matters Physically
Momentum is conserved in collisions (we'll explore this next). It's also the reason a light, fast object can be as hard to stop as a heavy, slow one. A tennis ball at 75 m/s and a 3 kg brick at 1.5 m/s both have roughly the same momentum (~4.5 kg m/s), so they hit you with similar impact.
Calculating Momentum
Question: A tennis ball (60 g) travels at 75 m/s, and a 3 kg brick moves at 1.5 m/s. Which has more momentum?
Step 1: Convert grams to kilograms.
Tennis ball: 60 g = 0.06 kg
Step 2: Calculate ball momentum.
p = mv = 0.06 × 75 = 4.5 kg m/s
Step 3: Calculate brick momentum.
p = mv = 3 × 1.5 = 4.5 kg m/s
Step 4: Compare.
Both have the same momentum. Despite the mass difference, they deliver equal impact on collision.
Practice: Momentum Calculation
A car (1200 kg) travels at 20 m/s. What is its momentum?
💡 Exam Tip: Always Convert Units
Mass given in grams? Convert to kg first. Velocity in km/h? Convert to m/s. One unit mistake = wrong answer. Take 10 seconds to check.
Momentum in Collisions
When objects collide, they exchange momentum. The tennis ball bouncing off a wall is a perfect example.
The Vector Story: Before and After a Bounce
Say a tennis ball (60 g) approaches a wall at 75 m/s (leftward = positive). After bouncing, it rebounds at 75 m/s to the right (now negative).
- Before collision: p = 0.06 × (+75) = +4.5 kg m/s
- After collision: p = 0.06 × (−75) = −4.5 kg m/s
The momentum completely reversed sign because the direction flipped. This is why choosing your positive direction early is essential in exam questions.
⚠️ Common Mistake: Ignoring Direction
Conservation of Momentum