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1.6.2Motion, Forces and Energy

Impulse — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

5 min read 9 parts Lesson 56 of 91 1 quiz

📚 Cambridge IGCSE Physics 0625 Hub / Topical Revision
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Lesson summary

1.6.2 Impulse — IGCSE PHYSICS notes: Define impulse, relate it to change in momentum and force, and explain how a longer collision time reduces the force.A resultant force changes the momentum of an object.From F = ma and a = (v − u)/t, F = (mv − mu)/t = Δp/t.The resultant force …

  • Impulse equals the change in momentum
  • What the symbols mean
  • For a fixed change in momentum, F
  • Triangular graph
  • Average force
  • Newton second (N s)

Part 1 of 7Force and change in momentum

A resultant force changes the momentum of an object.
From F
From F ma and a = (v − u)/t, F = (mv − mu)/t = Δp/t.
The resultant force equals the rate of change of momentum.
This is the more general form of Newton's second law and works even when mass changes.

Part 2 of 7Impulse

1Impulse
Impulse the product of force and the time for which it acts (impulse = FΔt).
2Impulse equals the change in momentum
  • FΔt = Δp = mv − mu.
    What the symbols mean: F = force (N) · Δt = time the force acts (s) · Δp = change in momentum (kg m s−1); FΔt is the impulse (N s)
3Unit
  • Unit the newton second (N s), which is the same as kg m/s.
4
Impulse is a vector in the direction of the force.
5
Different forces and times can give the same impulse if the product F × t is the same.
6
Do not use N/s as a unit of impulse.

Part 3 of 7Impulse in everyday situations

Kicking, catching and colliding objects all involve impulse.
Rain and hail on an umbrella:
  • Raindrops splatter, so their velocity falls to near zero.
  • Hailstones bounce back, so their velocity reverses and the change in momentum is greater.
  • The impulse on hail is greater, so a larger force is needed to hold the umbrella.
Rocket
  • Rocket the same thrust applied for longer gives a larger gain in momentum.

Part 4 of 7Why collision time matters

Why collision time matters
For a fixed change in momentum, F
Δp/t shows that a longer time gives a smaller force.
Crumple zones collapse gradually and lengthen the stopping time.
Seat belts stretch slightly and air bags inflate to spread the stopping over a longer time.
Fielders move their hands back when catching a hard ball.
Bending the knees on landing or using a crash mat increases the stopping time.
Helmets contain foam that compresses.
Hammering a nail needs a short stopping time so that the force on the nail is large.

Part 5 of 7Force–time graphs

When the force changes during a collision, the impulse is the area under the force–time graph.
Triangular graph
  • Triangular graph the area is ½ × Fmax × t.
A tall narrow peak and a low wide peak can have the same area, so the same impulse and change in momentum.
Average force
Average force impulse ÷ contact time.
A bounce changes momentum from +mv to −mv, a change of 2mv.

Part 6 of 7Method

1
Choose a positive direction.
2
Find the initial momentum mu and the final momentum mv with signs.
3
Calculate Δp = mv − mu, which is the impulse.
4
Find the average force using F = Δp/t if the time is known.
Method
Change direction signs when an object reverses, as with a ball returning from a racket.
Convert times in milliseconds to seconds.

Part 7 of 7More from the specification

More from the specification
Impulse and change in particle speed
  • Impulse and change in particle speed applying a resultant force over time provides a means to alter vehicle momentum and speed.
Impulse
  • Impulse force multiplied by time taken, equal to the change in momentum, measured in newton seconds
Momentum
  • Momentum mass times velocity, a vector, so reversing direction gives a negative value, for example a tennis ball hit back
Conservation of momentum
  • Conservation of momentum total momentum of a closed system is constant before and after collision
Force from impulse
  • Force from impulse force equals change in momentum divided by time, so a longer collision time reduces force, as with crumple zones
Closed system
  • System with no net external force or exchange.

Key terms

TermMeaning
ImpulseForce × time for which it acts; equals the change in momentum.
Newton second (N s)Unit of impulse, equal to kg m/s.
Change in momentumFinal momentum minus initial momentum, Δp = mv − mu.
Crumple zoneCar structure designed to collapse and lengthen collision time.
Rate of change of momentumMomentum change divided by time taken; equals the resultant force.
Average forceImpulse divided by contact time.

Avoid these mistakes

1
Treating a rebound as a change from mv to zero — the sign flips, so the change is bigger.
2
Saying impulse is force only — it is force multiplied by time.
3
Saying a safety device reduces the change in momentum — it reduces the force by increasing the time.
4
Forgetting to convert milliseconds to seconds.
5
Treating N s and kg m/s as different quantities — they are the same.

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