Lesson 7 – Momentum & Collisions

Stage 1: Intermediate Complete Notes
Page 2 – Impulse & Impulse–Momentum Theorem

In many real situations, forces act for a very short time but produce a large change in momentum. Such situations are explained using the concept of Impulse.

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1️⃣ What is Impulse?

Impulse is a physical quantity that measures the effect of a force acting over a short time interval.

Impulse (I) = Force × Time

I = F Δt

Impulse is a vector quantity and its direction is the same as the direction of the applied force.

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2️⃣ Units and Dimensions of Impulse

Since impulse = force × time:

• SI unit of force = Newton (N)
• SI unit of time = second (s)

SI Unit of Impulse: N s
Dimensions: [M L T⁻¹]

👉 Dimensions of impulse are the same as momentum.

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3️⃣ Impulse–Momentum Theorem

According to Newton’s Second Law:

F = dp/dt

Multiplying both sides by time interval Δt:

F Δt = Δp

This relation is called the Impulse–Momentum Theorem.

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4️⃣ Statement of Impulse–Momentum Theorem

The impulse of a force acting on a body is equal to the change in momentum of the body.

This theorem is valid for:

• Constant force
• Variable force
• Very short duration forces

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5️⃣ Practical Examples of Impulse

• A cricket player pulls his hands backward while catching a ball to increase time of contact and reduce force.
• Airbags increase collision time and reduce injury.
• A karate player breaks a brick by applying a large force in a very short time.

👉 For the same change in momentum: increasing time reduces force.

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6️⃣ Impulse vs Force (Exam Comparison)

✔ Force depends on instantaneous action
✔ Impulse depends on total effect over time
✔ Large force for small time can produce same impulse

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7️⃣ Important Exam Points

✔ Impulse and momentum have same dimensions
✔ Zero force does not mean zero impulse (time matters)
✔ Impulse can change direction of motion

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📌 Page 2 Summary

✔ Impulse = F × Δt
✔ Impulse equals change in momentum
✔ Used in collisions and safety design
✔ Basis for conservation of momentum

👉 In the next page, we will study Conservation of Linear Momentum.