Impulse (physics)

In classical mechanics, impulse (symbolized by J or Imp^[1]) is the product of a force, F, and the time, t, for which it acts. The impulse of a force acting for a given time interval is equal to the change in linear momentum produced over that interval.^[2] Impulse is a vector quantity since it is the result of integrating force, a vector quantity, over time. The SI unit of impulse is the newton second (N·s) or, in base units, the kilogram meter per second (kg·m/s).

A resultant force causes acceleration and a change in the velocity of the body for as long as it acts. A resultant force applied over a longer time therefore produces a bigger change in linear momentum than the same force applied briefly: the change in momentum is equal to the product of the average force and duration. Conversely, a small force applied for a long time produces the same change in momentum—the same impulse—as a larger force applied briefly.

J = F_{average} (t_2 - t_1)

The impulse is the integral of the resultant force (F) with respect to time:

J = \int F dt

Mathematical derivation in the case of an object of constant mass

The impulse delivered by the sad^[3] ball is mv₀, where v₀ is the speed upon impact. To the extent that it bounces back with speed v₀, the happy ball delivers an impulse of mΔv=2mv₀.

Impulse J produced from time t₁ to t₂ is defined to be^[4]

\mathbf{J} = \int_{t_1}^{t_2} \mathbf{F}\, dt

where F is the resultant force applied from t₁ to t₂.

From Newton's second law, force is related to momentum p by

\mathbf{F} = \frac{d\mathbf{p}}{dt}

Therefore

\begin{align} \mathbf{J} &= \int_{t_1}^{t_2} \frac{d\mathbf{p}}{dt}\, dt \\ &= \int_{p_1}^{p_2} d\mathbf{p} \\ &= \mathbf{p_2} - \mathbf{p_1} = \Delta \mathbf{p} \end{align}

where Δp is the change in linear momentum from time t₁ to t₂. This is often called the impulse-momentum theorem.^[5]

As a result, an impulse may also be regarded as the change in momentum of an object to which a resultant force is applied. The impulse may be expressed in a simpler form when the mass is constant:

\mathbf{J} = \int_{t_1}^{t_2} \mathbf{F}\, dt = \Delta\mathbf{p} = m \mathbf{v_2} - m \mathbf{v_1}

where

F is the resultant force applied,

t₁ and t₂ are times when the impulse begins and ends, respectively,

m is the mass of the object,

v₂ is the final velocity of the object at the end of the time interval, and

v₁ is the initial velocity of the object when the time interval begins.

The term "impulse" is also used to refer to a fast-acting force or impact. This type of impulse is often idealized so that the change in momentum produced by the force happens with no change in time. This sort of change is a step change, and is not physically possible. However, this is a useful model for computing the effects of ideal collisions (such as in game physics engines).

A large force applied for a very short duration, such as a golf shot, is often described as the club giving the ball an impulse.

Impulse has the same units (in the International System of Units, kg·m/s = N·s) and dimensions (M L T⁻¹) as momentum.

Variable mass

The application of Newton's second law for variable mass leads to the Tsiolkovsky rocket equation.

See also

• Specific impulse
• Wave–particle duality defines the impulse of a wave collision. The preservation of momentum in the collision is then called phase matching. Applications include:
  • Compton effect
  • nonlinear optics
  • Acousto-optic modulator
  • Electron phonon scattering

Notes

Bibliography

• Serway, Raymond A.; Jewett, John W. (2004). Physics for Scientists and Engineers (6th ed.). Brooks/Cole.  
• Tippler, Paul (2004). Physics for Scientists and Engineers: Mechanics, Oscillations and Waves, Thermodynamics (5th ed.). W. H. Freeman.