Impulse: Wikis

  
  

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In classical mechanics, an impulse is defined as the integral of a force with respect to time. When a force is applied to a rigid body it changes the momentum of that body. A small force applied for a long time can produce the same momentum change as a large force applied briefly, because it is the product of the force and the time for which it is applied that is important. The impulse is equal to the change of momentum.

Contents

Mathematical derivation

Impulse I produced from time t1 to t2 is defined to be[1]

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

where F is the force applied dt denotes an infinitesimal amount of time.

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

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

Therefore

\begin{align} \mathbf{I} &= \int_{t_1}^{t_2} \frac{d\mathbf{p}}{dt}\, dt \ &= \int_{t_1}^{t_2} d\mathbf{p} \ &= \Delta \mathbf{p}, \end{align}

where Δp is the change in momentum from time t1 to t2. This is often called the impulse-momentum theorem.[2]

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

\mathbf{I} = \mathbf{F}\,\Delta t = m \,\Delta \mathbf{v} = \Delta\mathbf{p}

where

F is the constant total net force applied,
Δt is the time interval over which the force is applied,
m is the constant mass of the object,
Δv is the change in velocity produced by the force in the considered time interval, and
m Δv = Δ(mv) is the change in linear momentum.

It is often the case that not just one but both of these two quantities vary.

In the technical sense, impulse is a physical quantity, not an event or force. The term "impulse" is also used to refer to a fast-acting force. 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. This is a useful model for computing the effects of ideal collisions (such as in game physics engines).

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

Impulse can be calculated using the equation

\mathbf{F}\,\Delta t = \Delta p = mv_1 - mv_0 \,

where

F is the constant total net force applied,
t is the time interval over which the force is applied,
m is the constant mass of the object,
v1 is the final velocity of the object at the end of the time interval, and
v0 is the initial velocity of the object when the time interval begins.

See also

Notes

  1. ^ Hibbeler, Russell C. (2010), Engineering Mechanics, 12th edition, Pearson Prentice Hall, p. 222, ISBN 0-13-607791-9 
  2. ^ See, for example, section 9.2, page 257, of Serway (2004).

Bibliography

  • Serway, Raymond A.; Jewett, John W. (2004). Physics for Scientists and Engineers (6th ed. ed.). Brooks/Cole. ISBN 0-534-40842-7. 
  • Tipler, Paul (2004). Physics for Scientists and Engineers: Mechanics, Oscillations and Waves, Thermodynamics (5th ed. ed.). W. H. Freeman. ISBN 0-7167-0809-4. 

External links


In classical mechanics, an impulse is defined as the integral of a force with respect to time. When a force is applied to a rigid body it changes the momentum of that body. A small force applied for a long time can produce the same momentum change as a large force applied briefly, because it is the product of the force and the time for which it is applied that is important.

Contents

Mathematical derivation

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

where

I is impulse (frequently marked J),
F is the force, and
dt is an infinitesimal amount of time.
t1 and t2 denote a time interval

A simple derivation using Newton's second law yields:

\mathbf{I} = \int_{t_1}^{t_2} \frac{d\mathbf{p}}{dt}\, dt
\mathbf{I} = \int_{t_1}^{t_2} d\mathbf{p}
\mathbf{I} = \Delta \mathbf{p}

where

p is momentum

This is often called the impulse-momentum theorem.[1]

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

\mathbf{I} = \mathbf{F}\Delta t = m \Delta \mathbf{v} = \Delta\mathbf{p}

where

F is the constant total net force applied,
\Delta t is the time interval over which the force is applied,
m is the constant mass of the object,
Δv is the change in velocity produced by the force in the considered time interval, and
v = Δ(mv) is the change in linear momentum.

It is often the case that not just one but both of these two quantities vary.

In the technical sense, impulse is a physical quantity, not an event or force. The term "impulse" is also used to refer to a fast-acting force. 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. This is a useful model for certain purposes, such as computing the effects of ideal collisions, especially in game physics engines.

Impulse has the same units and dimensions as momentum (kg m/s = N·s).

Impulse can be calculated using the equation:

\mathbf{F}\Delta t = \Delta\ p

\Delta\ p can be calculated, if initial and final velocities are known,

\mathbf{F}\Delta t = mv_1 - mv_0

where

F is the constant total net force applied,
t is the time interval over which the force is applied,
m is the constant mass of the object,
v_1 is the final velocity of the object at the end of the time interval, and
v_0 is the initial velocity of the object when the time interval begins.

See also

Notes

  1. ^ See, for example, section 9.2, page 257, of Serway (2004).

Bibliography

  • Serway, Raymond A.; Jewett, John W. (2004). Physics for Scientists and Engineers (6th ed. ed.). Brooks/Cole. ISBN 0-534-40842-7. 
  • Tipler, Paul (2004). Physics for Scientists and Engineers: Mechanics, Oscillations and Waves, Thermodynamics (5th ed. ed.). W. H. Freeman. ISBN 0-7167-0809-4. 

External links and references


Wiktionary

Up to date as of January 15, 2010

Definition from Wiktionary, a free dictionary

See also impulse

German

Noun

Impulse

  1. Plural form of Impuls.

Simple English

Simple English Wiktionary has the word meaning for:

In classical mechanics, an impulse is defined as the integral of a force with respect to time:

\mathbf{I} = \int \mathbf{F}\, dt

where

I is impulse (sometimes marked J),
F is the force, and
dt is an infinite amount of time









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