# Why impulse?

1. Mar 3, 2012

### mishima

Is impulse only really used in real life because of the instruments you might have? I'm having trouble understanding why this idea is a useful physical construct. The best I found is that just like work can be thought of as a "transfer mechanism" of energy, impulse is the "transfer mechanism" of momentum (I don't recall where that quoted term is from).

I was really just wondering why in an experiment someone would choose to talk about impulse rather than momentum. I have a hunch its because of what you can measure with what you have.

2. Mar 4, 2012

### zhermes

Impulse is the term for a change in momentum, just like work is the term for a change in energy. Impulse is not just momentum (just like work isn't just energy).

Impulse is also equal to the applied force integrated over time---i.e. the force times time (if its constant). The concept of impulse is useful in that it eliminates the need for an explicit force by itself.

Consider bouncing a tennis ball off of the ground. To calculate the force acting on the ground during the bound would be very difficult, and involve modeling the tennis ball as some sort of complex spring, and taking into account its structural properties.... etc etc. The force can be approximated, however, by calculating the impulse (which is just the change in momentum---easy), and dividing by the total time of the encounter.

Another example is in propellants (e.g. rocket fuel). Often a situation will be such that the impulse delivered is roughly constant (e.g. for a certain amount of rocket fuel)---if the fuel is used over a short amount of time, it produces a large force (but the same impulse); or if its used over a long time it produces a small force (but the same impulse). Again the impulse is very useful for making calculations.

3. Mar 5, 2012

### Philip Wood

Zhermes has it, in my opinion, spot-on.

A semantic question remains, perhaps. Why do we talk about 'impulse' rather than 'change in momentum' if they're equal to each other? For the same reason, perhaps, that we talk about 'force' rather than 'rate of change of momentum'. 'Impulse' and 'force' are both terms we apply with an agent in mind, originally, perhaps, a human being. We distinguish what we do (initiated by an act of will?) from what happens as a result.

4. Mar 5, 2012

### mishima

Ok, thanks for the responses. The idea of agency attached to these concepts is interesting. I've also heard it often that things like force and impulse are considered "causes" of the effects of rate of momentum change and momentum change.

5. Mar 6, 2012

### Philip Wood

Exactly. Consider, though, the stance of Ernst Mach: "There is no cause nor effect in nature. Nature simply is.