# Find the amount of stored energy in a mouse trap

1. May 24, 2004

### ValiantKhan

Hello, I'm trying to find the amount of stored energy in a mouse trap when it is set. I've read up on Potential energy.. but all i've found is gravitational, electrical, or elastic potential energy.. and that E = MGH .. but i don't have any height..

but if anyone can point me in the right direction , i'd be much appreciative.

Thanks!

--Val

2. May 24, 2004

Have you got the actual trap to measure it? You could use:

Work done =Force x distance moved in direction of force

and then measure the force with a Newtonmeter and see how far you have to pull it back to set it.
Not very accurate, but close enough...

3. May 24, 2004

### HallsofIvy

Staff Emeritus
With a mouse trap (I assume you are talking about one of the standard mouse traps you can buy in a store), it's not "mgh" but "kx" that is important. The mouse trap arm, when released, is moved by a spring. If k is the spring constant (the amount of force necessary to contract the spring a unit distance) then the work done in moving the spring through a distance x is kx and that is the energy involved in releasing the spring.

Since you probably are not given "k", you will need to measure it as Adrian Baker said.

4. May 24, 2004

### ZapperZ

Staff Emeritus
I'm sure HallsofIvy was in a hurry when that was typed, but what he actually meant was that the PE of a spring with spring constant k is

$$\frac{1}{2} kx^2$$

:)

Zz.

Last edited: May 24, 2004
5. May 24, 2004

### arildno

Wouldn't it be more "natural" to express the potential energy in terms of the rotated angle from equilibrium?

6. May 24, 2004

### ValiantKhan

Okay.. So I have to use the newtonmeter to find out how much force the spring is pushing down on the bar.. that gives me the K right.. and then the X distance would be the distance the end of the bar goes ? would it be the distance in a semicircle.. or the distance from the spring loaded position directly to the unloaded position? if that makes any sense.. then i can go and use the PE = 1/2KX^2 correct??

i thank you all for your help. much appreciated.

--Val

7. May 25, 2004

### pmb_phy

A mouse trap is basically a lever arm which has a tourque on it caused by the spring. The spring coil has a torsional constant. Multiply the tortional constant by the angle that the arm is moved through from equilibrium and you get the potential energy of the system. This treats the mouse trap as if the board is nailed to the floor (i.e. remains fixed).

Pete

8. May 25, 2004

### Staff: Mentor

torsional spring constant

Right. If the rotational (torsional) spring constant is k (units of torque per angle), then the potential energy of the spring (at an angle θ from equilibrium) would be:
$$\frac{1}{2} k\theta^2$$
However, the spring does not start off in equilibrium (the closed position of the mousetrap is not θ = 0). So you'd have to measure k, and figure out θ1 and θ2.

Interesting!