# Q & A: Measuring forces and speeds

Q:
How can you measure the force on an object? Is there any way to measure the speed of an object without using a radar gun?
- Anonymous
A:
There are lots of ways to measure the force on an object and the speed of an object. Which way you might use depends on the details of what you're doing. I'll give some examples of how to measure force and speed -- perhaps you can think of some more!

Measuring force:

1) A scale will directly read the force applied to it. A bathroom scale usually has a spring inside attached to a dial which reads how much the spring has either compressed or stretched, depending on how the scale is constructed. Scales read forces in units of pounds in the United States.

2) You can hook up a spring directly to the object you would like to measure the force on, and pull on the other end of the spring. The amount by which the spring stretches is directly proportional to the force applied. The constant of proportionality depends on the spring and should be calibrated with a known force, like a known weight.

3) You can use Newtons law F=mA. If you know the mass of an object and can measure its acceleration, the force is the product of these two. But be careful -- this force is the sum of all forces on the object. Be careful with friction! Measuring the acceleration just means measuring the speed at different times and seeing how it changes. Acceleration is the change in speed divided by the change in time.

Measuring speed without a radar gun:

1) The speed of a car is most easily determined with the cars own speedometer. Modern digital ones count the revolutions of some shaft which is mechanically coupled to the cars wheels and divide the revolution rate by the time interval used to count.

2) You can determine the speed of a person on a racetrack of known length by measuring the time he takes to run the track's length with a stopwatch. Dividing distance by time gives his average speed on the course.

3) You can make a movie of the object in motion. It works best if you have something next to the objects path like a ruler which is marked off in distance units. (if the object is farther away, you can put a smaller ruler closer to the camera and then use the laws of similar triangles to compute the distance traveled by the object ). If you know the time difference between frames of the movie, you can use the distance traveled between frames to determine the objects speed between each frame of the movie.

4) Police sometimes use lasers instead of radar guns to determine the speed of cars along the road. These lasers are easier to point directly at a single car and are less often fooled by other cars. They also cannot be detected by radar detectors. If the light is infrared, they cannot be seen either. The idea is the same as the radar gun -- the laser light is doppler shifted when it bounces off of the moving vehicle, and comparing the outgoing to the incoming light frequencies gives the speed of the car.

5) Doppler radar uses the technique for measuring moving water droplets in storms for accurate weather reports showing where it is raining.

Can you think of more ways to measure speed and force?

Tom

(published on 10/22/2007)