Word games

Physics has a reputation for being a difficult subject.  I think that's partly because of the focus on problem solving -- which is an inherently high-level activity -- and partly because of the pride of place it gives to mathematics -- which makes it like studying two subjects at the same time.

But much of what a physics student is asked to do (at least in high school and first year uni) is not difficult.  Instead it's the relatively simple business of learning vocabulary, and showing you know what the word means.

Consider the following question:

A ball (mass 120g) travelling in a straight line at 18 m/s bounces off a wall, returning at the same speed. Calculate the change in momentum of the ball.

This question will neatly divide students into two groups: those who can define "momentum" and those who can't*.  For the first group (who know that the momentum of an object is the product of its mass and velocity) this will be easy marks (the answer is -4.3 kgm/s, assuming the initial velocity of the ball is in the positive direction).  The second group will be left scratching their heads.

The question was not difficult to formulate or visualise, required making no connections between different physical principles, did not require chaining together multiple steps to complete, nor even required particularly challenging (or even fiddly) mathematics.  It was just a matter of definition.

Physics is a relatively "low memory" subject (that's one of the reasons it appealed to me).  Learning a language requires memorising a vast vocabulary.  And other sciences -- particularly biology, but even chemistry -- can require their students to internalise many facts.  But even in physics there are some things that just have to be learned.

It can be useful to borrow a technique from the languages: flash cards.  Write the word on one side, and the definition on the other.  It could look something like this:

You can test yourself, and you and your friends can test each other. And never miss out on those easy marks again.

 

* Ok, there is one more word to know: "change".  In this context it is the difference between the final value and the initial value.

Comments

Post a Comment

Popular posts from this blog

Make it dynamic: Relating angles

Image
How often have I walked into a lecture theatre planning to talk about some deep secret of physics only to end up arguing with students about which angle is which in a problem?  To be fair, I also often struggled to follow geometrical arguments when I was a student.  One trick I learned was to try to see the problem as dynamic rather than static, by which I mean imagine the angles changing so I can see how they relate to each other. For example, consider the inclined plane.  In these problems, some object is placed on a surface that is at an angle to horizontal: something sliding down a ramp, for example. We have the usual free-body diagram, indicating the relevant forces (weight of the object, normal force, friction), and the angle of the slope is labelled θ (theta).  To determine the acceleration (if any) of the object we need the net force acting on it.  And one of the things that will require is the angle between the normal force (N) and the vertical.  It turns out to be the same as
Read more

Choose your own angle

Image
The relativity principle says that we get to choose the coordinate system we use when describing physical problems, and that includes deciding which way is up.  To show how useful that can be I'm going to use the inclined plane. We have an object sitting on a surface which is inclined at some angle: a box on a ramp or some such.  We want to know the object's acceleration.  Drawing the free-body diagram is straight-forward: there is weight, a normal force, and usually there is friction as well. We can write down the equations of motion and solve them, but it won't be pretty!  The acceleration has horizontal and vertical components, and because the friction depends on the normal force, which depends on the weight and the angle, the trig functions are going to build up.  Also, it's not so easy to tell what the normal force should be. And so on.... A much better approach is to rotate the coordinate system so that motion of the object is along the x-axis. We can immediately
Read more

Choose your own velocity

Image
When solving physics problems we always have the benefit of choosing our frame of reference.  Not only can we chose where the origin of our coordinate is, but we can also chose which part of our system is stationary.  Consider the following simple kinematic problem: A car is travelling along, and we want to know how long it takes to travel some distance.  We can plug numbers into a kinematic equation and get the answer.  If the car is accelerating then the problem is mathematically a bit more fiddly (a quadratic equation, or do it in two steps with two equations) but it's not conceptually harder. But now consider a similar problem with two moving bodies: Now there's two cars at different speeds, and the question is: how long will it take the red car to overtake the blue car?  This looks rather harder because the distance the red car has to travel is not immediately clear.  But that's only because of our choice of reference frame, which up till now has been implicit. We inst
Read more