Patrick Teaches Physics

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, ass

You grind your way through a problem, get an answer, and want to know if it is correct.  How can you tell? In the previous post I talked about changing the angle in a problem (the inclined plane) to see how that affected other angles.  It can help you see how the angles relate to each other.  In this post I will use the same technique, but this time to test if my answer could be wrong. Let's return to the inclined plane.  Suppose a block of mass m = 2.5 kg slides down a frictionless plane at a slope of θ = 35 degrees to the horizontal.  You need to determine the block's acceleration.    You calculate an acceleration of a = 8.0 m/s/s down the slope.  Your friend, attempting the same problem, gets an answer of a = 5.6 m/s/s.  Who is right? After both checking that your calculators are set to degrees (not radians), you look at each other's work.  You have a = g cos θ while your friend has a = g sin θ where g is the usual acceleration due to gravity.  At least one of

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