06 August

Using Socrative for Pop Quizzes

Seng Kwang Tan 08 Gravitational Fields, Teaching Resources 0 Comments

Since almost every student owns a smartphone in SIngapore now, it has become possible for us to gather instant feedback or conduct an on-the-spot formative assessment to identify common misconceptions during class time.

A simple true-and-false quiz conducted during my recent lesson on Gravitation, after the lecture series was completed, showed that students are confused about the different treatment between vectors (force and field strength) and scalars (potential and potential energy).

The questions asked, along with the percentage answered correctly, were:

  1. If you were sitting half as far from your nearest classmate, the gravitational force of attraction between the two of you would be four times as strong. (81.0% correct)
  2. At the midpoint of the distance from the center of the earth to the center of the moon, the gravitational field strength is zero. (90.5% correct)
  3. At the midpoint of the distance between two identical masses, the gravitational potential is zero. (23.8% correct)
  4. Work done by gravitational force in bringing an object from infinity to the surface of a planet is negative. (38.1% correct)
  5. Satellites in a geostationary orbit must move directly above the equator. (52.4% correct)

This quiz was effective in helping students realise that potential cannot cancel out.

I also took the opportunity to explain to the class why the definition of gravitational potential (as well as potential energy) was based on the work done by an external force, rather than work done by gravitational force.

The tool that I used was Socrative and if anyone would like to use my quiz, you can follow the steps below:

  1. Create a teacher account (not a student account) at socrative.com and log in.
    socrative1

  2. Click on Manage Quizzes.
    socrative2

  3. Select “Import Quiz”
    socrative3

  4. Key in the SOC number 12289667

Since you have imported the quiz, you have effectively duplicated the quiz for yourself and are at liberty to edit the questions or even modify the quiz altogether as it does not affect mine.

To conduct the quiz, all you need to do is to go back to the dashboard and click on “Start a Quiz”, selecting the quiz you have saved. Follow the rest of the instructions to select the type of quiz you want (whether at students’ own pace or at a pace determined by yourself).

Once you have started the quiz, get your students to log in at m.socrative.com on their smartphone browsers and key in your Room Number (shown at the top of the screen). You will be able to see the number of students logging in at the top left corner and the attempts of your students in the main area.

socrative physics

 

If you have any questions regarding the use of Socrative in teaching, feel free to leave a comment below and I’ll try to answer it. Have fun!

05 August

Breaking Glass with Sound

Seng Kwang Tan 09 Oscillations, Blog 0 Comments

This video shows yet another demonstration for the effect of resonance, when the sound frequency is equal to the natural frequency of the glass. I have yet to conduct this demonstration because of the potential risks involving flying pieces of glass. A video should suffice though, especially one that is taken at slow motion.

05 August

Tacoma Narrows Bridge

Seng Kwang Tan 09 Oscillations 0 Comments

This is a video that we usually will show during a lecture on the topic of Resonance, under the unit “Oscillations”.  It was taken in 1940 at the Tacoma Narrows Bridge in Washington, USA. One of the main reasons (not the only reason – the other being aeroelastic flutter) for its collapse is the effect of resonance, which occurs when the driving frequency of the wind that hits the bridge matches the natural frequency of vibration of the bridge.

29 May

Can a Liquid Freeze and Boil Simultaneously?

Seng Kwang Tan 13 Thermodynamic Systems 0 Comments

Witness the freezing and boiling of a liquid take place at the same time. A thorough explanation for this observation is found here.

Cyclohexane can both freeze and boil simultaneously under specific conditions when it is in a state known as triple point.

The triple point of a substance is the unique combination of temperature and pressure where all three phases (solid, liquid, and gas) coexist in equilibrium. For cyclohexane, at its triple point:

  • Temperature: Around 6.5°C
  • Pressure: Around 0.053 atm (40 mm Hg)

At this point, cyclohexane can freeze (turn from liquid to solid) and boil (turn from liquid to gas) simultaneously. This occurs because the conditions allow the substance to transition between all three phases at the same time. The energy added or removed from the system can cause some molecules to leave the liquid phase as a gas (boiling), while others enter the solid phase (freezing).

27 May

Which Equations Apply Only to Ideal Gases?

Seng Kwang Tan 12 Temperature and Ideal Gases 0 Comments

Students are sometimes unclear about which of the equations taught in the topic of Thermal Physics apply to ideal gases and which apply to all systems (whether ideal or real gas, even liquids and solid). The following table should help to clarify:

 

Applies to Ideal Gas only Applies to all systems
Gas Laws $$pV=nRT$$ $$pV\approx nRT$$ only for gases at low pressure and high temperatures
Average Kinetic Energy $$=\dfrac{3}{2}kT$$ $$\propto T$$
Internal Energy $$U$$ = sum of KE of molecules
$$U=\dfrac{3}{2}NkT=\dfrac{3}{2}nRT=\dfrac{3}{2}pV$$		 $$U$$ = sum of KE and PE of molecules
First Law of Thermodynamics applies to all systems $$\Delta U=Q+W$$
24 May

3 Common Dynamics Mistakes

Seng Kwang Tan 03 Motion and Forces, IP3 03 Dynamics 0 Comments

After going through the topic of Dynamics in the A-level syllabus recently, it strikes me that there are several common mistakes or misconceptions that students whom I have taught over the years have always struggled with. I shall lay them out here in the hope that students who read this do not repeat the same mistakes.

1. Mixing up free-body diagrams involving multiple bodies in motion

Students are often confused about the forces in drawing free-body diagrams, especially so when they have to consider the different parts of multiple bodies in motion. A common mistake that students make is to draw all the forces, including the internal forces within a system, on a single diagram.

wrong freebody diagram

To solve a two-body dynamics problems, students need to learn how to draw the 3 possible free-body diagrams. A detailed set of instructions is found in an earlier post.

2. Adding instead of subtracting momenta to find impulse

Students who do not have a strong foundation in Math may not appreciate the fact that the addition and subtracting of vectors is not done by simply adding or subtracting the magnitudes. Sometimes, students are asking to find the force acting on an object when given the initial and final momenta.  The correct way to find the vector of the change in momentum is to place the tails of the initial and final momenta together, and to join the arrowhead of the initial momentum with that of the final momentum with a third vector in that direction.

Vector triangle to find change in momentum
Vector triangle to find change in momentum

They are often confused between finding the change in momentum and the act of adding two vectors together. When that happens, what teachers see is the following triangle instead.

Wrong vector diagram to find change in momentum
Wrong vector diagram to find change in momentum

While the magnitude of the vector found in both diagrams are the same, students will get the wrong answer if asked for the direction of the change in momentum. This is actually an important notion because the direction of change in momentum is really the direction of the net force applied on the body.

3. Using wrong signs for momentum for head-on collisions

In A-level physics, in the study of collision problems, we will deal only with head-on collisions where two objects collide along the same line. Even then, students can get confused with the sign convention associated with the direction of motion of the objects.

Applying Conservation of Momentum

For a two body head-on collision problem, always start by writing down the following equation for the conservation of momentum:

[latex]m_Au_A +m_Bu_B = m_Av_A + m_Bv_B[/latex]

where [latex]m_X[/latex] is the mass of X, [latex]u_X[/latex] is the initial velocity of X,  [latex]v_X[/latex] is the final velocity of X, where X is either A or B.

before collision

Follow this up by deciding on which direction you want to define as positive. For example, if you decide that the rightward direction is positive, the value of initial velocity for mass A is positive and that for mass B is negative.

What students sometimes do is to include the negative signs into the equation before substitution, e.g. [latex]m_Au_A +m_B(-u_B) = m_Av_A + m_Bv_B[/latex]. This causes confusion as they do not know where the directions of the final velocities are and hence, do not know which vectors to assign as negative.

What we should do instead is to determine the final direction of the velocities after we have found the values. Before we are able to do so, we shall assume that the final velocities are to the right. If the final value of velocity of a mass is negative, it means that its final velocity is to the left.

Substitute the values into equation.

Assuming that the initial speeds are both 4.0 m s-1;, the mass of A is 3.0 kg, mass of B is 5.0 kg, and that the collision is perfectly inelastic, we have:

perfectly inelastic collision

[latex]3.0 \times 4.0+5.0 \times(-4.0)=(3.0+5.0) \times v[/latex]
[latex]-8.0=8.0v[/latex]
[latex]v=-1.0 \text{ m s}^{-1}[/latex]

This way, since the final velocity of the objects has a negative value, we can tell that the direction of the final velocity is to the left.