Explanations
Today I asked my class, of which Adriana from Romania was a member
, to explain why the temperature of an insulated cylinder of gas will increase when the volume is reduced. The rather fragmented explanations led me to spend some time talking about what we mean by an explanation. Actually I should say what I mean, I can't speak for everyone.
An explanation is a series of true statements that follow each other logically to give an understanding of a process that enable us to make predictions. Each statement should be a law, definition or obvious. Here's an example.
1. To reduce the volume the piston is moved. (obvious)
2. The gas exerts a pressure pushing the piston out so moving it in requires an unbalanced force (Newt 1)
3. The point of application of the force moves in the direction of the force so work is done. (def. work)
4. If work is done energy is transferred. (def. energy)
5. There are no forces between gas atoms (assumptions of kinetic theory)
6. The gas atoms have no PE (def PE)
7. KE of atoms increases (obvious)
8.The temperature increases (3/2kT)
I think such an explanation would be awarded 3 marks in an exam so some steps are maybe not so important, i'd chose 2 4 and 7 for my mark scheme I think.
Making explanations takes some practice and you have to understand the physics in each step for them to make sense. To help you on your way I thought I would make a list of some possible explanations. If I have time I might even write the explanations.
So, I will start my list:
- Explain why a ball dropped from a 20 storey building falls through the air with constant velocity as it passes the last two floors of the building.
- A box is placed on a wooden plank. One end of the plank is lifted making an inclined plane. Explain why the box does not move at first but when it starts to move it accelerates.
- Ball A collides with stationary ball B, after the collision the balls have the same velocity. If the balls are isolated explain how you know that the collision is inelastic.
- Explain why a helium balloon accelerates upwards.
- Explain why a car travelling at constant velocity on a horizontal road uses fuel.
I've just had an idea. I could write the steps in the explanation and jumble them up. Students then have to put them in order. Try these.
A box is placed on a wooden plank. One end of the plank is lifted making an inclined plane. Explain why the box does not move at first but when it starts to move it accelerates
As the end is lifted mgsinθ increases and the normal force decreases. The forces are unbalanced The box is in contact with the plank so there is a normal force up. The maximum friction force = μN The friction is now dynamic friction which is less than static friction. As the plank is lifted there is a component of weight down the slope mgsinθ. The plank is not frictionless so there is a friction force opposing mgsinθ. The block accelerates The box is in a gravitational field so there is a force down. At some point mgsinθ > μN so the block starts to move. Answer
Ball A collides with stationary ball B, after the collision the balls have the same velocity. If the balls are isolated explain how you know that the collision is inelastic.
KE before = 1/2mAu^2 After collision (mA + mB)v v = mAu/(mA + mB) The KE is not conserved If elastic then KE is conserved KE after = 1/2 (mA + mB) mA^2u^2/(mA + mB)^2 The collision is non elastic. Before the collision total momentum =mAu so if momentum is conserved mAu =(mA + mB)v
Explain why a helium balloon accelerates upwards in air.
According to Newton's 1st law the balloon will accelerate up. Helium is less dense than air. Archemedes states that the balloon will experience a buoyant force equal to the weight of air displaced. There is a resultant force up. A balloon displaces an amount of air equal to it's volume. The volume of air displaced is greater than the weight of the balloon. The buoyant force is greater than the weight.
Explain why a car uses fuel when travelling at constant velocity on a horizontal road.
Drag = forward force Velocity is constant so forces are balanced (Newt. 1) The car is travelling through air so experiences drag. If work is done energy is transferred. Work is done (definition). The energy comes from the fuel. The car is moving in the direction of the forward force.