We started this week’s class by talking about Newton’s law (again) and how objects in motion continue in motion, as I gave a block on the table a push and we watched it come to a stop. So why didn’t the block continue in motion? Because the force of friction is also acting on the block. We also talked about air resistance and how it increases the faster you move. Think about putting your hand out the car window as you drive slowly in a parking lot – there is very little air resistance on your hand, but if you do this while traveling at 70mph on the freeway you will feel a very large force from air resistance. Similarly, walking in knee deep water isn’t too bad, but running is almost impossible because of the resistance of the water. The students had heard of the term ‘terminal velocity’ but they weren’t sure what it meant so I explained how the force of air resistance increases as you fall through the air because your speed is increasing and at some point the force of gravity pulling you down and the force of air resistance pushing up will be equal. If the net force on an object is zero than the acceleration is zero and the object will be moving at constant speed, its terminal velocity. We went on to talk a bit more about sky diving, flying squirrels and wing suits!
The lab for this class was “Friction lab #1: How Much is Enough?” from Real Science Odyssey Physics. Basically the kids put books or blocks on a ramp (plank of wood) and then raised the ramp until the book started to slide. One student would hold the ramp while another student measured the height and base of the triangle created by the ramp. The lab uses a ratio of these measurements to determine the coefficient of friction for the set up. I’m not sure the students ‘got’ the coefficient of friction part, but they certainly understood that if the friction force was greater then they had to raise the plank higher before the object would slide. They used books with and without dust jackets and covered the plank with aluminum foil. We also used some friction blocks I had made by gluing sand paper to one side on a wooden block. We had to hold the ramp at almost 70 degrees before the block started to move!! The lab handout also asked them to devise a way to determine if the weight of the block/book would change the coefficient of friction, which is kind of tricky because the weight of the block does increase the FORCE of friction, but not the coefficient of friction which is a characteristic of the surface. We found the coefficient to be constant even when we doubled the weight (adding another book or block).
We watched “A World Without Friction” by MITK12Videos on Youtube in class:
Here are a few other vidoes including the friction of curling and a clip from Bill Nye.