Introduction Have you ever driven up a mountain, seen a sign reading “Hazard! Icy Roads Ahead,” and wondered why ice makes roads dangerous to drive on? The answer has to do with friction—specifically, the lack of it.
Specifically, in the case of driving a car down the road, the friction that allows the car to move occurs between the tires and the road. This friction normally allows the car to “grip” the road—keeping the tires in contact with the relatively rough road surface, and the driver of the car in control. However, when the road is icy, the friction between the two surfaces decreases, allowing the car to slip—rather than grip. You can replicate these situations much more safely at home using some common objects—regardless of the weather.
Background Friction is the resistance to motion when two objects rub together. An example of low friction is when you sled down an icy hill. It’s easy because both the sled and the ice on the hill are smooth and the surfaces of the sled’s runners and the ice slide past one another with little resistance. On the other hand, sledding down a rough, concrete driveway is almost impossible due to the rough surface of the driveway, which provides resistance on the sled’s runners. This is an example of high friction.
When two objects rub together, there is always friction, and the surfaces of the two objects determine the amount that is generated. In this activity, you should have seen that there was less friction between the tub and the icy wood than from the dry plank. In other words, a smaller ramp angle was needed when using the icy wood than when using the dry wood for the tub to slip.
To calculate the force of friction, one must multiply the coefficient of friction by the force generated by the object, which was the mass of the tub in the pull of gravity. Although the mass of the tub and gravity did not change, the coefficient of friction of the two ramp systems did. The icy plank and the tub bottom had a lower coefficient of friction than the dry wood and the tub. You can easily feel this difference by carefully running your hand along the icy and then dry wood surfaces. The icy plank’s frozen coating makes it much smoother than the dry wood.
More to explore “Friction” from Kidipede, History and Science for Kids, Portland State University “Moti” from Motion Basics: Friction from Rader’s Physics4Kids.com “Finding an Angle in a Right Angled Triangle” from Math is Fun Advanced “Slip Sliding Away: Experimenting with Friction” from Science Buddies
This activity brought to you in partnership with Science Buddies
