Forces on a playground slide
To start moving, you need a force. Maybe you press a bit more on the ground or pull with your hand in the bar on the slide on the way up. Accordning to Newton's second law, a force is required to change motion. On the way up in the stairs, you may notice that the steps in the ladder bend slightly: As the step pushes on you, you push back on the step – by exactly the same amount. Newton's thir law.
When you sit down on the top of the slide and let go, you start to move down the slide. The force of gravity pulls you down, but since the slide surface is in the way, you move down the inclined plane. The friction between you and the slide slows you dpwn. How much depends on your clothes, but also whethter there is water or sand on slide (e.g.). When you reach the ground, the force from the ground will be larger than when you are standing still – it has to change your motion.
The force up from the slide is referred to as a normal force, N, since it is orthogonal ("normal") to the surface, preventing you from moving through the surface.
The force of friction is instead parallel to the surface and slows your motion. In the figure it has exactly the right size to cancel the component of gravity along the slide. Your speed will then be constant (Newton's first law). If the friction force is smaller, you will move faster and faster. If it is larger than the component of gravity, the motion will slow down, coming to a stop.
The slide as an inclined plane
Preschool children know that it is easier to walk up a slide in parts where it is less steep or that objects sliding down may come to rest where the slide is less steep. They know what clothes let them move fastest down a playground slide and may have learned that this is related to friction. They are also likely to have discovered the effects of rain on the slide, and possibly also of sand. But there is more to discover and learn about friction than the term itself and the differences between different materials.
Experimenting on a playground slide can be combined with more systematic investigations of wooden blocks covered in different fabric. Some of the investigations by middle school students are shown to the right.
Read more about their investigation in Motion on an inclined plane and the nature of science, in Physics Education, 49, 180 2014, with a Video abstract. (Manuscript copy)