Imagine a crash test dummy sitting in a car during a collision test with its seat belt securely fastened. The force experienced by this dummy isn’t just a number on a screen – it’s the intense push and pull of the impact, revealing the true nature of forces in action.  When force is exerted on an object, it can cause the motion of an object to change.  In a scenario with a crash test dummy, a downward force keeps the dummy anchored to its seat, whereas an upward force could potentially lift it into the air.  When two or more forces act on an object simultaneously, the forces combine to form a net force.

Friction is the force that opposes the sliding motion of two surfaces in contact.  This opposition arises from microscopic bumps on surfaces called microwelds.  There are four types of friction.  Firstly, static friction occurs when two surfaces don’t slide past each other, as seen when pushing a crate across the floor.  Secondly, sliding friction involves surfaces moving against each other, as can be seen when you sled down a hill.  Thirdly, rolling friction occurs between a rolling object and the surface it rolls on, such as a basketball rolling down a hill.  Finally, fluid friction occurs when an object moves through a fluid, meaning either a liquid or a gas.  Skydiving is an example of fluid friction.

Not all objects fall to the earth at the same time, and this is because of air resistance, which acts as a friction-like force opposing the motion of objects moving through air.  When an object collides with the air, it experiences deceleration caused by the resistance of the air molecules.  Whether an object falls faster or slower depends on factors such as its speed, size, and shape.  For instance, a flat sheet of paper will fall slower than a crumpled-up sheet of paper because the flat sheet of paper collides with more air particles.

Review:

1. What is a net force?
2. Compare static friction to sliding friction.
3. Explain air-resistance.