Friction, Air Resistance, and Gravity

Friction

  • Friction is the force that opposes motion between two surfaces that are touching one another.

    • It is the force that occurs when two things rub together

      • Ex. when rubbing your hands together, it produces heat

    • It the force that tends to stop things sliding past each other.

Types of Friction

There are 3 types of friction:

  • Static Friction

    • Static friction is the friction between two surfaces that are not moving past each other.

      • ex. Pushing a heavy box that won’t move.

  • Sliding Friction

    • Sliding friction is the force that opposes the motion of two surfaces sliding past one another

      • It is caused by mircowelds constantly breaking and then forming again as the box slides along the floor

        • ex. Sliding a desk from one place to another

  • Rolling Friction

    • Rolling friction occurs when one surface rolls on or across another surface

      • ex. Train wheels rolling on the tracks

    • Rolling friction is usually less than static or sliding friction. That’s why it is easier to pull a load in a wagon rather than drag it

Air resistance

  • Air resistance is caused by the friction between the surface of a moving object and the air molecules.

    • The # of air resistance on an object depends on its mass and shape as well as on the speed with which it moves

Characteristics

  • Objects with larger surfaces can experience greater air resistance, thus slowing an object’s speed

  • As an object’s speed increases, so does the air resistance

    • The designing of cars takes into account air resistance!

Gravity

  • Gravity is a force!

    • Every object exerts a gravitational force on every other object in the universe

    • The force of gravity is different from forces such as friction; the force of gravity acts even when the objects do not touch!

      • The Earth and moon do not touch, yet both have a gravitational attraction to each other. (Ex. Tides)

Factors Affecting Gravitational Force

Mass and distance affect gravitational force:

  • The force of gravity between two objects depends on their masses and on the distance between the two objects.

  • The greater the mass of an object is, the larger the gravitational force it exerts on other objects.

    • Example: Earth is so large that you don’t notice it’s gravitational force!

  • The force of gravity changes as the distance between objects changes.

    • Gravitational force weakens the further apart two objects are. Ex. Earth and Pluto

  • Gravitational force strengthens the closer two objects are. Ex. The moon’s affect on the tides.