Chapter 10 Section 2: Friction and Gravity Key concepts: What factors determine the strength of the friction force between two surfaces? What factors affect the gravitational force between two objects? Why do objects accelerate during free fall? Key terms: Friction, static friction, sliding friction, rolling friction, fluid friction, gravity, mass, weight, free fall, air resistance, terminal velocity, projectile

Friction Friction – the force that two surfaces exert on each other when they rub against each other Causes of friction – – Strength and force of friction depends on two factors: how hard the surfaces push together and the types of surfaces involved – Smooth surfaces generally have less friction (duh) Friction acts in a direction opposite to the object’s movement

Types of friction Static – not moving objects take extra force to get them to move Sliding – when two solid surfaces slide over each other Rolling – object rolls along a surface (easier to overcome than sliding) Fluid – when a solid object moves through a fluid

Gravity Gravity is a force that pulls objects toward each other Law of Universal gravitation – the force of gravity acts between all objects in the universe. Any two objects in the universe, without exception, attract each other.

Law of universal what? XYFU XYFU m7E m7E

Factors affecting gravity Two factors affect gravity between objects: mass and distance Mass: amount of matter in an object – SI unit is the kilogram More mass is more gravitational force Distance – greater distance, less force

Weight and mass Mass is amount of matter Weight is the measure of gravitational force on an object – it varies with gravity, mass does not!

Gravity and motion Free fall – when the only force acting on an object is gravity. The force of gravity is unbalanced, which causes an object to accelerate. Acceleration due to gravity on Earth is 9.8m/s 2 This means that for every second an object is falling, its velocity increases by 9.8 m/s

Air resistance Objects falling through air experience a fluid friction known as air resistance (an upward force for falling objects). Air resistance is not the same for all falling objects. Air resistance increases with velocity. The greatest velocity something can reach is called its terminal velocity. It is reached when the force of air resistance equals the weight of the object.

Why does air resistance change for different objects? Imagine a feather falling. Now a brick. Yayy!

Projectile motion An object that is thrown is called a projectile. When you throw a projectile, gravity reduces its vertical velocity and it will eventually fall to the ground