Friction and Gravity

What is Friction?

• Friction is the resistance to the sliding, rolling, or flowing motion of an object due to its contact with another object. 

• Without friction, an object would continue to move at constant speed forever.

• The strength of the force of friction depends on the type of surfaces and how hard the surfaces push together.

• Rough surfaces produce greater friction than smooth surfaces.

Three Types of Friction

• Static Friction

• Sliding Friction

• Fluid Friction

Static Friction

• When the force on an object and the friction are balanced.– An object is not in motion when there is

static friction

Sliding Friction

•A resistance to movement that is created when two things rub together.

Fluid Friction

• The force that tries to slow objects down when they move through a liquid or a gas. It's also known as "drag", or "air resistance".

• All gases and liquids are fluids.

• An airplane and a swimmer both experience fluid friction.

Ways to Reduce Friction• Smooth the surface

• Put ball bearings in wheels

• Replace sliding with rolling

• Add oil or another type of lubricant

Gravity

• The attractive force between all objects that have mass

Universal Gravitation

The law of universal gravitation states that the force of gravity acts between all objects in the universe.

Gravity and Free Fall

• An object is in free fall when the only

force acting on the object is gravity.

• Free-falling objects do not encounter

air resistance.

• In free fall, the force of gravity is an

unbalanced force.

Calculating Free Fall

All objects in free fall accelerate at the same rate – 9.8 m/s² - regardless of their mass.

in one second = 9.8 m/s²

in two seconds = 19.6 m/s²

in three seconds = 29.4 m/s²

in four seconds = ________

The velocity continues to increase as the object falls.

Gravity and Air Resistance• Most objects falling through air

experience a type of fluid friction called air resistance.

• Air resistance is not the same for all objects.

• The greater the surface area of the object the greater the air resistance.

• However, since the elephant has more mass, it has more downward force of gravity and falls faster.

Weight

• The gravitational force exerted on an object.– Mass: amount of matter in an object.

Contact vs. Noncontact Forces

• Contact Force: a push or pull by touching– Ex: Friction

• Noncontact Force: a force exerted on an object without touching it– Ex: Gravity