Section 2 – pg 380 Friction, Gravity, and Elastic Forces Chapter 10 Section 2 – pg 380 Friction, Gravity, and Elastic Forces

Pg 381 Friction Friction: a force that two surfaces exert on each other when they rub against each other Acts in a direction opposite to the motion of objects

The Causes of Friction Pg 381 Smooth surfaces produce less friction than rough surfaces Even the smoothest objects have irregular, bumpy surfaces When the irregularities of one surface come into contact with those of another surface, friction occurs The strength of the force of friction depends on the types of surfaces involved and on how hard the surfaces push together Iron under microscope

Pg 382 Static Friction Static Friction: the friction that acts on objects that are not moving B/c of static friction, you must use extra force to start the motion of stationary objects

Pg 382 Sliding Friction Sliding Friction occurs when two solid surfaces slide over each other When an object is pulled at a constant velocity across a level surface, the pulling force equals the sliding force Bike brakes work using sliding friction: the rubber pads push against the metal rim of the wheel, causing the wheels to slow and eventually stop

Pg 382 Rolling Friction When an object rolls across a surface, rolling friction occurs Rolling friction is less than sliding friction for similar materials Engineers use ball bearings to reduce the friction between wheels and the rest of the product. These ball bearings are small, smooth steel balls that reduce friction by rolling between moving parts

Pg 382 Fluid Friction Fluid Friction occurs when a solid object moves through a fluid Fluid friction is usually less than sliding friction This is why many machine parts need to remain oiled

Gravity Pg 384 Gravity: a force that pulls objects towards each other Gravity acts everywhere in the universe Reason moon orbits earth and planets orbit the sun Universal Law of Gravitation states that the force of gravity acts between all objects in the universe All objects in the universe are attracted to each other If the objects are small then the attraction force is also small

Pg 384 - 385 Mass and Distance The force of gravity between objects increases with greater mass and decreases with greater distance Mass: the measure of the amount of matter in an object The more mass an object has, the greater its gravitational force The farther apart two objects are, the lesser the gravitational force between them

Pg 385 Gravity and Weight Weight: the gravitational force exerted on a person or object at the surface of a planet Objects that have a greater mass will have a greater weight Weight = Mass x Acceleration due to gravity Weight is measured in Newtons and mass in kilograms Unlike mass, weight varies with the strength of the gravitational force

Pg 386 Gravity and Motion Gravity and air resistance are the two forces responsible for how things fall

Pg 386 Free Fall Free Fall: when the only force acting on an object is gravity An object in free fall is accelerating In free fall, the force of gravity alone causes an object to accelerate in the downward direction Near the surface of the Earth, the acceleration due to gravity is 9.8 m/s2 After 1 sec, falling at 9.8 m/s After 2 secs, falling 19.6 m/s After 3 secs, falling 29. 4 m/s

Pg 387 Air Resistance In a vacuum, where there is no air, all objects fall with exactly the same rate of acceleration A type of fluid friction called air resistance acts on objects falling through airs A upward force exerted on falling objects Objects with greater surface area experience more air resistance as they fall A leaf falls more slowly than an acorn because the leaf has a larger surface area

Projectile Motion Projectile: an object that is thrown Pg 387 A projectile that is thrown horizontally will land on the ground at the same time as an object dropped

Pg 388 Elastic Forces Matter is considered elastic if it returns to its original shape after it is squeezed or stretched Compression and tension are two types of elastic forces Compression: an elastic force that squeezes or pushes matter together Tension: an elastic force that stretches or pulls matter

Chapter 10 Section 2 Homework – pg 388

1A. What factors affect the friction force between two surfaces?

1B. What are the four types of friction?

1C. What types of friction occur when you ride a bike through a puddle?

2A. What is the law of universal gravitation?

2B. How do mass and distance affect the force of gravity between objects?

2C. How would your weight change on the surface of an Earth-sized planet whose mass was greater than Earth’s? Why?

3A. Why does an object accelerate when it falls toward Earth’s surface?

3B. How does the mass of an object affect its acceleration during free fall?

3C. What force changes when a sky diver’s parachute opens 3C. What force changes when a sky diver’s parachute opens? What force stays the same?

4A. What are two types of elastic forces?

4B. Describe the forces on a rope that support a tire swing when a child sits on a tire.