4-3 Friction And Gravity

Friction and Gravity Chapter 4, Section 3

Friction The force that one surface exerts on another when the two rub against each other is called friction

The Nature of Friction Friction acts in a direction opposite to the object’s direction of motion The strength of the force of friction depends on two factors: the types of surfaces involved and how hard the surfaces push together Rough surfaces produce greater friction than smooth surfaces

Controlling Friction Is Friction Useful or Not? Walking Automobiles

Sliding friction- when solid surfaces slide over each other Rolling friction- when an object rolls over a surface The force needed to overcome rolling friction is much less than the force needed to overcome sliding friction Fluid friction- the friction that occurs when an object moves through a fluid Force needed to overcome fluid friction is less than needed to overcome sliding friction

Gravity Free Fall Gravity is the force that pulls objects toward each other When the only force acting on a falling object is gravity, the object is said to be in free fall Objects in free fall accelerate 9.8 m/s2

Projectile Motion Projectile- An object that is thrown A projectile is in free fall Horizontal motion of a projectile does not interfere with its free fall Projectiles and objects dropped will hit the ground at the

Air Resistance Objects falling through air experience a type of fluid friction called air resistance Air resistance is an upward force Greater surface area of an object the greater the air resistance Air resistance increases with velocity Eventually air resistance equals force of gravity Terminal velocity- the greatest velocity the object reaches

Growth of Seedlings Plants’ growth responds to the force of gravitygravitropism

Weight Weight- the measure of the force of gravity on a person or object at the surface of a planet Mass- the measure of the amount of matter in that object Weight = Mass X Acceleration

Universal Gravitation Gravity acts between objects everywhere in the universe Law of universal gravitation- the force gravity acts between all objects in the universe; any 2 objects in the universe attract each other The strength of gravitational force depends on the masses of the objects involved Gravitational force also depends on the distance between the objects; farther apart the objects, weaker