Section 3: Friction and Gravity Objectives: describe friction and identify the factors that determine the friction force between two surfaces explain how mass differs from weight state the universal law of gravitation describe the effects of gravity and air resistance on an object in free fall

I. Friction A.Friction is a force that opposes motion. i. strength depend: 1.type of surfaces 2.How hard surfaces push together.

B. Is friction useful? i. Without friction motion would not be possible.

FYI: Sometimes we need to increase friction. Friction is quite often helpful. For example, gymnasts use chalk on their hands to increase friction. Cyclists rely on friction to hold their bicycles on the ground during turns.

II. Types of Friction A. Fluid Friction

B. Sliding friction C. Rolling friction

III. Controlling Friction A. Ways to reduce friction: 1.Smooth the surface (sanding) 2.Use lubricants. (oil, graphite, air on air hockey tables) 3. Change from sliding friction to rolling friction

B. Increase Friction: 1.Roughen up the surface. i. adding gravel to icy roads ii. use studded snow tires instead of radials 2. Use different type of surface. i. carpet vs smooth tiles. 3. Increase pressure upon the surface.

IV. Gravity A.force that pulls objects towards Earth FYI: Without the force of gravity, these sky-diving acrobats would simply float in the sky. Thanks to gravity, however, they receive a thrilling adventure as they fall to Earth.

B.Free fall- i. when the only force acting on a falling object is gravity. (Objects in free fall accelerate as it falls, because it of unbalanced force. Unbalanced forces accelerate.) ii. On Earth the rate of acceleration if 9.8 m/s/s

C.Projectile motion i. curved path object follows when thrown or propelled near surface of Earth.

ii. Projectile motion has two components: horizontal and vertical

FYI: Although gravity pulls both a leaf and a rock toward the earth, the two objects do not accelerate at the same rate. The leaf and rock do not strike the earth at the same time. On the moon, however, they would. Why?

D.Air resistance i. type of fluid friction ii. caused by object falling through air. FYI: Friction is the force that is opposite to motion, so air resistance is an upward force. The greater the surface area of an object, the greater the air resistance. Explain why the ball of paper reaches the ground before the flat sheet.

FYI: Without air resistance objects of different mass will fall and accelerate that the same rate and land on earth at the same time.

V. Terminal Velocity A. Terminal velocity i. point when upward force of air resistance equals downward pull of gravity.

FYI: Air resistance acts on the parachute, allowing the parachutist to fall at terminal velocity that is slow enough to allow a safe landing. FYI: Terminal velocity is reached when the pull of gravity equals the air resistance and the falling object no longer accelerates.

VI. Weight A. Weight is a measure of force of gravity on object, B.mass is measure of amount of matter in that object. C. Weight = Mass x Acc due to gravity Example: 50 Kg person weighs 50 Kg x 9.8 m/s/s = 490 N on Earth.

VII. Universal Gravitation A.The law of universal gravitation: force of gravity acts between all objects in the universe. i. amount of gravity depends on mass of object.