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. FrictionA. 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 FrictionA. 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 roadsii. 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 thrillingadventure 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 unbalancedforce. 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 donot accelerate at the same rate. The leaf and rock do not strike the earth atthe 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 resistanceis an upward force.

The greater the surface area of an object, the greater the airresistance.

Explain why the ball of paper reachesthe 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 onthe parachute, allowingthe parachutist to fallat terminal velocitythat is slow enough to allow a safe landing.

FYI:Terminal velocity is reached when the pullof gravity equals the airresistance and the fallingobject 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 weighs50 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.