Chapter 6 Forces In Motion

Section 1 – Gravity and Motion

pp. 150 - 157

Terms to Learn

Air resistance – the fluid friction that opposes the motion of objects through the air

Terminal velocity – the constant velocity of a falling object when the size of the upward force of air resistance matches the size of the downward force of gravity

Terms to Learn

Free fall – the condition an object is in when gravity is the only force acting on it

Projectile motion – the curved path an object follows when thrown or propelled near the surface of the Earth

All objects fall with the same acceleration Objects fall to the ground at the same rate

because the acceleration due to gravity is the same for all objects.

ACCELERATION DUE TO GRAVITY All objects accelerate toward earth at a

rate of 9.8 m/s every second, no matter what the mass. This means that for every second an object falls, its downward velocity increases by 9.8 m/s.

Complete Math Focus

on p. 151

AIR RESISTANCEDrop a feather and a coin. Which hits the ground first? Why?

Air resistance slows down the acceleration.

AIR RESISTANCE

Air resistance is also known as fluid friction.

Air resistance depends on the size and shape of an object.

Air resistance slows down the acceleration of falling objects.

ACCELERATION

Acceleration stops at the terminal velocity.

As long as the net force on a falling object is not zero (0), the object accelerates downward.

As the speed of the object increases, the amount of air resistance also increases.

ACCELERATION

As an object falls, the upward force of air resistance continues to increase until it matches the downward force of gravity (the net force is zero).

When this happens, the object stops accelerating, then falls at a constant velocity called the terminal velocity.

FREE FALL

Free fall only happens if gravity is the only force acting on an object.

It can only occur where there is no air (in a vacuum or in outer space).

FREE FALL

Sky divers are NOT in free fall because air resistance is always acting on them.

Orbiting objects are in free fall.

ORBITS AND ORBITING

An orbit is formed by combining forward motion and free fall.

Objects in orbit appear to be weightless because they are in free fall.

ORBITS AND ORBITING

A centripetal force is needed to keep objects in circular motion. Gravity acts as this centripetal force.

PROJECTILE MOTION

Projectile motion is the curved path an object follows when thrown or propelled near the surface of the Earth.

PROJECTILE MOTION

Projectile motion has two components (parts): 1. horizontal 2. vertical

Gravity only affects the vertical motion of projectile motion

See Figure 9 on p. 155 of text.

EXAMPLES OF PROJECTILE MOTION