Chapter 6—Forces in MotionMaterial on the Final Exam

Falling Water• How does gravity affect our world?

Section 1—Gravity and Motion• Objectives

• Explain the effect of gravity on falling objects

• Explain the effect of air resistance on falling objects

• Explain why objects in orbit are in free fall and

appear to be weightless

• Describe how projectile motion is affected by gravity

Section 1—Gravity and Motion• Aristotle was wrong again!

• Aristotle thought that the rate at which on object falls depended on the object’s mass

• Corrected by Galileo Galilei• Experiment with cannonballs

• Supported by many scientists• NASA supports Galileo• Mythbusters support Galileo

Section 1—Gravity and Motion• Gravity and acceleration

• Objects fall to the ground at the same rate because the acceleration due to gravity is the same for all objects

• A golf ball may have more mass…• …but it will also be harder to accelerate

Section 1—Gravity and Motion• Acceleration due to

Gravity• Acceleration is the

rate at which velocity changes over time

• Due to gravity, all objects accelerate toward earth at a rate of 9.8 m/s/s, or 9.8 m/s2

• Every second, velocity increases by 9.8 m/s

Section 1—Gravity and Motion• Velocity of Falling Objects

• Formula

Δv = g x t• Δv = change in velocity• g =gravity• t = time

Section 1—Gravity and Motion• Math practice!

• A penny at rest is dropped from the top of a tall stairwell. What is the penny’s velocity after it has fallen for 2s?

• Write the formula• Identify the variables• Plug in the numbers• Solve and circle

Section 1—Gravity and Motion• Math practice!

• The same penny hits the ground in 4.5s. What is the penny’s velocity as it hits the ground?

• Write the formula• Identify the variables• Plug in the numbers• Solve and circle

Section 1—Gravity and Motion• An acorn at rest falls from an oak tree. The acorn

hits the ground with a velocity of 14.7 m/s. How long did it take the acorn to land?

• Write the formula• Identify the variables• Plug in the numbers• Solve and circle

Section 1—Gravity and Motion• An rock falls off of a cliff on Mercury. The rock

takes 5 seconds to fall and has a velocity of 18.5 m/s when it lands. What is the gravity on Mercury?

• Write the formula• Identify the variables• Plug in the numbers• Solve and circle

Section 1—Gravity and Motion

• Air Resistance and Falling Objects• Air resistance can

affect the speed at which something falls

• Air resistance is a force that opposes the motion of objects through air

• The amount of air resistance acting on an object depends on the size, shape, and speed of the object

Section 1—Gravity and Motion• Acceleration Stops at Terminal Velocity

• At some point, air resistance (upward force) and gravity (downward force) equal and cancel out

• Net force is 0N; object stops accelerating

Section 1—Gravity and Motion• Terminal velocity is the constant velocity of a falling

object when the force of air resistance is equal in magnitude and opposite in direction to the force of gravity

Section 1—Gravity and Motion• Free Fall occurs when there is no air resistance

• CAN ONLY OCCUR IN A VACUUM OR OUTER SPACE!

• Free fall is the motion of a body when only the force of gravity is acting on the object

Section 1—Gravity and Motion• Orbiting Objects are in Free Fall

• Astronauts float in orbiting space crafts because of free fall

Section 1—Gravity and MotionProjectile Motion

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

Section 1—Gravity and MotionProjectile Motion• It has two independent (have no effect on

each other) components• Vertical motion –force exerted from gravity.• Horizontal motion – the force exerted on the

object.• The combination of the two motions form the

curved path.• Examples: a frog leaping, water sprayed by

a sprinkler, an arrow from an archer. Projectile motion video clip

Section 1—Gravity and Motion• Objects going out horizontally and objects

being dropped will hit the ground at the same time

video demo

Projectile motion video clip 1

Section 2—Newton’s Laws of Motion• Objectives

• Describe Newton’s First Law of Motion• Explain how the first law relates to objects at rest

and objects in motion• State Newton’s Second Law of Motion• Explain the relationship between force, mass, and

acceleration• State Newton’s Third Law of Motion• Describe and give examples of force pairs

Section 2—Newton’s Laws of MotionNewton’s First Law of Motion• An object at rest remains at rest, and

an object in motion remains in motion at a constant speed and straight line unless acted on by an outside or unbalanced force.

• Also known as (AKA) the Law of Inertia

Section 2—Newton’s Laws of Motion• Examples of Newton’s First Law

• An object will not move unless you push it.• An object will stay in motion unless friction, or some

other force changes the objects velocity. • A car at a red light will stay stopped until you let go

of the brake. A moving car will continue to move until you hit the brakes.

INERTIA = LAZINESS

Section 2—Newton’s Laws of Motion• Inertia is the tendency of an object to

resist a change in its motion. • Mass is a measure of inertia. • An object that has a small mass has

less inertia than an object that has a large mass.

Section 2—Newton’s Laws of MotionNewton’s Second Law of Motion• Force = mass x acceleration

F = maSI Unit = Newton (N)

F = ma video clip

Section 2—Newton’s Laws of Motion• What is the

acceleration of a 7 kg mass if a force of 68.6 N is used to move it toward Earth?

• You try!• What force is

necessary to accelerate a 1,250 kg car at a rate of 40 m/s2?

Section 2—Newton’s Laws of Motion• You try!

• Zookeepers carry a stretcher that holds a sleeping lion. The total mass of the lion and the stretcher is 175 kg. The lion’s forward acceleration is 2 m/s2. What is the force necessary to produce this acceleration?

Section 2—Newton’s Laws of Motion

• The more mass an object has the slower it will move.

• In order to speed up an object with a large mass, a greater force must be applied.

Section 2—Newton’s Laws of MotionNewton’s Third Law• Whenever one object exerts a

force on a second object, the second object exerts an equal and opposite force on the first. • All forces act in pairs.

• For every action, there is an equal and opposite reaction

Section 3--Momentum• Objectives

• Describe and calculate the momentum of moving

objects

• Explain the law of conservation of momentum

Section 3--Momentum• What is momentum?

• The momentum of an object depends on the object’s mass and velocity.

• The more momentum an object has, the harder it is to stop the object or change its direction.

• If velocity increases, its momentum also increases.

Section 3--Momentum• Momentum symbol is p• Unit for momentum is kg· m/s• Formula

p = mv

Section 3--MomentumMomentum math!• A bowling ball has a mass of 10 kg and is traveling at 5

m/s. What is the momentum of the bowling ball?

• A bullet travels 300 m/s and has a mass of 50 g. What is the momentum?

• Which has greater momentum? The bowling ball or the bullet?

Section 3--Momentum• Law of Conservation of Mass

• States that when two objects collide, their combined momentum remains the same after the collision. • This law applies whether the objects stick

together or bounce off each other after they collide.

• Because action and reaction forces are equal and opposite, momentum is neither gained or lost in a collision.

Conservation of Momentum… In Space!

Section 3--Momentum