# Chapter 6 Forces and Motion Section 1 Gravity

• Slides: 67

Chapter 6 Forces and Motion Section 1 Gravity and Motion

Essential Questions for 6 -1 • • • What is a force? What are the effects of gravity and air resistance on falling objects? Why are objects in orbit in free fall? Why do they appear to be weightless? How is projectile motion affected by gravity?

Forces and Fields • A Force is a push or a pull • A Force Field is an area in which a force is experienced.

Vectors and Scalars • Vectors are quantities that have both magnitude and direction. – For example: Velocity: 45 mph West • Scalars are quantities that just have magnitude – For example: Speed 45 mph

Why use vectors • They show the forces acting on an object. We can add them or subtract them to determine the Net force acting on an object.

This is Galileo Galilei (15641642)

Galileo went to the Leaning Tower of Pisa…

… dropped two cannon balls of different masses to the ground…

…and this is what happened.

Force of Gravity • Gravity = A force that pulls objects together. • On Earth, gravity makes any object of any mass accelerate toward Earth at 9. 8 m/s 2.

Relationships in FG = m x 9. 8 m/s 2 • Heavier objects do not fall faster than lighter objects. • Heavier objects hit the ground with a Stronger force, but they accelerate toward Earth at the same rate.

Gravity and Falling Objects • Objects fall to ground at the same rate because acceleration due to gravity is the same for ALL objects • Why? Acceleration depends on both force and mass. – A heavier object experiences a greater gravitational force, BUT it is also harder to accelerate. – Galileo was a genius.

Acceleration and Velocity • Acceleration is the rate velocity changes over time. The acceleration of an object affected by gravity is 9. 8 m/s. • Change in velocity of falling objects can be measured by the following equation: ∆v = g x t OR 9. 8 m/s times the number of seconds an object falls…

Formula for Force of G where: • F is the force between the masses, • G is the gravitational constant, • m 1 is the first mass, • m 2 is the second mass, and • r is the distance between the centers of the masses. The force of gravity we experience on Earth is primarily due to the pull of the sun on the Earth. Although, we are far from it, its’ mass is so great it creates the strongest gravitational pull in our solar system

Which of these objects will fall at the fastest rate when dropped? A. The ball with a mass of 75 kg B. The ball with a mass of 25 kg C. The ball with a mass of 10 kg D. They all fall at the same rate.

Mr. I Earth a = 9. 8 m/s 2 m = 70 kg Moon a = 1. 6 m/s 2 m = 70 kg

Calculate Your Weight Earth a = 9. 8 m/s 2 m = your mass in kg Moon a = 1. 6 m/s 2 m = your mass in kg

Air Resistance • Air resistance – the force that opposes the motion of objects through air – Amount of air resistance depends on the size, shape, and speed of the object – A place where all the particles have been sucked out is called a vacuum. There is no Air resistance in a vacuum, because there is nothing to cause friction with.

Discussion • Why would a feather dropped from the same height as an acorn fall to Earth more slowly than the acorn?

Discussion • How can a sky diver benefit from air resistance?

Terminal Velocity • Terminal Velocity – when air resistance = gravity – 0 N or a balanced force! – It’s the fastest an object will fall – It’s also why rain drops don’t kill you… think about it…

Free Fall • Free fall - when gravity is the only force acting upon an object – Can only happen where there is NO air resistance: SPACE (or a vacuum) • - Orbiting objects are in free fall. Orbit is caused by two motions: - Orbiting objects move forward, but are also in free fall – so are the astronauts

Discussion • How does free fall in an orbiting spacecraft cause the appearance of weightlessness?

Orbiting and Centripetal Force • Orbit is caused by two things that create an unbalanced force – Gravity provides centripetal force to the orbiting object – Centripetal force – the unbalanced force that causes an object to move in a circular path – Ex: planets around the sun or the moon around the Earth

Discussion • What does free fall in an orbit look like? • Which is – The path of the orbiting spacecraft? – The path of free fall for the spacecraft? – The path the spacecraft would follow if there was no gravity? – The curve of the Earth’s surface?

Projectile Motion • The curved path an object follows when it is thrown near the surface of the Earth • Composed of horizontal motion and vertical motion – Horizontal motion (like throwing a ball) causes the object to go forward – Vertical motion (gravity) causes the object to fall • Together they create a curved path: BUT still hits the ground the same time as a dropped object – This is why you always have to aim above a target when trying to hit it…

An example of projectile motion

Quiz 6 -1: 4 Questions 1. If a baseball and a cannonball are dropped from the same height at the same time, and there is no air resistance, which ball will hit the ground first? a. The cannonball lands first. b. The baseball lands first. c. The balls land at the same time. d. The ball with the larger volume lands first.

2. What feature of an object does not affect air resistance? a. its size b. its chemical properties c. its shape d. its speed

3. Why do astronauts in space appear weightless? a. There is no gravity in space. b. Air resistance opposes gravity. c. They have no mass in space. d. They are in free fall.

4. In what directions is a projectile accelerated? a. both vertically and horizontally b. vertically downward c. vertically upward d. horizontally forward

Chapter 6 Forces and Motion Section 2 Newton’s Laws of Motion

Essential Questions for 6 -2 • • • What is Newton's first law of motion? How does it relate to objects at rest and objects in motion? What is Newton's second law of motion? What is the relationship between force, mass, and acceleration? What is Newton's third law of motion? What are some examples of force pairs?

1 st Law of Motion (Law of Inertia) An object at rest will stay at rest, and an object in motion will stay in motion at constant velocity, unless acted upon by an unbalanced force.

1 st Law • Inertia is the tendency of an object to resist changes in its velocity: whether in motion or motionless. These pumpkins will not move unless acted on by an unbalanced force.

1 st Law • Once moving, unless acted on by an unbalanced force (gravity and air – fluid friction), an object would never stop!

1 st Law • Unless acted upon by an unbalanced force, this golf ball would sit on the tee forever. • Ex. So far…

Discussion 1: • You are a passenger in a car that is moving rapidly down a straight road. As the driver makes a sharp left turn, you are pressed against the right side of the car. Explain why this happens.

Why then, do we observe every day objects in motion slowing down and becoming motionless seemingly without an outside force? It’s a force we sometimes cannot see – friction.

What is this unbalanced force that acts on an object in motion? • There are four main types of friction: – Sliding friction: ice skating – Rolling friction: skateboarding – Fluid friction (air or liquid): air or water resistance – Static friction: initial friction when moving an object

Newton’s 2 nd Law

2 nd Law The net force of an object is equal to the product of its mass and acceleration, or

Newton’s 2 nd Law proves that different masses accelerate to the earth at the same rate, but with different forces. • We know that objects with different masses accelerate to the ground at the same rate. (Gravity = 9. 8 m/s) • However, because of the 2 nd Law we know that they don’t hit the ground with the same force. F = ma 98 N = 10 kg x 9. 8 m/s/s 9. 8 N = 1 kg x 9. 8 m/s/s

Discussion 2: • How does Newton’s second law explain why it is easier to push a bicycle than to push a car with the same acceleration?

3 rd Law • For every action, there is an equal and opposite reaction.

3 rd Law There are two forces resulting from this interaction - a force on the chair and a force on your body. These two forces are called action and reaction forces.

Newton’s 3 rd Law in Nature • Consider the propulsion of a fish through the water. A fish uses its fins to push water backwards. In turn, the water reacts by pushing the fish forwards, propelling the fish through the water.

3 rd Law Flying gracefully through the air, birds depend on Newton’s third law of motion. As the birds push down on the air with their wings, the air pushes their wings up and gives them lift.

Other examples of Newton’s Third Law • The baseball forces the bat to the left (an action); the bat forces the ball to the right (the reaction).

3 rd Law • Consider the motion of a car on the way to school. A car is equipped with wheels which spin forward. As the wheels spin forward, they grip the road and push the road backwards.

3 rd Law The reaction of a rocket is an application of the third law of motion. Various fuels are burned in the engine, producing hot gases. The hot gases push against the inside tube of the rocket and escape out the bottom of the tube. As the gases move downward, the rocket moves in the opposite direction. So the third law says…

Discussion 3: • How does Newton’s third law explain how a rocket takes off?

Quiz 6 -2: 3 Questions 1. The reaction force of a chair you are sitting on a. is greater than your weight. b. is equal to your weight. c. is determined by many factors. d. varies.

2. Which of the following is the equation for Newton’s second law of motion? a. F= m × a b. m=F × a c. a=m ÷ F d. F=m ÷ a

3. Which is a common unbalanced force acting on objects in motion? a. inertia b. acceleration c. friction d. speed

Chapter 6 Forces and Motion Section 3 Momentum

Essential Questions for 6 -3 • • How is the momentum of a moving object calculated? How would you explain the law of conservation of momentum?

Momentum • Dependent on the objects mass and velocity • More momentum = harder to stop or change direction of the object • Momentum (p) = mass in kilograms times velocity in m/s. p = m x v • Momentum must have a direction, like velocity

Law of Conservation of Momentum • When objects collide, the total amount of momentum stays the same • Momentum before collision always = momentum after collision – This is called a transfer of momentum – Sometimes it is split between the objects – Sometimes one object stops and transfers all momentum to the object that was struck

Discussion • Use Newton’s third law to explain how a person hammering a nail into a block of wood is demonstrating conservation of momentum.

Problem • A 0. 151 kg baseball is pitched with a velocity of 43. 1 m/s. Ignoring air resistance, what is the baseball’s momentum after being pitched? Show your work. END

Quiz 6 -3: 2 Questions 1. When a train increases its velocity, its momentum a. decreases. b. remains unchanged. c. increases. d. is transferred to it surroundings.

2. The law of conservation of momentum states that when two objects collide, their combined momentum a. increases after the collision. b. remains the same after the collision. c. decreases after the collision. d. cannot be determined after the collision.