What are forces? Inertia Gravity Newton’s Laws

Key Points of the Lesson -Understand that unbalanced forces cause changes in motion (acceleration) -Understand Newton’s Laws of Motion - Understand how to draw and read a free body diagram -Use Newton’s Laws to calculate problems involving forces in one or two dimensions

Force Inertia Gravity Orbit Mass Newton’s Laws Free Body Diagram Forces Vocabulary Force Inertia Gravity Orbit Mass Newton’s Laws Free Body Diagram

What is a force? A push or a pull May be unbalanced or balanced

What does a force do? Gives energy to an object Can set an object in motion, make it stop, or change its motion

Unbalanced forces change motion

Unbalanced forces change motion The truck motion is changed, but the ladder keeps going

Unbalanced forces change motion crosswind tailwind headwind

Forces are represented by vectors motion of book friction

Forces are added with vectors

Friction sliding fluid rolling A force which opposes motion, bringing objects to rest Types: sliding fluid rolling

Inertia I have more inertia but we fall at the same rate The tendency of an object to stay at rest or in motion. Objects with greater mass have greater inertia but we fall at the same rate

Inertia keeps astronauts in motion

Seat belts protect against inertia

Gravity The attractive force between all objects in the universe

F = M1 M2 d2 More mass means greater gravity More distance means less gravity My gravitational attraction is small no matter what the distance! Massive objects have lots of gravity

The earth exerts a force on the student and the student exerts a force on the earth because of GRAVITY!

Gravity

gravity pulls it in inertia keeps it going What forces are acting on the moon? Inertia

Newton’s Laws of Motion 1. Inertia 2. F=ma 3. Action-reaction

Newton’s First Law of Motion The law of inertia: An object at rest stays at rest an object in motion stays in motion unless acted upon by an outside force.

Inertia keeps satellites in orbit

Newton’s Second Law of Motion Force = Mass x acceleration A force is needed for acceleration Acceleration will occur in the direction of the force.

Which cars are accelerating? If you watch carefully, the green and blue cars are accelerating. The red car is traveling at constant speed.

Newton’s 2nd Law F_____ M A Force = mass x acceleration F_____ M A Force is measured in newtons. A newton is a kg m s2 Example: A car with mass 1000kg hits a tree and decelerates at 50m/s/s. What force does it exert on the tree?

Answer: F_____ M A 1000kg * 50m/s/s = 50,000 kgm or 50,000N s2

F = ma A car is traveling at constant speed. Is a force acting on it? Answer: although there are forces acting on the car (gravity, friction, air resistance,etc…) since there is no acceleration, the net force in the direction of motion must be zero.

More 2nd Law Problems: 1. What is the acceleration on a mass of 50kg if a force of 10N is applied? 2. An object accelerates due to gravity at a rate of 10m/s/s. If its mass is 15kg, what force is acting on the mass?

Answers: _F_ M A 1. _10N_ 0.2 n/kg= 0.2 kg m = 0.2 m/s2 50kg A s2/kg 15kg 10m/s2 150 kgm/s2 or 150N

Gravity is a force that causes an acceleration On earth, ALL objects accelerate at 9.8m/s2 (ignoring air resistance) because of gravity. No matter what the mass, ALL objects on earth accelerate at 9.8 m/s2

Newton’s 2nd Law F__ m a An object of mass 3kg falls off a cliff. How much force is acting on the object? Answer: F_______ 3kg 9.8 m/s2 29.4N

Free fall without air resistance Both the elephant and the feather land at the same time!

Free fall with air resistance The feather takes lots longer to hit the ground!

Newton’s Third Law For every action, there is an equal and opposite reaction

For every action, there is an equal and opposite reaction In rocketry…… The gases being expelled is the action. The rocket propelling forward is the reaction.

How do Newton’s laws describe the motorcycle rider? 1st: Inertia keeps the rider moving forward. 2nd: The force with which the rider hits the ground will be equal to his mass times his deceleration. 3rd: The action of hitting the ground will equal the reaction of the dirt.