Chapter 12 Forces

http://www.youtube.com/watch?v=8zsE3mpZ6Hw http://www.youtube.com/watch?v=4NQbeZ0EXZQ http://www.youtube.com/watch?v=bkivr6bB-og&feature=related

Newton’s First Law of Motion 1st Law of Motion(Law of Inertia) Objects in motion will stay in motion in a straight line and objects at rest will stay at rest, unless acted on by an outside force Ex. Stack of books on a car seat. What happens when you suddenly stop?

Inertia - objects resistance to a change in motion Mass affects an objects inertia Increase mass = Increase inertia Lower mass = Lower inertia

Seat belts and inertia Seat belts work because they keep your body from flying forward. If you get hit from behind will seatbelts or air bags help? No, you will fly back into the seat.

Newton’s Second Law of Motion Force is equal to the mass of an object and its acceleration. Force = Mass x Acceleration F = m x a Force is measured in Newtons 1 N = 1 kg x 1 m/s2 or 1kgm/s2 ( 1 lb is = to 4.45 N or 1 N is = to .225 lb)

Newton’s Second Law of Motion Newton’s second law explains how acceleration is dependant on force and mass.

Newton’s Second Law of Motion

Newton’s Second Law of Motion Newton’s second law could also be written as: - When a force is applied to an object, the object accelerates in the direction of the greater force. OR The acceleration of an object is directly proportional to the net force on the object and inversely proportional to the object’s mass.

Newton’s Second Law of Motion Acceleration = net force/mass or a = F/m

Gravity Gravity - Force of attraction between two objects. - force of gravity depends on two variables: a. depends on mass larger = more gravity (attraction) smaller = less gravity (attraction)

Gravity - force of gravity depends on two variables:(continued) b. depends on distance closer = more gravity (attraction) - further = less gravity (attraction)

Weight A measurement that can be taken because of gravity is weight. Weight – force of gravity pulling on the mass of an object. - weight is a force that can change. Mass never changes!

Free Fall when only gravity is acting on a falling object. constant rate of acceleration. This is called gravitational acceleration (g) * g = 9.8 m/s² - which means everything accelerates as it falls at the rate of 9.8 m/s² each second!

Free Fall and Weight We can use this knowledge to mathematically find weight! Equation to find weight is same as force! Force (weight) = m a or weight = m X g (acceleration of gravity)

Weight Ex. Find the weight of a 58 kg man on Earth. Weight = 58kg x Weight = mg Weight = 58kg x 10 m/s2 = 580 kg m/s2 or 580 N Now find his weight on: Venus g=8.8 m/s2 Mars g= 3.7 m/s2 Neptune g=11.8 m/s2 58kg x 8.8 m/s2 = 510N 58kg x 3.7 m/s2 = 220N 58kgx11.8 m/s2 = 680N

Newton’s 2nd Law Newton’s 2nd law explains why heavy objects and light objects fall at the same rate. -because if the mass is large, it is harder to accelerate (has more inertia). So it takes more time to fall!

Terminal Velocity On Earth we have air resistance - causes friction, causing the falling objects to slow down Terminal Velocity - maximum velocity reached by a falling object. (320 km/h or 120 mph) - occurs when air resistance is equal to the gravitational acceleration.

Terminal Velocity

Projectile Motion curved path an object follows when projected near the Earth’s surface.

Projectile Motion - 2 components make up the path of projectile motion. Both are independent of each other. a. horizontal motion -force stays constant vertical motion -pulling it down at a rate of 9.8 m/s2

Projectile Motion What if your object you are shooting at is falling too. Where should you aim? http://www.physicsclassroom.com/mmedia/vectors/mzng.cfm

Projectile Motion -Orbiting objects are in projectile motion. - so they are NOT weightless or floating, they are just constantly falling!

Newton’s 3rd Law of Motion For every action there is an equal and opposite reaction. Forces act in pairs - don’t necessarily act on the same objects. Ex. Escaped balloon Action – air escaping from opening in balloon Reaction - balloon flies forward.

Momentum The product of the mass and velocity of a moving object. Momentum(p) = mass (kg) x velocity (m/s) p = kgm/s Ex. Calculate the momentum of a 6.00kg bowling ball Moving at 10.0 m/s down the lane. 6.00kg x 10 m/s = 60.0 kgm/s down the lane

The Law of Conservation of Momentum Total amount of momentum in a system is conserved. Ex.