Forces & Motion Chapter 12

 Newton ’ s first law of motion - an object at rest remains at rest and an object in motion maintains its velocity unless it experiences an unbalanced force.  Objects tend to maintain their state of motion.  Inertia - the tendency of an object to resist being moved or, if the object is moving, to resist a change in speed or direction until an outside force acts on the object.  Inertia is related to an object ’ s mass. Mass is a measure of inertia. Causes all objects to fall with the same acceleration regardless of mass

 Seat belts and car seats provide protection. Because of inertia, you slide toward the side of a car when the driver makes a sharp turn. When the car you are riding in comes to a stop, your seat belt and the friction between you and the seat stop your forward motion. Mass and Inertia hyperlinkMass and Inertia

 Force - an action exerted on a body in order to change the body’s state of rest or motion. Force has magnitude and direction. net force - combination of all forces acting on an object.  balanced forces - objects either do not move or move at constant velocity.  unbalanced force - must be present to cause any change in an object’s state of motion or rest.

Frictional Forces & Acceleration

 Forces and motion are connected  An object will have greater acceleration if a greater force is applied to it  The mass of an object and the force applied to it affect acceleration  Newton’s 2 nd Law of Motion – states that the unbalanced force acting on an object equals the object ’ s mass times its acceleration. It connects force, mass, and acceleration in the equation a = f / m, acceleration = net force / mass, same as f = ma  Force is measured in newtons (N). 1 N = 1 kg  1 m/s 2 Newton's 2nd Law of MotionNewton's 2nd Law of Motion hyperlink

Newton’s 2 nd Law

Newton ’ s Second Law Paramedics lift a stretcher that holds an unconscious man. The total mass of the man and stretcher is 175 kg, and the man’s upward acceleration is m/s 2. What is the unbalanced force necessary to produce this acceleration of the man and the stretcher? 1. List the given and unknown values. Given: mass, m = 175 kg acceleration, a = m/s 2 Unknown: force, F = ? N

2. Write the equation for Newton ’ s second law. force = mass  acceleration F = ma 3. Insert the known values into the equation, and solve. F = 175 kg  m/s 2 F = 115 kg  m/s 2 = 115 N

 Friction – force that opposes motion between two surfaces that are touching each other  Microwelds – areas where surface bumpers stick together, are the source of friction  Types of Friction:  Static friction – friction b/n two surfaces that are not moving past each other  Sliding friction – force that opposes the motion of two surfaces sliding past each other  Rolling friction – friction b/n a rolling object and the surface it roll on  Fluid friction – force that opposes motion in a fluid

 Air resistance - opposes the force of gravity  The amount of air resistance depends on an object’s shape, size, and speed Terminal velocity – forces on a falling object are balanced and the object falls with constant speed  air resistance is equal in magnitude and opposite in direction to the force of gravity

 Velocity is constant when air resistance balances weight.  All matter is affected by gravity. Two objects, whether large or small, always have a gravitational force between them. When something is very large, like Earth, the force is easy to detect.  Gravitational force increases as mass increases and decreases as distance increases.

 Free fall is the motion of a body when only the force of gravity is acting on the body.  Free-fall acceleration near Earth’s surface is constant. If we disregard air resistance, all objects near Earth accelerate at 9.8 m/s 2. Freefall acceleration is often abbreviated as the letter g, so g = 9.8 m/s 2.  Free Fall Motion hyperlink Free Fall Motion

Free fall and motion  Orbiting objects are in free fall.  The moon stays in orbit around Earth because Earth’s gravitational force provides a pull on the moon.  Two motions combine to cause orbiting.

 Law of Gravitation – any two masses exert an attractive force on each other, F = G (m 1 m 2 )/d 2 G = 6.67 x m 3 /kg. s 2 Acceleration due to gravity on Earth is 9.8 m/s 2 Mercury – 3.8 m/s 2 Jupiter – 25.8 m/s 2  Gravity is one of the four basic forces that also include the electromagnetic force, the strong nuclear force, and the weak nuclear force Gravity is a long-range force that gives the universe its structure

 Strong nuclear force – acts only on the neutrons and protons  A very strong force of attraction  Acts over short distances ( m)  100 times stronger than electric force  Weak nuclear force – weaker than strong force  Acts over short distances ( m)

 Weight is equal to mass times free-fall acceleration. weight = mass  free-fall acceleration w = mg  Weight is different from mass. Mass is a measure of the amount of matter in an object. Weight is the gravitational force an object experiences because of its mass.  Weight influences shape. Gravitational force influences the shape of living things. Mass & weight hyperlinkMass & weight

Law of Universal Gravitation

 Weight – gravitational force exerted on an object  Decreases as an object moves away from the Earth  Weight results from a force; mass is a measure of how much matter an object contains  Objects in the space shuttle float b/c they have no force supporting them

Two Motions Cause Orbiting

 Projectile motion is the curved path an object follows when thrown, launched, or otherwise projected near the surface of Earth.  applies to objects that are moving in two dimensions under the influence of gravity.  Has two components—horizontal and vertical. The two components are independent.  Projectile Motion hyperlink Projectile Motion

 Projectile motion has some horizontal motion. Horizontal motion is motion that is perpendicular (90º) to Earth’s gravitational field. The horizontal velocity is constant.  Projectile motion also has some vertical motion. The vertical motion is the same as downward free-fall motion.

Projectile Motion

 Projectiles have horizontal and vertical velocities due to gravity, and follow a curved path  Centripetal acceleration – acceleration toward the center of a curved path  Centripetal force – an unbalanced force, causes centripetal acceleration  Newton’s 3 rd Law of Motion – to every action force there is an equal and opposite reaction force

 Action-Reaction forces – act on different objects and differ from balanced forces  Rocket propulsion is based on Newton’s 3 rd law of motion  Neptune was discovered based on predictions of gravitational forces and Newton’s laws  Newton’s third law of motion states that for every action force, there is an equal and opposite reaction force.  Forces always occur in action-reaction pairs.  Action-reaction force pairs are equal in size and opposite in direction.  Newton's 3rd Law of Motion Newton's 3rd Law of Motion

 Force pairs do not act on the same object.  When one object exerts an action force on a second object, the second object exerts a reaction force on the first object.  Equal forces don’t always have equal effects. For example, the action force of Earth pulling on an object and causing it to fall is much more obvious than the equal and opposite reaction force of the falling object pulling on Earth

 Momentum – related to how much force is needed to change an object’s motion; p= mv, momentum = mass x velocity  F = (mv f – mv i ) / t - changing momentum formula  Law of Conservation of Momentum – momentum can be transferred between objects; momentum is not lost or gained in the transfer

 Moving objects have momentum. For a given velocity, the more mass an object has, the greater its momentum is. Likewise, the faster an object is moving, the greater its momentum is.

Momentum Calculate the momentum of a 6.00 kg bowling ball moving at 10.0 m/s down the alley toward the pins. 1. List the given and unknown values. Given: mass, m = 6.00 kg velocity, v = 10.0 m/s down the alley Unknown: momentum, p = ? kg m/s (and direction)

2. Write the equation for momentum. momentum = mass x velocity p = mv 3. Insert the known values into the equation, and solve. p = mv = 6.00 kg  10.0 m/s p = 60.0 kg m/s down the alley

 Force is related to change in momentum. When you force an object to change its motion, you force it to change its momentum.  Momentum is conserved in collisions. The law of conservation of momentum states that the total amount of momentum in an isolated system is conserved.  Conservation of momentum explains rocket propulsion.

Rocket Propulsion

 Momentum is transferred. When a moving object hits a second object, some or all of the momentum of the first object is transferred to the second object. Momentum can be transferred in collisions, but the total momentum before and after a collision is the same.  Action and reaction force pairs are everywhere.  Momentum Momentum