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.

 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 Second Law Zookeepers lift a stretcher that holds a sedated lion. The total mass of the lion and stretcher is 175 kg, and the lion’s upward acceleration is m/s 2. What is the unbalanced force necessary to produce this acceleration of the lion 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  Newton ’ s 2 nd law can also be stated as: The acceleration of an object is proportional to the net force on the object and inversely proportional to the object ’ s mass.

 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 that 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.  Gravitational force 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.

 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

 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.

 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

 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.

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

 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

 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