Chapter 3 Forces Section 1 Newton’s Second Law Newton’s 3 Laws 1. Law of Inertia 2. Newton’s second law of motion connects force, mass, and acceleration 3. For every action there is an equal and opposite reaction Newton’s second law • Force and motion are connected – an object will have greater acceleration if a greater force is applied to it • Ex. throwing a baseball – the mass of an object and the force applied to it affect acceleration • Ex. difference between throwing a baseball and a softball Force = mass X acceleration • Can also be written as…. • Force is calculated in Newtons • Mass will be calculated in kilograms • Acceleration will be in meters per seconds per seconds or m/s2 Lets do an example… • You push a friend on a sled. Your friend and the sled together have a mass of 70 kg. If the net force on the sled is 35 N, what is the sled’s acceleration? You can also calculated force • F = ma • A tennis player hits a ball. The acceleration is 5000 m/s2. The mass of a tennis ball is 0.06 kg. What would be the net force exerted on the ball? Other forces exerted… • Gravity • Friction • Air resistance Friction • Friction is a force that opposes motion between two surfaces that are touching each other Amount of friction… • Depends on two factors: – the kinds of surfaces that are touching – the force pressing the surfaces together Microwelds • Even apparently smooth surfaces still have microscopically rough surfaces • Microwelds are areas where surface bumpers stick together (these are the sources of friction) Friction • Static Friction – friction between two surfaces that are not moving past each other Friction • Sliding Friction – force that opposes the motion of two surfaces sliding past each other Friction • Friction between a rolling object and the surface it rolls on is called rolling friction Air Resistance • Air resistance opposes the motion of objects that move through the air – The amount of air resistance depends on an object’s shape, size, and speed Terminal Velocity • Gravity is a force and thus causes objects to accelerate towards earth • As something falls faster air resistance gets stronger and thus is able to balance the downward force of gravity. • Terminal velocity depends on the size, shape, and mass of a falling object. Terminal Velocity Section 2 Chapter 3 Gravity • Law of gravitation – any two masses exert an attractive force on each other • Gravity – an attractive force between two objects that depends on the masses of the objects and the distance between them Gravity depends on the size of the planet… Earth’s Gravitational Acceleration • Every object on earth falls at the same rate af acceleration due to gravity • acceleration due to gravity = 9.8 m/s2 What is weight? • Weight – is a gravitational force exerted on an object – Weight decreases as an object moves away from Earth – Weight results from a force – W = mass (kg) X acceleration of gravity (m/s2) – W = mg Difference between Weight and Mass • Weight is a force and mass is a measure of how much matter an object contains. • However… they are related. • Weight increases as mass increases… Projectile Motion • When something is thrown it has a velocity in the direction which it was thrown as well as a velocity towards earth due to gravity. • Therefore a projectile will have horizontal and vertical velocities due to gravity, and follow a curved path. Centripetal Force • Centripetal force is the force pulling toward the center of a curved path. • This is due to centripetal acceleration. • Centripetal force is an unbalanced force. Section 3 Chapter 3 Newton’s Third Law of Motion • Law: To every action force there is an equal and opposite reaction force. Momentum • A moving object has a property called momentum that is related to how much force is needed to change its motion. • The momentum of an object doesn’t change unless its mass, velocity, or both change. • Momentum – related to how much force is needed to change an object’s motion; momentum equals mass times velocity • momentum = mass (kg) X velocity (m/s) • p = mv Law of conservation of momentum • Momentum can be transferred between objects; momentum is not lost or gained in the transfer.