Chapter 3 Forces

Section 1 Newton’s Second Law

Newton’s 3 Laws Law of Inertia Newton’s second law of motion connects force, mass, and acceleration 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 Acceleration = net force (in newtons) mass (in kilograms)

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.