Chapter 3

Forces

Section 1

Newton’s Second Law

Newton’s 3 Laws

1. Law of Inertia2. Newton’s second law of motion connects

force, mass, and acceleration3. 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

Accelerati on = net force (in newtons) mass (in ki lograms)

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 areaswhere 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.