Unit 3 - Dynamics

Introduction to Forces and Newton’s three Laws of Motion

Unit Outcomes

Demonstrate an understanding of Newton's Three Laws of Motion. Describe various types of forces. Define constant velocity, net force and inertia in terms of Newton's Laws. Calculate net force using the formula derived from Newton's Second Law. Calculate the equilibrant force of an object. Identify the three fundamental types of forces. Define and calculate "weight." Compare and contrast mass and weight in terms of physics. Describe normal force. Accurately draw normal force and tension vectors in physics diagrams. Describe how pulleys affect tension forces. Identify the different types of friction. Calculate the force of kinetic friction. Solve friction problems using the formula for friction and the friction coefficient. Draw free-body diagrams to show forces on a given object. Use free-body diagrams with Newton's Second Law to solve mechanics problems. Define momentum. Demonstrate an understanding of changes in momentum and Newton's Second Law. Calculate momentum using the formula. Calculate impulse.

Inertia

The natural tendency of an object to resist any change in its motion

The amount of inertia is directly related to mass Seatbelts, bus, subway

Mechanics

Kinematics – predicting and describing motion (displacement, velocity and acceleration)

Dynamics – Explaining why objects move the way they move The study of dynamics involves forces –a

push of a pull on an object that causes change in motion (see table 4.2 page 128)

Inertial Mass/ Gravitational Mass Inertial Mass - The net external force needed to

act on a mass in order to change its motion

Gravitational Mass – the mutual force of attraction between every pair of masses in the universe. The forces vary directly with the magnitude of each mass and inversely with the square of the distance between the masses (Law of Universal Gravitation)

Common Forces

Two Categories of Force Contact Forces – friction, applied force,

tension, normal force

Non-Contact Forces – gravity, electrical and magnetic

attraction

Gravitational Force - Weight

Because the Earth is so big (huge mass), its exerts a strong force on you and any other object. You exert an equal force on the Earth. Gravitational forces always act in pairs

See figure 4.4 pg 131 The magnitude of gravitational force varies

Weight

Mass – the amount of matter in an object (constant)

Weight – force of gravity action on an object. It varies with the distance the object is away from the center if the Earth

Acceleration due to Gravity (g) 9.80 m/s2

Is influenced by both mass of Earth and distance from the Earth. The value of “g” varies with location

See table 4.3 page 132

Calculating Weight

Weight – Fg or W

Fg = mg Fg (W) – force of gravity (weight)

m – mass of object

g – acceleration due to gravity

Units – kg x m/s2

one newton (N)

Friction

Friction is a contact force Frictional forces inhibit relative motion between

objects in contact with each other Two types of friction:

Static frictional force – exists when you start to move an object from rest

Kinetic frictional force – exists while the object is moving

Static friction is greater that kinetic friction

Static Frictional Force

The strength of frictional force between two surfaces depend on the nature of the surfaces. All surfaces create some friction (no such thing as a smooth surface)

Force of friction is an electromagnetic force acting between the surface atoms of one object and those of another

When two surfaces are at rest, the surface atoms interact to form relatively strong attractive forces. A push on one object creates a static frictional force that pushes back with exactly the same magnitude of force until the applied force is great enough to break the attractive forces between the surface atoms

New bonds are formed when objects begin to move. The are continually being formed and broken as the object slides over the other.

Frictional force depends on two things: Type of atoms and molecules making up the

materials passing over each other Stickiness factor – coefficient of friction (μ)

The magnitude of the forces that are pressing the two surfaces together

Normal force (Fn )– equal and opposite to weight

Surprisingly friction is independent of velocity and area if contact (surface area) – three assumption are necessary for this to be true

Calculating Friction

Ff = μFn

Ff - force of friction (N) μ – coefficient of friction (none) Fn - normal force (N)