Forces A force is an interaction (push or pull) between objects. Forces can be divided into two categories: 1.Contact forces Push or pull between two objects that are touching each other. 2.Non-Contact / Field Forces Push or pull between two object that are not in contact. There is an invisible “field” around an object. When a second object enters the field it moves in the direction of the field. Example: Gravity, Magnets

The Newton Force is measured in Newtons. – Definition: A force that causes a mass of one kilogram to accelerate at a rate of one m/s 2 – 1 N = 1 kg m/s 2 A Newton is a derived unit because it is made up of fundamental units.

Newton’s Laws 1 st – Inertia – The tendency of an object to resist change – Car and Wall Car and Wall – Motorcycle and Wall Motorcycle and Wall – Truck and Car Truck and Car 2 nd – F = ma – Shows the relationships among force, mass, and acceleration 3 rd – All forces occur in action reaction pairs – These pairs are equal in size (magnitude) and opposite in direction.

Mass vs. Weight Weight is the gravitational attraction exerted by a large body, usually the Earth. – Like all other forces, weight is measure in Newtons. – English units for weight are pounds. Mass refers to the amount of matter in an object. – Mass is measured in kilograms (kg)

Mass vs. Weight (cont.) Newton’s 2 nd law gives us the relationship between weight and mass. – F = ma – W = mg (g represents the average acceleration due to gravity on Earth.) The average acceleration due to gravity on Earth is 9.8 m/s 2. – The value increases as the distance from the center of the Earth increases. Elevator Elevator Problem #1 Elevator Problem #2

Normal Force The force that pushes two surfaces together. F N Always acts perpendicular to the two surfaces in contact.

Net Force Sum of all forces acting on an object F NET If F NET = o, then the object is said to be in equilibrium.

Balanced and Unbalanced Forces

Free Body Diagrams A pictorial representation of the forces acting on an object. 1. Draw the object (or a shape to represent the object). 2. Use an arrow to represent each force acting on a the object. 3. At times it is helpful to add an arrow under the object to show the net force.

Frictional Force The force that opposes motion between two surfaces. Acts parallel to the two surfaces and in the direction that opposes the slipping/sliding of the two surfaces. Occurs because all surfaces are rough. Frictional force DOES NOT depend on surface area.

Static and Sliding Friction Static friction is the force that opposes the start of motion. Sliding friction is the force that opposes two surfaces that are rubbing/sliding across one another. Static friction is always bigger than sliding friction.

Coefficient of Friction A constant that relates the normal force to the frictional force. Represented with the Greek letter μ (mu) F f = μ F N Frictional force DOES NOT depend on surface area.

Terminal Velocity As a bodies falls through the air, its speed increases and the air drag increases in the direction that opposes the fall. The velocity at which the air drag equals the object’s weight is called its terminal velocity. – The terminal velocity of an object is dependant on the objects weight and surface area. Without Air Resistance With Air Resistance Skydive

Friction Problems 1.Your new motorcycle weights 2450 N. What is its mass? 2.When you drop a 0.40-kg apple, the Earth exerts a force on it that accelerates it at 9.8 m/s 2 towards the Earth’s surface. According to Newton’s third law, the apple must exert an equal and opposite force on the Earth. If the mass of the Earth is 5.98 x kg, what is the magnitude of the Earth’s acceleration?

Friction Problems 3.A 20-kg Radio Flyer wagon is at rest until little Johnny begins to pull on it. a.If Johnny has to pull with a force of N to get the wagon rolling, what is the coefficient of static friction? b.If the coefficient of sliding friction between the wagon and the sidewalk is 0.31, how much force does Johnny have to pull with to keep the wagon rolling at a constant speed?

Vector A vector is a quantity that involves two parts: a magnitude and a direction. Vectors are represented with an arrow. – The length of the arrow represents the magnitude of the vector. – The direction the arrow points represents the direction of the vector. Force is an example of a vector.

Head–to–Tail The graphical method of vector addition is called the head-to-tail method. The order in which vectors are added does not matter. Head Tail

Steps of Head-to-Tail o Draw the 1 st vector to scale. o Draw the 2 nd vector so that its tail begins on the head of the 1 st vector. o Repeat step 2 until all vectors have been drawn. o Draw the resultant vector from the tail of the 1 st vector to the head of the last vector. A resultant (R) is the sum of two or more vectors.

Try It! Allie went to the store and walked 6 blocks south and 4 blocks East. She remembered needed to get stop and get some money from her aunt and went 3 blocks west. She continued on her way 3 blocks East and another 2 blocks South. How far did she go?