Unit 6 Forces & Newton’s Laws

A Force is… A “push” or “pull” Measured in Newtons (N) in the metric Measured in pounds (lbs) in the English system A vector quantity – Direction & Magnitude Represented by drawing arrows on a diagram

Mass - Refresher The amount of matter an object is made up of. Measured in kilograms. NOT a force. The same everywhere.

Weight F = m a --> Fg = wt = m g -Weight always acts down! -The force of gravity acting on a mass. -Weight always acts down! Weight = mass (kg) * acceleration due to gravity F = m a --> Fg = wt = m g Weight is a force…so this is a special case of F=ma and the unit is a Newton.

Newton’s 1st Law Inertia An object at rest will stay at rest, and an object in motion will stay in motion at constant velocity unless acted on by an unbalanced force. Think  Seat Belts!! If you are NOT wearing your seatbelt and you get hit from behind, your body will propel forward hitting the windshield.

Newton’s 2nd Law Fnet = ma If an unbalanced force acts on a mass, that mass will accelerate in the direction of the force. Since 8N is greater than 2N, the unbalanced force is to the right so the acceleration is to the right. 2 N 8 N a What is the resultant force?

Newton’s 3rd Law Action - Reaction For every action force there is an equal and opposite reaction force. Example: If you punch a wall with your fist in anger, the wall hits your fist with the same force. That’s why it hurts! Action-reaction forces cannot balance each other out because they are acting on different objects.

Types of Forces Weight - force of gravity Normal force – surface pushing back Friction - resistance force Applied force - force you exert Tension - applied through a rope or chain Net force – total vector sum of all forces Balanced forces – equal and opposite Unbalanced forces – not equal and opposite

Normal Force (FN) Defined as the force of a surface pushing back on an object. Always directed perpendicular to the surface. This is a contact force. No contact…no normal force. NOT always equal to weight. See diagrams below. FN Examples: FN Wa l l Table

Friction A contact/resistance force is usually caused by two surfaces moving past each other. Always in a direction that opposes motion. Measured in Newtons Depends on surface texture and how hard the surfaces are pressed together (Fn  or Fg if sliding horizontally). AND Surface texture determines the coefficient of friction (μ) which has no units.

Types of friction Static friction is the force an object must overcome to start moving. Kinetic friction is the force an object must overcome to keep moving. Static friction is always greater than kinetic friction!

Calculating the Force of Friction Where f is the force of friction: 1. μ is the coefficient of friction 2. FN is the normal force

Additional info about μ static & μ kinetic reference data: μ static and μ kinetic are…. usually given for common surfaces in reference books However μ static & μ kinetic can be derived if the surfaces are not common.  

Take out your Formula Chart Let’s add the Force and Friction formulas to your chart.

Applying Force - Pressure P = F /A  F = P * A Pressure = Force / Area Pressure is measured in Pascal ( Pa ) 1 Pa = 1 N per m2 OR 1 lb per in2 ( psi ) ____Area , ____Pressure

Let’s work a Pressure Problem using your feet!!! 1. You need 1 piece of graph paper. 2. Place your foot in the center of the graph paper and trace around it. * you may leave your shoe on !!! 3. Count the # of squares on the graph paper contained within your footprint. 4. Divide your weight in lbs by the # of squares. 5. The answer will be Pressure ( psi ) ( lbs per in2 )

NEXT… Calculate and Compare your answer for PRESSURE standing on 1 foot to the PRESSURE of standing on 2 feet. What is your conclusion?? 2

Refresher on …. Free-Fall Acceleration 2 objects with different masses fall at the same time. The only Force acting equally on each is Gravity ( 10 m/s2 ) and we leave out air resistance. Here is a Formula you can use to remember this concept: F / m EX: Obj. 1 : 20 N / 2 kg = 10m/s2 Obj. 2 : 50 N / 5 kg = 10m/s2

Oh yeah – Air Resistance ! Its back! When we do introduce air resistance into Falling objects there are 2 things to consider. 1. when air resistance = weight of object NET FORCE = O = equilibrium = no accel. This is called reaching: TERMINAL SPEED 2. if concerned w/ DIRECTION – which is down for falling objects This is called reaching: TERMINAL VELOCITY

Free-body diagrams Free-body diagrams are used to show the relative magnitude and direction of all forces acting on an object.

There are 7 Primary Types of Forces. 1. Applied Force 2. Gravitational Force 3. Normal Force 4. Frictional Force 5. Air Resistance Force 6. Tension Force 7. Spring Force

What makes a free-body diagram FREE? It has only the lines of Force AND it has only the direction of Force There are no FORCE #’s & units!

Free-body diagrams cont. This diagram shows four forces acting upon an object. There aren’t always four forces, For example, there could be one, two, or three forces. Or more than four.

Problem 1 A book is at rest on a table top. Diagram the forces acting on the book.

Problem 1 In this diagram, there are normal and gravitational forces on the book.

Problem 2 An egg is free-falling from a nest in a tree. Neglect air resistance. Draw a free-body diagram showing the forces involved.

Gravity is the only force acting on the egg as it falls.

Problem 3 A flying squirrel is gliding (no wing flaps) from a tree to the ground at constant velocity. Consider air resistance. A free body diagram for this situation looks like…

Gravity pulls down on the squirrel while air resistance keeps the squirrel in the air for a while.

Problem 4 A rightward force is applied to a book in order to move it across a desk. Consider frictional forces. Neglect air resistance. Construct a free-body diagram. Let’s see what this one looks like.

Note the applied force arrow pointing to the right Note the applied force arrow pointing to the right. Notice how friction force points in the opposite direction. Finally, there is still gravity and normal forces involved.

Problem 5 A skydiver is descending with a constant velocity. Consider air resistance. Draw a free-body diagram.

Gravity pulls down on the skydiver, while air resistance pushes up as she falls.

Problem 6 A man drags a sled across loosely packed snow with a rightward acceleration. Draw a free-body diagram.

The rightward force arrow points to the right The rightward force arrow points to the right. Friction slows his progress and pulls in the opposite direction. Since there is not information that we are in a blizzard, normal forces still apply as does gravitational force since we are on planet Earth.

Problem 7 A football is moving upwards toward its peak after having been booted by the punter. Draw a free-body diagram.

The force of gravity is the only force described The force of gravity is the only force described. It is not a windy day (no air resistance).

Problem 8 A car runs out of gas and is coasting down a hill.

Even though the car is coasting down the hill, there is still the dragging friction of the road (left pointing arrow) as well as gravity and normal forces.

Net Force Now let’s take a look at what happens when unbalanced forces do not become completely balanced (or cancelled) by other individual forces. An unbalanced force exists when the vertical and horizontal forces do not cancel each other out.

Example 1 Notice the upward force of 1200 Newtons (N) is more than gravity (800 N). The net force is 400 N up.

Example 2 Notice that while the normal force and gravitation forces are balanced (each are 50 N) the force of friction results in unbalanced force on the horizontal axis. The net force is 20 N left.

Determine the Resultant Net Force for each Situation.