FORCESPhysics 1:

CHHS Physics;Mr.Puckett

What is a Force? A Force is a push or a pull on an object. Forces are vectors because they have

magnitude and direction. Force is equitable to energy in later

chapters. Make a list of all the forces you can think

of…. Think back to the elementary playground – your first physics lab.

The 4 Forces in Nature: There are four basic forces in

nature:1. Gravity- the attractive force between

all matter in the universe.2. Electromagnetic – the forces of

attraction in atoms and magnets.3. The Strong nuclear force that keeps the

nucleus together.4. The Weak nuclear force that causes

radioactive decay.

Dynamics and Statics Dynamics is the study of the effects of

forces on matter in motion. When two objects interact, forces are the cause of the interaction. This includes a baseball being hit by a bat and a rocket ship blasting off.

Statics is the study of forces acting on objects in equilibrium (stationary). Hanging signs are examples here. Both are major engineering areas of study.

Newton's First Law of Motion – The Law of Inertia The name for this tendency is INERTIA - the

tendency for an object to remain in the same state of motion (or rest).

An object will continue in its state of rest or uniform motion in a straight line unless an outside unbalanced force acts upon it. When forces are balanced, there is no acceleration.

All situations on Earth’s surface are called inertial reference frames. A non-inertial reference frame is one in which an object is in freefall or accelerating and may not obey the law precisely.

Newton’s Second Law of Motion: Force Acceleration One of the most important formulas in all of

physics is F = ma . This says that the summation of the forces on an object is directly proportional to the acceleration of the object. It says if you push hard on a basketball it goes fast and when you push easy, it goes slow….. You already knew that !

Force is a vector quantity and is the net force of the sum of all acting forces in all directions. The acceleration will vary directly with the net applied force and inversely with the mass.

Newton’s Third Law of Motion: Forces in Pairs. Forces in Pairs states that every

balanced force in nature is accompanied by an equal and opposite force. Forces always are in pairs between pairs of objects. The action and reaction forces act on different bodies.

 Examples are a rocket blasting off, a hang glider flying in air and you sitting in your desk. You push down on the chair and it pushes back up on you.

Units of Force: The Newton A force is measured in terms of the acceleration

it gives a standard mass. The SI unit of force in physics is named after the father of physics; Newton. The Newton (N) is defined as the force that will cause a 1.00 kg mass to accelerate at 1.00 meter per second squared.

Proof: F = m a =(1.00 kg)(1.00 m/s2) = 1 kg x m/s2

= 1 N

Weight and Mass: Mass is a measure of the inertia of a body. It

depends upon the amount of matter contained within an object. The relationship between mass and inertia is direct. The unit of measurement for mass is the Kilogram (kg). Formula: m = F/a. Mass is CONSTANT.

Weight is the gravitational force exerted object. Weight is a vector force and a negative symbol shows force in a downward direction. A medium apple weighs about one Newton. Formula: W (weight) or Fg = m x g. Weight changes according to where you are and the acceleration of gravity changes (outer space).

Ways to Measure Mass a. Triple beam balance (or electronic version)

will compare the weights of two objects. One weight is placed on the pan and then the masses on the beams are adjusted to equal the force on both sides of a pivot point.

b. A second method to measure mass is that a known force is applied to an object on a frictionless surface and then the acceleration is measured. The formula used is : mass = Force divided by acceleration. m = F/a

Free Body DiagramsFree Body

Diagrams are pictures of how forces act upon an object or system.

W is the weight and Fn is the normal force.

T is tension.

The Normal Force is the Earth’s response to Weight.

Newton’s 3 law of action- reaction forces can be shown in the Normal force of the Earth pushing up against our weight.

Typical Problems for Forces:

Vertical Forces in an Elevator

Note how the motion increases and decreases the apparent weight on the scale.

Elevator Vertical Analysis

The “UP” side.

The “Down” side.

Tension in Ropes and Life Tension in physics is the forces we

put through ropes – not the tests we give.

Vector Analysis of Pulling a Sled with a 25o Angle

Force and Acceleration of an 80 kg Sled pulled at 25o with 150N

Tension analysis of a Sled

Forces on a Hanging Sign Note how the

two ropes have unequal angles. This means they will support unequal amounts of the weight of the sign.

Analysis of a Hanging Sign

Frictional Force Friction is a force that opposes the motion of two

objects that are touching each other. It does this by creating temporary electromagnetic forces between the contact points of the two surfaces.

Friction acts in a direction parallel to the surfaces in contact and opposing the motion. The force exerted by a two surfaces touching is called a contact force. When a contact force acts perpendicular to the common surface of contact it is called a Normal Force (meaning perpendicular). Friction is independent of the areas of the surfaces in contact but is directly proportional to the mass.

Friction Opposes Motion Note that the

vectors of force / motion are opposite from friction.

Types of Frictional Force 1.        Static friction is the force of friction that

resists the start of motion. This is always greater than the sliding frictional force.

2. Sliding friction (also called Kinetic Friction) is the force that resists the existing or continuing motion.

3. Air resistance is a special case of sliding friction. This is due to air molecules colliding with a moving object. Ff air v2. The air friction force is a square function of the velocity and in reality a function of the air density altitude. We ignore it in this course.

Friction in Life Kinetic Friction

is opposing continued motion.

Static friction is opposing the starting of motion.

Relationship between Static and Kinetic Friction.

What Causes Friction? Rough surfaces-

a side view of smooth steel.

Electrostatic attraction between atoms.

Steps to Solve Force Problems

a.       Read and label the problem. b.       Draw an accurate diagram (free body diagram).

Show all forces exerting force on the object. c.       Choose a convenient x-y coordinate system for

resolution of vectors. d.       Determine knowns and unknowns to assist in

choosing equations. e.       Solve it roughly to get an idea of the range of

reasonable answers. f.        Solve with equations. g.       Keep track of units throughout the equation and

check the answer.

Special Case Problem: The Inclined Plane Please note the special case with the skier

going downhill (Inclined plane) . When the axis is chosen for vector resolution it is slanted to a normal Cartesian system. This means that the vertical and horizontal components in the vector switch trig functions: Fx (parallel to

surface) = mg sin and Fy

(perpendicular to surface) = mg cos . This switch allows us to identify the force causing the sliding down force as the vertical sine.

The Inclined Plane Note that

when you have an incline that the normal force is no longer directly opposite to weight

Analysis of an Inclined Plane

Now Add Friction to an Inclined Plane.

Analysis of Friction on an Inclined Plane. Note the

change in axis for vertical and horizontal in this special case.

Inclined Plane with Pulley

InclinedPlanes withPulley Analysis

May the Force Be With You

Mr. Puckett flying from Clinch Mountain Tennessee 1979