Force in Mechanical Systems

http://www.uhigh.ilstu.edu/tech/PT/http://www.uhigh.ilstu.edu/tech/PT/

Objectives TLW evaluate and predict what happens to an object when TLW evaluate and predict what happens to an object when

balanced and unbalanced forces are applied (TEKS 6.C)balanced and unbalanced forces are applied (TEKS 6.C) TLW measure mechanical force TEKS 6.D)TLW measure mechanical force TEKS 6.D)

Define force, and describe how forces are measured. Define force, and describe how forces are measured. Describe what happens when forces on an object are balanced Describe what happens when forces on an object are balanced

and when they are unbalanced. and when they are unbalanced. Explain the meaning of Newton's first law of motion.Explain the meaning of Newton's first law of motion. Define scalar, vector, weight, mass, and torque. Define scalar, vector, weight, mass, and torque. Determine the resultant force on an object when two or more Determine the resultant force on an object when two or more

forces act on it. forces act on it. Solve problems involving force, lever arm, and torque. Solve problems involving force, lever arm, and torque.

Ways to transmit mechanical forces!

Measuring Force Metric System (SI- Systems International)Metric System (SI- Systems International) EnglishEnglish

QuantityQuantity UnitUnit SymbolSymbol

LengthLength metermeter mm

MassMass kilogramkilogram kgkg

TimeTime secondsecond ss

Electric CurrentElectric Current ampereampere AA

TemperatureTemperature KelvinKelvin KK

Amount of substanceAmount of substance molemole molmol

Luminous intensityLuminous intensity candelacandela cdcd

Units

EnglishEnglish Length – foot (ft)Length – foot (ft) Force - Pound (lb) Force - Pound (lb)

SISI Length – meter (m) Length – meter (m) Force - Newton (N)Force - Newton (N)

Conversions

1 pound = 16 ounces

1 pound = 4.45 newtons

1 kilogram = 1000 grams

1 kg = 9.8 N = 2.2 lb

1 slug = 32.2 lb

1 slug = 14.59 kg

Definitions

ForceForce WeightWeight MassMass Vector – has magnitude and directionVector – has magnitude and direction Scalar – quantity described by only a Scalar – quantity described by only a

magnitude. (temp., time, pressure, mass)magnitude. (temp., time, pressure, mass)

Not the same thing

Amount of matter in an object

Takes into account gravity

Drawing Forces

Draw as an arrow.Draw as an arrow. Length of the arrow is proportional to the Length of the arrow is proportional to the

magnitude of the vector.magnitude of the vector. Head points in the direction of the vector.Head points in the direction of the vector.

20º

3”

1 inch = 20 lb.

Terms

Balanced forcesBalanced forces Unbalanced forcesUnbalanced forces Net force = sum of all forces (+ or -)Net force = sum of all forces (+ or -) Equilibrium – no net force, object in equilibriumEquilibrium – no net force, object in equilibrium Newton’s First Law of Motion (law of inertia)Newton’s First Law of Motion (law of inertia)

Every object will remain at rest, or will continue to Every object will remain at rest, or will continue to move in a straight line with constant speed, unless move in a straight line with constant speed, unless the object is acted on by a net force.the object is acted on by a net force.

Inertia – the property of an object to resist changes in Inertia – the property of an object to resist changes in its motion.its motion.

Adding Forces

1. Draw the 40 lb. Force

2. Add the 30 lb. Force to the 40 lb. Force (head to tail method)

3. Draw the resultant force

4. Determine magnitude and direction of resultant force

5. Form conclusion.

Example

Terms

Torque – the quantity that causes rotation in Torque – the quantity that causes rotation in mechanical systems. (cw or ccw)mechanical systems. (cw or ccw)

=F x L =F x L (Greek letter “tau”)(Greek letter “tau”)

Torque = Applied Force x Length of lever armTorque = Applied Force x Length of lever arm

=F x r =F x r (where r = radius for gears, etc.)(where r = radius for gears, etc.)

has units of foot-pounds or Newton-metershas units of foot-pounds or Newton-meters

Summary A force is a push or a pull. A force is a push or a pull. Force is a vector. It has both magnitude and direction. Force is a vector. It has both magnitude and direction.

Its magnitude is measured in pounds or in newtons. Its magnitude is measured in pounds or in newtons. Newton's first law says that an object will remain at Newton's first law says that an object will remain at

rest or will continue to move in a straight line unless rest or will continue to move in a straight line unless it is acted on by a net force. it is acted on by a net force.

Unbalanced forces result in a net force acting on an Unbalanced forces result in a net force acting on an object. Balanced forces result in no net force acting object. Balanced forces result in no net force acting on an object. on an object.

  If two forces act on an object and the forces act in a If two forces act on an object and the forces act in a straight line, the magnitude of the resultant is either straight line, the magnitude of the resultant is either the sum of or the difference between the two forces' the sum of or the difference between the two forces' magnitudes.magnitudes.

Summary (cont.) If two forces act on an object and the forces do not act in If two forces act on an object and the forces do not act in

a straight line, the resultant can be found using the head-a straight line, the resultant can be found using the head-to-tail method of vector addition. to-tail method of vector addition.

The mass of an object is a measure of the object's The mass of an object is a measure of the object's inertia. The weight is a measure of the force exerted on inertia. The weight is a measure of the force exerted on the object by gravity. the object by gravity.

A torque is exerted on a body when a force is applied A torque is exerted on a body when a force is applied and the line of action of the force does not pass through and the line of action of the force does not pass through the body's axis of rotation. The torque equals the force the body's axis of rotation. The torque equals the force times the lever armtimes the lever arm

  If no net torque is exerted on a body, it will remain at If no net torque is exerted on a body, it will remain at rest or will continue to rotate at a constant rate. rest or will continue to rotate at a constant rate.

(TLS # 16)

Practice Problems

Physics in ContextPhysics in Context textbook pages 23 – 26 textbook pages 23 – 26 (selected problems)(selected problems)