Force in Mechanical Systems

Objectives

 Define force, and describe how forces are measured.  Describe what happens when forces on an object are balanced and when they are unbalanced.  Explain the meaning of Newton's first law of motion.

 Define scalar, vector, weight, mass, and torque.  Determine the resultant force on an object when two or more forces act on it.  Solve problems involving force, lever arm, and torque.

Ways to transmit mechanical forces!

Measuring Force

 Metric System (SI- Systems International)  English Quantity Unit Symbol Length meter m Mass Time Electric Current Temperature Amount of substance Luminous intensity kilogram second ampere Kelvin mole candela kg s A K mol cd

Units

 English  Length – foot (ft)  Force - Pound (lb)  SI  Length – meter (m)  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

Takes into account gravity  Force    Weight Not the same thing Mass Amount of matter in an object Vector – has magnitude and direction  Scalar – quantity described by only a magnitude. (temp., time, pressure, mass)

Drawing Forces

 Draw as an arrow.

 Length of the arrow is proportional to the magnitude of the vector.

 Head points in the direction of the vector.

1 inch = 20 lb.

20º

Terms

 Balanced forces  Unbalanced forces  Net force   Equilibrium Newton’s First Law of Motion (law of inertia)  Every object will remain at rest, or will continue to move in a straight line with constant speed, unless the object is acted on by a net force.

 Inertia – the property of an object to resist changes in 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 5. Form conclusion.

Example

Terms

 Torque – the quantity that causes rotation in  mechanical systems. (cw or ccw)  =F x L (Greek letter “tau”) Torque = Applied Force x length of lever arm

 A force is a push or a pull.

Summary

 Force is a vector. It has both magnitude and direction. Its magnitude is measured in pounds or in newtons.  Newton's first law says that an object will remain at rest or will continue to move in a straight line unless it is acted on by a net force.  Unbalanced forces result in a net force acting on an object. Balanced forces result in no net force acting on an object.  If two forces act on an object and the forces act in a straight line, the magnitude of the resultant is either the sum of or the difference between the two forces' magnitudes.

Summary (cont.)

 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 to-tail method of vector addition.  The mass of an object is a measure of the object's inertia. The weight is a measure of the force exerted on the object by gravity.  A torque is exerted on a body when a force is applied and the line of action of the force does not pass through the body's axis of rotation. The torque equals the force times the lever arm  If no net torque is exerted on a body, it will remain at rest or will continue to rotate at a constant rate. (TLS # 16)