Chapter 4 Dynamics: Newton’s Laws of Motion

Objectives Apply the Law of Inertia to explain physical phenomena. Compare and contrast weight and mass. Use mass and weight units.

Units of Chapter 4 Force Newton’s First Law of Motion Mass Newton’s Second Law of Motion Newton’s Third Law of Motion Weight – the Force of Gravity; and the Normal Force

Solving Problems with Newton’s Laws: Free-Body Diagrams Applications Involving Friction, Inclines Problem Solving – A General Approach Units of Chapter 4

Newton’s Contributions Calculus Light is composed of rainbow colors Reflecting Telescope Laws of Motion Theory of Gravitation

4-1 Force A force is a push or pull. An object at rest needs a force to get it moving; a moving object needs a force to change its velocity. The magnitude of a force can be measured using a spring scale.

4-2 Newton’s First Law of Motion Newton’s first law is often called the law of inertia. Every object continues in its state of rest, or of uniform velocity in a straight line, as long as no net force acts on it.

Demo – 1 st law Period 1 stopped here

Forces may be balanced or unbalanced Balanced forces – all forces acting on an object are equal –There is NO MOTION Unbalanced forces – one or more forces acting on an object are stronger than others –There is MOTION A NET FORCE

Balanced Force Equal forces in opposite directions produce no motion

Unbalanced Forces Unequal opposing forces produce an unbalanced force causing motion

If objects in motion tend to stay in motion, why don’t moving objects keep moving forever? Things don’t keep moving forever because there’s almost always an unbalanced force acting upon them. A book sliding across a table slows down and stops because of the force of friction. If you throw a ball upwards it will eventually slow down and fall because of the force of gravity.

4-2 Newton’s First Law of Motion Inertial reference frames: An inertial reference frame is one in which Newton’s first law is valid. This excludes rotating and accelerating frames.

15 Inertial frames of reference Inertial reference frames are reference frames in which Newton’s laws hold From Newton’s 1 st law: –If an object experiences no net force, the object either remains at rest or continues with constant speed in a straight line The Earth is not an inertial frame (it rotates) but is close enough to be considered an inertial frame

16 Inertial frames of reference Inertia is a property of matter It is that property of matter which opposes changes in velocity So, one must centre one's physics reasoning on these thoughts: –An object's velocity will not change all on its own. –Pushes, or pulls, are necessary to change an object's velocity.

17 Inertial frames of reference There are several ways to describe an inertial frame. Here are a few descriptions: –An inertial frame of reference is a frame of reference with constant velocity. –An inertial frame of reference is a non-accelerating frame of reference. –An inertial frame of reference is a frame of reference in which the law of inertia holds. –An inertial frame of reference is a frame of reference in which Newton's laws of motion hold. –In an inertial frame of reference no fictitious forces arise.

First Law  Newton’s first law says that with no net force there is no acceleration. Objects at restObjects at rest Objects at constant velocityObjects at constant velocity  If there is no observed acceleration on an object with no net force, the observer is in an inertial reference frame. Newton’s laws of mechanics apply equallyNewton’s laws of mechanics apply equally No absolute motionNo absolute motion

Inertial Frame  An observer on the table sees two ball fall. First straight down Second in a parabola  An observer with speed v x0 sees the reverse. Second straight down First in a backwards parabola  Both frames are inertial. Motion consistent with Newton

Accelerated Frame  A rotating observer throws a ball across a merry-go-round. Ball veers to the side No external force  This is a non-inertial frame. Observed motion inconsistent with Newton’s laws Fictitious forces

He believed that there existed an absolute (not accelerating) reference frame, and an absolute time. The laws of physics are always the same in any inertial reference frame. His laws applied only when measurements were made in this reference frame….. Newton clarified the mechanics of motion in the “ real world ”. … or in any other reference frame that was at rest or moving at a constant velocity relative to this absolute frame. Inertial reference frame

4-3 Mass Mass is the measure of inertia of an object. It is the measure of the amount of matter in an object. In the SI system, mass is measured in kilograms. Mass is not weight: Mass is a property of an object. Weight is the force exerted on that object by gravity. If you go to the moon, whose gravitational acceleration is about 1/6 g, you will weigh much less. Your mass, however, will be the same.

Mass: A measure of Inertia Kick the can. Kick the same can full of soda. Kick the same can full of concrete. Mass is not volume! Mass is not weight! The more mass an object has, the greater its inertia and the more force it takes to change its state of motion.

Objects at Rest Objects at rest tend to stay at rest unless acted upon by a force. [push or pull] Newton described this tendency as inertia. Inertia can be described as the tendency of an object to keep doing whatever ’ s it ’ s doing.

Mass & Inertia Mass is the amount of matter in an object. The more MASS an object has, the more INERTIA the object has. Bigger objects are harder to start & stop Which vehicle has more inertia? Slide from class.netwww.science- class.net

Newton ’ s 1 st Law (also known as the law of inertia) A moving object moves in a straight line with constant speed unless a force acts on it. The tendency of an object at rest to remain at rest and an object in motion to remain in motion unless acted upon by an unbalanced force. Objects do not change their motion unless a force acts on them

The truck is in motion. What is the force that causes it to stop? The push of the stopped car. The car is at rest. What is the force that causes it to move? The push of the truck. Slide from class.netwww.science- class.net

Newton’s First Law (law of inertia) INERTIA is a property of an object that describes how ______________________ the motion of the object more _____ means more ____ much it will resist change to massinertia

1 st Law Unless acted upon by an unbalanced force, this golf ball would sit on the tee forever.

There are four main types of friction: –Sliding friction: ice skating –Rolling friction: bowling –Fluid friction (air or liquid): air or water resistance –Static friction: initial friction when moving an object What is this unbalanced force that acts on an object in motion?

1 st Law Once airborne, unless acted on by an unbalanced force (gravity and air – fluid friction) it would never stop!

Inertia

Newton ’ s First Law When the motorcycle stops, the rider continues his motion.

Terminal Velocity

Exploration Mass and Weight Lab (No lab write up.)

Homework Newton’s 1 st law hand-out Questions p. 97 1, 2

Closure When the pellet fired into the spiral tube emerges, which path will it follow? (neglect gravity)

Answer While in the tube, it is forced to curve, but when it gets outside, no force is exerted on the pellet and (law of inertia) it follows a straight – line path…B!

Elaboration Computer Activity for Newton’s First Law ewtlaws/u2l1a.cfmhttp:// ewtlaws/u2l1a.cfm

Inertia Evaluation Carpenters use different hammers for different jobs. A tack hammer is a light hammer used for small nails in delicate situations. A regular hammer is heavier and is used for larger nails in building walls. Compare the two hammers using Newton’s 1 st law in terms of the hammers’ function, advantages and disadvantages.