Presentation on theme: "Forces and Newton’s Laws of Motion"— Presentation transcript:
1 Forces and Newton’s Laws of Motion Sections 4.1 – 4.3
2 4.1 The Concepts of Force and Mass Force Push or PullContact Forces pulling on rope, pushing on ballNon-contact Forces gravity pulling objectMass How much matterSir Isaac Newton studied force and massMade Newton’s Laws of MotionHave students give more examples of contact and noncontact forces.
3 4.2 Newton’s First Law of Motion Ice HockeyWhen nothing happening puck sits on iceAfter hit, puck moves in straight line until hits something
4 Newton’s First Law of Motion An object continues in a state of rest or in a state of motion at a constant speed along a straight line, unless compelled to change that state by a net force.Show Eureka! Video (1. Inertia)
5 Net Force Net Force Vector sum of all forces Net Force required to change velocityAll forces often include friction, air resistance, etc.
6 Inertia and Mass Takes more force to change velocity of some objects Inertia tendency to stay at rest or in constant motionMass measurement of inertiaKilogram (kg)Show Eureka! Video (2. Mass)Example: Marble and Bowling Ball
7 Physics of Seat BeltsLocking mechanism consists of ratchet wheel with belt wrapped around it and a pendulum attached to a locking bar.Normally the pendulum hangs straight down and the locking bar is out of the way of the ratchet wheel.When the car stops suddenly, the pendulum continues moving forward, so the pendulum swings on its pivot.This pushes the locking bar into the ratchet wheel so it can’t turn.
8 An Inertial Reference Frame Reference frame in which Newton’s law of inertia is valid.Noninertial frame reference frames that are accelerating.For example you are in a car that suddenly accelerates. You don’t move compared to the car, but there is a net force pushing on you.
9 4.3 Newton’s Second Law of Motion What happens when a net force is applied to a hockey puck?Accelerates (changes velocity)More net force More accelerationShow Eureka! Video (3. Speed, 4. Acceleration 1)Remember that net force is the sum of all the forces acting on the object.
10 Newton’s Second Law Components ForceMassForce more force make more accelerationMass more mass, more force required to change velocity
11 Newton’s Second Law of Motion When a net external force F acts on an object of mass, m, the acceleration, a, that results is directly proportional to the net force and has a magnitude that is inversely proportional to the mass. The direction of the acceleration is the same as the direction of the net force.OR
13 Force External force Come from the environment Internal force One part of object pushing (or pulling) another part of object
14 Unit of Force Newton (N) Force = mass x acceleration N = kg x m /s2 N = kg m /s2This is a derived unit (not a base unit)
15 Calculations with Newton’s 2nd Law F neededFree-body diagramDraw only forces acting on the objectRepresent the forces are vector arrows
16 ExampleFour people are having a tug-o-war game. Ashley pulls left with 200 N, Bert pulls left with 278 N, Charlie pulls right with 304 N, and Dannie pulls right with 189 N. What is the net force, acceleration of the 5 kg rope, and who wins the game?Free-body diagramA = -200 NC = +304 NB = -278 ND = +189 N
17 ExampleWhat is the net force, acceleration of the 5 kg rope, and who wins the game?F = +15Na= 3 m/s2Charlie and Dannie win!F = -200 N N = +15 NF = ma 15 N = 5 kg (a) a = 3 m/s2A = -200 NC = +304 NB = -278 ND = +189 N
18 Practice Work You should be able to try 115 #CQ 1- 5, P 1- 7 Total of 13 problems