# Forces and Newton’s Laws of Motion

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Forces and Newton’s Laws of Motion
Sections 4.1 – 4.3

4.1 The Concepts of Force and Mass
Force  Push or Pull Contact Forces  pulling on rope, pushing on ball Non-contact Forces  gravity pulling object Mass  How much matter Sir Isaac Newton studied force and mass Made Newton’s Laws of Motion Have students give more examples of contact and noncontact forces.

4.2 Newton’s First Law of Motion
Ice Hockey When nothing happening puck sits on ice After hit, puck moves in straight line until hits something

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)

Net Force Net Force  Vector sum of all forces
Net Force required to change velocity All forces often include friction, air resistance, etc.

Inertia and Mass Takes more force to change velocity of some objects
Inertia  tendency to stay at rest or in constant motion Mass  measurement of inertia Kilogram (kg) Show Eureka! Video (2. Mass) Example: Marble and Bowling Ball

Physics of Seat Belts Locking 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.

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.

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 acceleration Show Eureka! Video (3. Speed, 4. Acceleration 1) Remember that net force is the sum of all the forces acting on the object.

Newton’s Second Law Components
Force Mass Force  more force make more acceleration Mass  more mass, more force required to change velocity

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

Newton’s Second Law of Motion
F = ma

Force External force Come from the environment Internal force
One part of object pushing (or pulling) another part of object

Unit of Force Newton (N) Force = mass x acceleration N = kg x m /s2
N = kg m /s2 This is a derived unit (not a base unit)

Calculations with Newton’s 2nd Law
F needed Free-body diagram Draw only forces acting on the object Represent the forces are vector arrows

Example Four 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 diagram A = -200 N C = +304 N B = -278 N D = +189 N

Example What is the net force, acceleration of the 5 kg rope, and who wins the game? F = +15N a= 3 m/s2 Charlie and Dannie win! F = -200 N N = +15 N F = ma  15 N = 5 kg (a)  a = 3 m/s2 A = -200 N C = +304 N B = -278 N D = +189 N

Practice Work You should be able to try 115 #CQ 1- 5, P 1- 7
Total of 13 problems