Unit 4 – Lecture 3

Force Force: the influence that produces a change in a physical quantity your book: a push or a pull all objects exert forces on each other if more force is applied to an object than the object can provide, the object is moved. net force = total force on an object measured in units called “Newtons” = N we will discuss what these are on another slide

Force – cont’d Force: the influence that produces a change in a physical quantity balanced force = equal but opposite forces = no movement [equilibrium]

Force – cont’d Force: the influence that produces a change in a physical quantity unbalanced force = unequal forces in opposite directions = movement / acceleration add the forces mathematically this square will be pushed with a net force of 20 Newtons to the left.

Force – cont’d Showing “equal and opposite” – the observer determines the direction of motion left, right, degrees, etc. directions opposite to that motion are NEGATIVE forces moving in the direction opposite to the motion can also be considered NEGATIVE

Practice What would the net force be if you pushed against the wall with 122,616N? 122,616 N -122,616 N 0 N What would the net force be if two players kick a soccer ball from opposite directions according to the diagram? 60 N, to the left 60 N, to the right 0 N 180 N, upwards

Practice – cont’d What would the net force be if you pushed a hockey puck with 200 N? 0 N 200 N against the push 200 N in the direction of the push

Inertia Inertia – tendency of an object to resist change in its motion [another contribution of Galileo] Law of Inertia: objects will remain at the same speed and direction [velocity] unless some other force acts on it objects in motion stay in motion objects at rest stay at rest

Inertia – cont’d Why don’t objects continue moving when we push them over a surface? the force of friction

Sir Isaac Newton explained effects which seemed to have no cause revolutionalized math & science with his book, Principia determined many physical laws based on math Three Laws of Motion

Newton’s First Law Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it. a restatement of Galileo’s Law of Inertia objects in motion stay in motion objects at rest stay at rest this is an inherent property of matter

Practice Which has more inertia.. an empty dump truck OR a full dump truck? a quarter OR a dime? a freight train OR a full dump truck? a moving car or a stopped car? So…is the inertia of an object dependent on its mass? YES

Practice Which has more inertia.. a moving car or a stopped car? a moving plane or a stopped freight train? So…is the inertia of an object dependent on its acceleration? YES

Recap Summarize Newton’s First Law with a partner. for every push, there is a pull if one or the other is greater, there will be movement if there is movement, there will continue to be movement until another force acts upon the object

Newton’s Second Law Mass influences the inertia of an object – but so does acceleration. Newton’s Second Law: Force = Mass (multiplied by) Acceleration F = m *a force = any push or pull that can affect motion mass in kg acceleration in m/s 2

Newton’s Second Law Force is measured in a unit called a Newton 1 N = 1 kg*m/s 2 [mass *acceleration] Example: What is the force applied by a mass of 2 kg with an acceleration of 6 m/s 2 ? equation = F = m * a F = (2 kg) * (6 m/s 2 ) F = 12 kg*m/s 2 or more simlply N

Practice What is the acceleration of a mass of 6 kg applying a force of 24 N? 4 m/s m/s m/s 2 What is the force applied by the acceleration of a 3.5 kg mass to 7 m/s 2 ? 2 N 24.5 kg ● m/s kg ● m/s 2

Practice What mass is required to apply a force of 28 N after an acceleration of 7 m/s 2 ? 196 kg 4 kg 0.25 kg What acceleration would cause a 12 kg mass to produce a force of 90 kg ● m/s 2 ? 7.5 N 7.5 m/s m/s

Answer the Following… If you have… a 55 kg couch moving to the right at 3 m/s 2 a 1.5 kg cat flying to the left at 125 m/s 2 Which has the greater force/inertia? 55*3 = 165N (couch) vs.15*125 = 187.5N (cat) If one collided with the other, what would be the net force? 187.5N – 165N = 22.5N to the left 55 kg 3 m/s kg 125 m/s 2