Unit 4 – Lecture 3

ForceForce: the influence that produces a change

in a physical quantityyour book: a push or a pullall 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” = Nwe will discuss what these are on another

slide

Force – cont’dForce: the influence that produces a change

in a physical quantitybalanced force =

equal but opposite forces = no movement [equilibrium]

Force – cont’dForce: the influence that produces a change

in a physical quantityunbalanced force =

unequal forces in opposite directions = movement / accelerationadd the forces mathematically

this square will be pushed with a net force of 20 Newtons to the left.

Force – cont’dShowing “equal and opposite” –

the observer determines the direction of motion left, right, degrees, etc.

directions opposite to that motion are NEGATIVEforces moving in the direction opposite to

the motion can also be considered NEGATIVE

PracticeWhat 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’dWhat 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

InertiaInertia – 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 motionobjects at rest stay at rest

Inertia – cont’dWhy don’t objects continue moving when we

push them over a surface?the force of friction

Sir Isaac Newtonexplained effects which

seemed to have no causerevolutionalized math &

science with his book, Principia

determined many physical laws based on math

Three Laws of Motion

Newton’s First LawEvery 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 Inertiaobjects in motion

stay in motionobjects at rest

stay at restthis is an inherent

property of matter

PracticeWhich 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

PracticeWhich 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

RecapSummarize Newton’s First Law with a

partner.

for every push, there is a pullif one or the other is greater,

there will be movementif there is movement,

there will continue to be movement until another force acts upon the object

Newton’s Second LawMass influences the inertia of an object –

but so does acceleration.

Newton’s Second Law:Force = Mass (multiplied by) AccelerationF = m *a

force = any push or pull that can affect motion

mass in kg acceleration in m/s2

Newton’s Second LawForce is measured in a unit called a Newton

1 N = 1 kg*m/s2 [mass *acceleration]

Example: What is the force applied by a mass of 2 kg with an acceleration of 6 m/s2?equation = F = m * a F = (2 kg) * (6 m/s2)F = 12 kg*m/s2 or more simlply... 12

N

PracticeWhat is the acceleration of a mass of 6 kg

applying a force of 24 N?4 m/s2

144 m/s2

0.25 m/s2

What is the force applied by the acceleration of a 3.5 kg mass to 7 m/s2? 2 N24.5 kg●m/s2

0.5 kg ●m/s2

PracticeWhat mass is required to apply a force of 28 N

after an acceleration of 7 m/s2? 196 kg4 kg0.25 kg

What acceleration would cause a 12 kg mass to produce a force of 90 kg●m/s2?7.5 N7.5 m/s2

7.5 m/s

Answer the Following…If you have…

a 55 kg couch moving to the right at 3 m/s2

a 1.5 kg cat flying to the left at 125 m/s2

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 kg3 m/s2

1.5 kg125 m/s2