Forces & Forces & MotionMotionApplied Physics and Chemistry Force Applied Physics and Chemistry Force Lecture 1Lecture 1

www.hse.k12.in.us/staff/tjohnsonwww.hse.k12.in.us/staff/tjohnson

ForceForce

ForceForce a push or pull that one body exerts on anothera push or pull that one body exerts on another What forces are being What forces are being

exerted on the football?exerted on the football?

Fkick

Fgrav

ForceForce

Force is a vectorForce is a vector We need to know magnitude AND directionWe need to know magnitude AND direction

Two categories of forceTwo categories of force Contact forcesContact forces Field forcesField forces

Measuring ForcesMeasuring Forces

Forces are measured Forces are measured in newtons in newtons

(kg (kg .. m/s m/s22).). Forces can be Forces can be

measured using a measured using a spring scale.spring scale.

ForceForce Balanced ForcesBalanced Forces

forces acting on forces acting on an object that are an object that are opposite in opposite in direction and direction and equal in sizeequal in size

no change in no change in velocityvelocity

equilibriumequilibrium

ForceForce Net ForceNet Force

Sum of all the forces on an objectSum of all the forces on an object

If forces are unbalanced (not opposite If forces are unbalanced (not opposite and equal) velocity changes (object and equal) velocity changes (object accelerates)accelerates)

If forces are balanced, velocity does not If forces are balanced, velocity does not changechange

Ffriction

W

Fpull

Fnet

NN

ConcepTest 1ConcepTest 1

TRUE or FALSE?TRUE or FALSE?

The object shown in the diagram must be at rest The object shown in the diagram must be at rest since there is no net force acting on it.since there is no net force acting on it.

FALSE! A net force does not FALSE! A net force does not cause motion. A net force causes cause motion. A net force causes a a changechange in motion, or in motion, or acceleration.acceleration.

Taken from “The Physics Classroom” © Tom Henderson, 1996-2001.

ConcepTest 2ConcepTest 2You are a passenger in a car and not wearing your You are a passenger in a car and not wearing your seat belt. seat belt.

Without increasing or decreasing its speed, the car Without increasing or decreasing its speed, the car makes a sharp left turn, and you find yourself makes a sharp left turn, and you find yourself colliding with the right-hand door. colliding with the right-hand door.

Which is the correct analysis of the situation? ...Which is the correct analysis of the situation? ...

ConcepTest 2ConcepTest 21. Before and after the collision, there is a rightward 1. Before and after the collision, there is a rightward

force pushing you into the door. force pushing you into the door.

2. Starting at the time of collision, the door exerts a 2. Starting at the time of collision, the door exerts a leftward force on you. leftward force on you.

3. both of the above 3. both of the above

4. neither of the above4. neither of the above

2. Starting at the time of collision, the door 2. Starting at the time of collision, the door exerts a leftward force on you.exerts a leftward force on you.

Newton’s First LawNewton’s First Law

Newton’s First Law of MotionNewton’s First Law of Motion An object at rest will remain at rest and An object at rest will remain at rest and

an object in an object in motionmotion will continue moving will continue moving at a at a constant velocityconstant velocity unless acted upon unless acted upon by a by a net forcenet force..

Newton’s First LawNewton’s First Law Newton’s First Law of MotionNewton’s First Law of Motion

““Law of Inertia”Law of Inertia”

InertiaInertia tendency of an object to resist any change in its tendency of an object to resist any change in its

motionmotion increases as mass increasesincreases as mass increases