Chapter 11Chapter 11ForcesForces

Newton’s Laws of Newton’s Laws of Motion (1Motion (1stst 2) 2)

GravityGravity Newton’s 3Newton’s 3rdrd Law Law

Newton’s Laws of MotionNewton’s Laws of Motion1st: An object at rest will remain at rest : An object at rest will remain at rest

or an object in motion will remain in or an object in motion will remain in motion at constant speed unless a motion at constant speed unless a net force acts upon it.net force acts upon it.

Examples: Examples: 1.1. A book at restA book at rest2.2. A person restrained by a seat belt A person restrained by a seat belt

after a car suddenly stopsafter a car suddenly stops3.3. The engine of a car being the net The engine of a car being the net

force to cause it to moveforce to cause it to move

Newton’s Laws, Cont.Newton’s Laws, Cont.2nd:2nd: The greater the force of an object, the The greater the force of an object, the

greater the acceleration. The greater greater the acceleration. The greater the mass of an object, the greater the the mass of an object, the greater the force needed to accelerate the object.force needed to accelerate the object.

Examples:Examples:1.1. More force is needed to pull a wagon full More force is needed to pull a wagon full

of bricks rather than an empty wagon.of bricks rather than an empty wagon.2.2. Lighter cars use less fuel.Lighter cars use less fuel.3.3. More force can cause balls to be thrown More force can cause balls to be thrown

farther.farther.

Newton’s Second Law (cont)Newton’s Second Law (cont)F=ma

m= F/am= F/a

a=F/ma=F/m

Units for force: Newtons!Units for force: Newtons!

One One newtonnewton is equal to the force is equal to the force required to accelerate a 1 kg required to accelerate a 1 kg object 1 m/sobject 1 m/s22

Section 2: GravitySection 2: GravityDefine: force of attraction Define: force of attraction between 2 objectsbetween 2 objects

Depends on:Depends on:–Mass of the objectsMass of the objects–Distance between the 2 Distance between the 2 objectsobjects

Falling Objects

Free Fall: the motion of a body when only Free Fall: the motion of a body when only the force of gravity is acting on it.the force of gravity is acting on it.The acceleration of a falling object due to The acceleration of a falling object due to the force of gravity between the object the force of gravity between the object and the Earth is: and the Earth is: 9.8 m/s2

The final speed or time that it takes for The final speed or time that it takes for objects to fall can be calculated:objects to fall can be calculated:

VVff=at=at t=vt=vff/a/a use 9.8m/suse 9.8m/s22

for (a)for (a)

Air ResistanceAir ResistanceDo all objects continue to accelerate Do all objects continue to accelerate until they hit the ground?until they hit the ground?

Air resistanceAir resistance opposes downward opposes downward motion.motion.

Some objects are affected more than Some objects are affected more than others depending on their shape.others depending on their shape.

Once an object stops accelerating, it Once an object stops accelerating, it reaches reaches terminal velocity. . Gravity and air resistance are equal are equal at this point.at this point.

Projectile Motion and Gravityand Gravity

Projectile motion is the curved path of an Projectile motion is the curved path of an object that is thrownobject that is thrown2 components – 2 components – vertical and horizontalThe downward accelerations of a thrown The downward accelerations of a thrown object and a falling object are identical object and a falling object are identical (see page 359).(see page 359).You should aim above targets for objects You should aim above targets for objects in projectile motion.in projectile motion.

Weight and MassWeight and MassWeightWeight is the measure of the force of is the measure of the force of gravity on an object.gravity on an object.Measured in newtonsMeasured in newtonsEquation:Equation:

FFww=ma=ma (a=acceleration (a=acceleration due to gravity)due to gravity)

Remember! Mass is the amount of Remember! Mass is the amount of matter in an object and cannot matter in an object and cannot change… (measured in kg)change… (measured in kg)

Newton’s Laws, cont.Newton’s Laws, cont.3rd: For every action, there is an equal : For every action, there is an equal

but opposite reaction – all forces but opposite reaction – all forces come in pairs.come in pairs.

Examples:Examples:1. Stepping off a canoe while pushing, while pushing,

will cause the canoe to move back.will cause the canoe to move back.2.2. Pushing on a deskPushing on a desk3.3. The reaction engine of a rocketThe reaction engine of a rocket

Summary of Newton’s LawsSummary of Newton’s Laws

1.1.InertiaInertia

2.2.F=maF=ma

3.3.Action-Reaction Action-Reaction forcesforces

MomentumMomentum

A quantity that is the product of the A quantity that is the product of the mass and velocity of an objectmass and velocity of an object

Momentum=mass x velocity or Momentum=mass x velocity or p=mvp=mv

Momentum is conserved.Momentum is conserved.

Momentum is transferred.Momentum is transferred.

Examples:Examples: