Acceleration & Momentum

Newton’s 2 nd Law of Motion states “a net force acting on an object causes an object to accelerate in the direction of the force” In other words….

if you push or pull an object, it will move in whatever direction you push it or pull it:

How much will it move (accelerate)? That depends on two things-

1.) the MASS of the object And… 2.) the amount of FORCE that is applied

The amount of force can be calculated using the formula: force = mass x acceleration or… f = ma

force is measured in (You guessed it) Newtons (n) (Yea, him again. I told you he was important…)

By the way, because MASS is measured in kilograms (kg) and ACCELERATION is measured in meters per second, squared (m/s 2 ) then… 1 Newton = 1kg x m/s 2

For example… How much force is placed on a 25kg object accelerating at 4 m/s 2 ? F = ma 25kgx4 m/s 2 = =100 kg x m/s 2 = 100 N

BTW- (and you probably learned this in math class… Formulas can be re-arranged if you need to find the other variables So... Since f=ma then...

If you want to find the MASS, the formula would be... M = FAFA A = F M If you want to find the ACCELERATION, the formula would be... The unit for mass is GRAMS (g) The unit for acceleration is METERS PER SECOND, SQUARED (m/s 2 )

So how do I know which formula to use? Yes we did. So what? Hey wait a minute… Didn’t we ALREADY learn a formula for acceleration? Use the Force, and look at the information you’re given.

If the problem talks about speeds and times, use this formula: FS- OS A= t If the problem talks about forces and masses, use this formula: F A= M In other words…

Falling Objects

In a vacuum, any object dropped will accelerate towards the earth at the same rate- 9.8 m/s 2 Question- why does the number “9.8” sound familiar?

Because… In the last chapter, you learned that one kilogram of mass on earth weighs 9.8N Here’s why: f = ma =1kgx9.8 m/s 2 = 9.8 kg x m/s 2 or 9.8N

In the “real world”, objects don’t fall at the same rate. Why not? Air Resistance the force air exerts on a moving object

this force always acts in the opposite direction of the motion: OBJECT  gravity   air resistance 

the amount of air resistance on an object depends on the SPEEDSIZE and of the object DENSITY SHAPE

as an object falls, it will continue to go fasterand faster go any faster until the AIR RESISTANCEis equal to the pull of GRAVITY.At that point, the object will notandcannot

Terminal Velocity  the highest velocity reached by a falling object air resistance = gravity occurs when

Gravity Air Resistance = Terminal Velocity

Projectile & Circular Motion

Projectile- - anything that is shot or thrown through the air examples include:

bullets balls rockets arrows

Projectiles do NOT travel in a straight line this is because… Projectiles have two types of motion:

Horizontal Motion - motion that is parallel (  ) to the ground - usually caused by the throwing or shooting action

Vertical Motion - motion that is perpendicular (  ) to the ground - usually caused by gravity Gravity, I came up with that!

horizontal and vertical motion work independently of each other By the way… In other words- the horizontal motion of an object has no effect on its vertical motion and vice versa Latin for, “and the opposite is also true”. vice versa

Motion Along Curves

As an object travels around a curve (or in a circle), two new forces will begin to act upon it:

1.) Centripetal Force - Motion towards the center of a curved or circular path - usually caused by friction

2.) Centrifugal Force - Motion towards the outside of a curved or circular path - usually due to inertia

centrifugal force

for example… centripetal force centrifugal force

Weightlessness & Freefall

To be truly “weightless”, an object must be free from the gravitational pull of all objects (earth, sun, etc.). This cannot happen in our solar system! You are here

The appearance of weightlessness in space is due to a combination of two things: 2.) lack of an opposing force 1.) gravity

for example… 17,500 mph

inertia gravity

Freefall- when an object is influenced only by gravity

Force Pairs Remember from the last chapter that Newton described these in… which are called “action-reaction” pairs. forces always act in pairs

Newton’s 3 rd Law of Motion Me again! “ When one object exerts a force on a second object, the second one exerts a force on the first that is equal in size and opposite in direction” in other words…

For every action, there is an EQUAL and OPPOSITE reaction ACTION REACTION

Examples: ACTIONREACTION Me hitting you You hitting me back Air coming out of a balloon Balloon flying away Bullet coming out of a gun Gun “kicking back”

Wait a minute… If these forces are EQUAL in size and OPPOSITE in direction, doesn’t that make them BALANCED forces? No, and here’s why…

the action and reaction forces are acting on different objects, so the forces are not usually “balanced”. This is what causes motion to occur. Action force applied here Reaction force applied here

Momentum - a property of moving objects - can be calculated with the formula - based on its mass and velocity

Momentum = mass x velocity or p = mv Question- Why do we use “p” to represent momentum? Answer- because the letter “m” is already being used for mass

- the unit of measure for momentum is “kg x m/s” - the momentum of an object can be increased (or decreased) by changing either the mass or the velocity (or both) - an object that is not moving is said to have “zero momentum” - the more momentum an object has the harder it is to stop

The Law of Conservation of Momentum “The total amount of momentum of a group of objects will not change unless an outside force acts upon the objects” For example- P = 1000 kg x m/s

However… Momentum can be transferred between objects within a group