Chapter 12Forces

http://www.youtube.com/watch?v=8zsE3mpZ6Hwhttp://www.youtube.com/watch?v=4NQbeZ0EXZQhttp://www.youtube.com/watch?v=bkivr6bB-og&feature=related

Newton’s First Law of Motion

1st Law of Motion(Law of Inertia)

-Objects in motion will stay in motion in a straight line and objects at rest will stay at rest, unless acted on by an outside force

-Ex. Stack of books on a car seat. What happenswhen you suddenly stop?

Inertia - objects resistance to a change in motion

Mass affects an objects inertia

Increase mass =

Lower mass =

Increase inertia

Lower inertia

Seat belts and inertia

Seat belts work because they keep your body from flying forward.

If you get hit from behind will seatbelts or air bags help?

No, you will fly back into the seat.

Newton’s Second Law of Motion

Force is equal to the mass of an object and its acceleration.

Force = Mass x AccelerationF = m x a

Force is measured in Newtons1 N = 1 kg x 1 m/s2 or 1kgm/s2

( 1 lb is = to 4.45 N or 1 N is = to .225 lb)

Newton’s Second Law of Motion

Newton’s second law explains how acceleration is dependant on force and mass.

Newton’s Second Law of Motion

Newton’s Second Law of Motion

Newton’s second law could also be written as: - When a force is applied to an object, the object accelerates in the direction of the greater force.

OR The acceleration of an object is directly

proportional to the net force on the object and inversely proportional to the object’s mass.

Newton’s Second Law of Motion

Acceleration = net force/mass or a = F/m

Gravity

Gravity - Force of attraction between two objects.

- force of gravity depends on two variables:

a. depends on mass

-larger = more gravity (attraction)- smaller = less gravity (attraction)

Gravity

- force of gravity depends on two variables:(continued)

b. depends on distance

-closer = more gravity (attraction)

- further = less gravity (attraction)

WeightA measurement that can be taken because of

gravity is weight.

Weight – force of gravity pulling on the mass of an object.

- weight is a force that can change. Mass never changes!

Free Fall

when only gravity is acting on a falling object.constant rate of acceleration. This is called

gravitational acceleration (g)

* g = 9.8 m/s²

- which means everything accelerates as it falls at the rate of 9.8 m/s² each second!

Free Fall and Weight

We can use this knowledge to mathematically find weight!

Equation to find weight is same as force!

Force (weight) = m a or weight = m X g (acceleration of gravity)

WeightEx. Find the weight of a 58 kg man on

Earth.Weight = mg

Weight = 58kg x

Now find his weight on:Venus g=8.8 m/s2

Mars g= 3.7 m/s2

Neptune g=11.8 m/s2

58kg x 8.8 m/s2 = 510N

58kg x 3.7 m/s2 = 220N

58kgx11.8 m/s2 = 680N

10 m/s2

= 580 kg m/s2 or 580 N

Newton’s 2nd Law

Newton’s 2nd law explains why heavy objects and light objects fall at the same rate.

-because if the mass is large, it is harder to accelerate (has more inertia). So it takes more time to fall!

Terminal Velocity

On Earth we have air resistance

- causes friction, causing the falling objects to slow down

Terminal Velocity - maximum velocity reached by a falling object. (320 km/h or 120 mph)

- occurs when air resistance is equal to the gravitational acceleration.

Terminal Velocity

Projectile Motion

curved path an object follows when projected near the Earth’s surface.

Projectile Motion

- 2 components make up the path of projectile motion.

Both are independent of each other.

a. horizontal motion-force stays constant

b. vertical motion -pulling it down at a rate of 9.8 m/s2

Projectile Motion

http://www.physicsclassroom.com/mmedia/vectors/mzng.cfm

What if your object you are shooting at is falling too.

Where should you aim?

Projectile Motion

-Orbiting objects are in projectile motion.

- so they are NOT weightless or floating, they are just constantly falling!

Newton’s 3rd Law of Motion

For every action there is an equal and opposite reaction.

Forces act in pairs

- don’t necessarily act on the same objects.

Ex. Escaped balloon

Action – air escaping from opening in balloon

Reaction - balloon flies forward.

Momentum

The product of the mass and velocity of a moving object.

Momentum(p) = mass (kg) x velocity (m/s)p = kgm/s

Ex. Calculate the momentum of a 6.00kg bowling ballMoving at 10.0 m/s down the lane.

6.00kg x 10 m/s = 60.0 kgm/s down the lane

The Law of Conservation of MomentumTotal amount of momentum in a system is

conserved.

Ex.