# Chapter 8 Motion & Forces - lcboe.net

How do you know an object is moving? An object is moving if its position changes against some background that stays the same.

The stationary background is called the reference frame. The change in position is measured by the distance traveled by an object from a fixed point.

Speed describes how fast an object moves. Speed is measured by calculating the distance traveled by the time. SI

units meters per second (m/s) Speed can also be measured in Kilometers per hour (km/h) and miles per hour (mi/h) Constant speed is the

simplest type of motion. An object is moving at a constant speed when it covers equal distances in equal amounts of time Speed can be determined

from a distance-time graph Time is given on the X axis Distance is given on the Y axis Speed can be determined by calculating the slope of the line Speed

= Distance Time V = d/t http://www.youtube.com/watch?v=DRb5PSxJerM Velocity describes both speed & direction of the motion

Example: The car was traveling at 50 mi/h headed north The velocity of an object changes if its speed or direction changes.

Does a cars velocity change when it turns a corner? Which is harder to stop A speeding train or a speeding

bicycle? A grown man or a small child Stampede of elephants or a stampede of mice?

Why? Momentum is a quantity defined as the product of an objects mass and its velocity Moving objects have momentum

If an object is moving in a straight line, momentum is calculated by multiplying an objects mass by the velocity An objects momentum is in

the same direction as its velocity. Measured in kilograms X meters per second kg X m/s Momentum

= mass X velocity P =mv The total amount of momentum in a system is conserved

Practice Problems Calculate the momentum for.. A 135 kg speed racer moving forward at 16m/s A 25 kg ostrich running north at 16.2 m/s

A 48.5kg passenger on a train stopped on the tracks Section 2 Acceleration & Force Acceleration is the change in velocity divided by the time

Acceleration = Final velocity Initial velocity Time If the acceleration is small, the speed in increasing gradually. If the acceleration is large, the speed is increasing rapidly. Positive acceleration

means the objects velocity will __________. (speed up) Negative acceleration means the objects velocity will decrease (slow down) Increase Practice Problem

A flowerpot falls off a second-story windowsill. The flowerpot starts from rest & hits the sidewalk 1.5s later with a velocity of 14.7 m/s. What is the average acceleration? Velocity-time graph Yay!

Acceleration can be determined from a velocity-time graph. What information is on the X axis? What information is on the Y axis?

The Acceleration of an object is O if its velocity is constant Why? Force Force is the cause of acceleration, or change

in an objects velocity. Net Force Many forces can act on an object at a given time. Net Force is the combination of all the forces acting on an object

They determine whether the velocity of an object will change An objects acceleration is in the direction of the net force.

Objects will not accelerate if the net force is zero. Balanced forces do not change motion Tug- of war Balanced forces are forces

acting on an object that combine to produce a net force equal to zero The forces cancel each other out. Unbalanced Forces Do Not Cancel Completely

Will produce a net nonzero force. If there are unbalanced forces in different directions they will act like a single force on the object. The net force will cause the object to accelerate

Friction & Air Resistance Friction is the force between 2 objects in contact that opposes the motion of either object. Example: The forces to move a car forward must be unbalanced.

The force moving forward must be greater than the friction opposing the cars motion. Frictional Force varies depending on the surface in contact.

Example: Tennis shoes on a waxed floor versus shoes with very little tread. (Someone is going to fall) Air Resistance is a form of friction Air resistance opposes the

motion of an object. Air Resistance is caused by the interaction between the surface of the object & the surrounding air molecules Depends on the objects size & shape

Key Vocabulary: Speed Unbalanced Forces Velocity Balanced Forces

Momentum Inertia Acceleration Gravity Force Friction Gravity

Gravity is the attraction between 2 particles of matter due to their mass. The force of gravity will act on objects that do not touch. Mass & distance affect

gravitational force The greater the mass of an object, the larger the gravitational force it is exerts on other objects. Earths gravitational force is very large due to Earths

massive size. What would happen if the force wasnt as strong? The force of gravity changes as the distance between two objects changes.

Gravitational force between two object will ____________as the distance between 2 object increases. Chapter 8 Section 3 Newtons Laws of Motion! Sir Isaac Newton

described the relationship between motion & force in 3 laws Newtons First Law An object at rest remains at rest & an object in motion maintains its velocity unless it

experiences an unbalanced force. Example Compare sliding your book across carpet & ice On which surface will it slide longer? Why?

Smooth surfaces provide less friction to oppose motion. Inertia Inertia is the tendency of an object to remain at rest or in motion with a constant velocity.

All object have inertia because they resist changes in motion. The greater the mass of an object, the greater the force that has to be applied to cause the object to move. Newtons 2nd Law The

unbalanced force acting on an object equals the objects mass times its acceleration. F = ma Force = mass x acceleration

Example: Pushing an empty shopping cart vs. pushing a really full one. Which one would need more force?

Acceleration will occur in the direction of the net force. Force is measured in Newtons 1N = 1kg X m/s2

Practice Problem Mass = 175 kg Acceleration = .657m/s2 What 115N is the force? Free Fall & Weight

When the force of gravity is the only force acting on an object it is said to be in free-fall. The free-fall acceleration of an object is directed toward the center of the Earth. g= 9.8 m/s2

Free-fall acceleration near Earths surface is constant. In the absence of air resistance, all objects near Earths surface accelerate at the same rate regardless of their mass. 9.8m/s2

The force on an object due to gravity is called Weight. Weight = mass x free-fall acceleration w=mg

(g = free-fall acceleration) The SI unit for weight is the Newton Weight is the gravitational force on an object due to its mass.

Weight influences shape (structure) Example: Skeletons in elephants Velocity is constant when air resistance balances weight

For a falling object, when the force of air resistance becomes equal to the gravitational force on the object it stops accelerating & reaches Terminal Velocity For every action force, there

is an equal and opposite reaction force. The law of action & reaction Forces always occur in pairs

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