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
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