how do you know an object is moving? an object is moving if its position changes against some...
TRANSCRIPT
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.
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
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 car’s 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 object’s mass and its velocity
Moving objects have momentum
If an object is moving in a straight line, momentum is calculated by multiplying an object’s mass by the velocity
An object’s momentum is in the same direction as its velocity.
Measured in kilograms X meters per second
kg X m/s
Practice ProblemsCalculate 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
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 object’s velocity will __________.(speed up)
Negative acceleration means the object’s 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?
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 object’s 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 car’s 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 object’s size & shape
Key Vocabulary:
Speed
Velocity
Momentum
Acceleration
Force
•Unbalanced Forces
•Balanced Forces
•Inertia
•Gravity
•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.
Earth’s gravitational force is very large due to Earth’s massive size.
What would happen if the force wasn’t 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.
Newton’s 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.
Newton’s 2nd Law
The unbalanced force acting on an object equals the object’s 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
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 Earth’s surface is constant.
In the absence of air resistance, all objects near Earth’s 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