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Motion: Speed, Velocity, Acceleration, and Forces
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What’s the difference between distance and displacement?
• Distance: the total magnitude traveled • Displacement: the shortest distance from
the starting point to the ending point
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What is speed?– Speed–The rate of change in distance with respect to
time.– Even objects that are not moving have a speed = 0
m/s– Units: m/s, cm/s, mph– Since speed is built from distance, a scalar quantity,
then speed is also a scalar quantity. – Scalar Quantities carry no direction information with
them.
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What is Velocity?– The rate of change in displacement with respect to
time. – Since displacement is a vector quantity, then
velocity is also a vector quantity. – Vector Quantities have both magnitude and
direction.– v=d/t• v= velocity (m/s)• d= distance (m)• t= time (s)
D
V T
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Speed & Velocity Examples
• Speed is distance divided by time– 2 cm/s
• Velocity includes the direction– 2 cm/s west
West
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Acceleration“Acceleration” occurs
when your speed changes up or down (or when you change direction).
• It is the rate speed changes– Level ground- no
acceleration due to gravity
– Steep down hill- acceleration due to gravity
– Steeper hill= more acceleration
Deceleration is slowing down
Zero acceleration is at a constant speed
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Acceleration
• Can use a graph to show acceleration–+ slope = +acceleration (speed up)– - slope = - acceleration (slow down)– 0 slope = no acceleration
Acceleration= change in speed (cm/sec)
time (sec)
a = vf-vi
t
Units: cm/sec*sec or cm/sec2
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Forces
• Force- Ability to change motion(push or pull)–Units of lb, N=kg.m/sec2
• If forces are balanced then the object won’t move and it is said to be in equilibrium
• Unbalanced forces=move• Forces always occur in pairs!!!!!
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Newton’s Laws of Motion• Newton’s First Law: – “AKA” the Law of Inertia– Inertia- property of an object to resist
change in state of motion
– an object will stay at rest until acted on by an unbalanced force
– In other words, things tend to keep on doing what they were doing in the first place unless you apply a force.
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Newton’s Laws of Motion• Newton’s Second Law: – force causes an object to accelerate
while the object’s mass resists acceleration
– Fnet=ma• a=acceleration (m/sec2), F= force
(N), m=mass (kg)• Fnet= Right – Left
• Fnet= Top - Bottom
– EX. Lighter cars go faster than heavier ones pushed with equal force. Lighter cars resist the force of acceleration less allowing them to move faster than heavier ones.
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Newton’s Third Law• Newton’s 3rd Law: – For every action there is a reaction force equal in
strength & opposite in direction– For every action there is an equal & opposite reaction– Can be + (move to right) or – (move to left)– Examples: rockets, stepping into a boat, throw a ball on a
a skate board
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Gravity, Weight & Mass• Gravity- – Force pulling toward mass of
planet– Depends on mass– Mass- amount of matter in an object
compared to a standard
– Changes on other planets– On Earth= -9.8m/s2
• Weight- force created by gravity depends on mass– It is a force acting on object in units of
lb or N
•Fw=mg
•Fw= weight force (N)
•m=mass (kg)
•g=acceleration due to gravity (-9.8 m/sec2)
•Fw=mg
•Fw= weight force (N)
•m=mass (kg)
•g=acceleration due to gravity (-9.8 m/sec2)
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• Friction: A force that…– Reduces acceleration– Works against motion– Occurs when surfaces move against on another– Causes wear on parts
• Examples of friction:– Air friction- (air resistance)- aka – “drag”– Sliding friction (rub hands together)– Viscous friction- (oil in car engines & joint fluid)– Rolling friction- (wheel on road, ball bearings)
What is friction?
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Practice Problems
What is the speed of a truck that travels 450 miles in 7.5 hours?
What is the acceleration of a car that goes from 55 mph to 35 mph in 0.1 hrs?
What is the mass of an object that is accelerated at a rate of 5.3 m/s2 when a 90 N force is applied to it?
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Practice Problems pg 2
How far did you ride your bike if you rode for 20 min at a speed of 3 m/min?
What is the force applied to a box with a mass of 45 kg and it causes it to accelerate at a rate of 4.5 m/s2?
What is the acceleration of an object that has a force of 350 N and a mass of 92 kg?
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Practice Problems pg 3
What is the amount of force used to move a 25 kg box if it accelerates at a rate of 3 m/s2?
What is the difference between speed and velocity?
What is the acceleration of a rabbit that speeds up from 10 m/s to 45 m/s in 5 seconds?
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Practice Problems pg 4
What is the velocity of a rabbit that travels 50 m in 2.5 seconds?
Which will accelerate more when dropped off the top of a building: a feather or a heavy rock?
Which has more mass: a kilogram of bricks or a kilograms of feathers?
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Practice Problems pg 5
If you use cruise control while traveling down I-75, are you accelerating?
What is a common term for Newton’s 1st Law: An object at rest will stay at rest unless a force acts on it. (or An object in motion will stay in motion unless a force acts on it.)
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Practice Problems pg 6
Which of Newton’s Laws is what we know as the equation F=ma.
How long does it take a turtle to go to the lake if he travels a distance of 150 m at a velocity of 0.5 m/s?
What is the mass of a rock that is accelerated at a rate of 5 m/s2 if a force of 45 N is used?
What is the acceleration of a car that is has a mass of 9 kg if a force of 63N is used?