Transcript Motion: Speed, Velocity, Acceleration, and Forces

Motion: Speed, Velocity,
Acceleration, and Forces
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
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.
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
D
• v= velocity (m/s)
• d= distance (m) V
T
• t= time (s)
Speed & Velocity Examples
• Speed is distance divided by time
– 2 cm/s
• Velocity includes the direction
– 2 cm/s west
West
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 hillacceleration due to
gravity
– Steeper hill= more
acceleration
Deceleration is slowing
down
Zero acceleration is at
a constant speed
Acceleration
Acceleration= change in speed (cm/sec)
time (sec)
a = vf-vi
t
Units: cm/sec*sec or cm/sec2
• Can use a graph to show acceleration
– + slope = +acceleration (speed up)
– - slope = - acceleration (slow down)
– 0 slope = no acceleration
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!!!!!
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.
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.
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
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)
What is friction?
• 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)
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?
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?
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?
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?
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.)
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?