Download 05 Notes

Measuring Motion
Chapter 5
Observing Motion by Using
a Reference Point
O How do you know an object is moving?
O When you watch the motion of an object,
you are actually watching the object in
relation to another object that appears to
stay in place.
O The object that appears to stay in place is a
reference point.
Observing Motion by Using
a Reference Point
O When an object changes position over time
relative to a reference point, the object is in
motion.
O Common Reference Points:
O tree
O building
O door
O objects in motion – a bird flying while in a hot air
balloon.
Speed Depends on Distance
and Time
O Speed-Distance traveled by an object
divided by the time taken to travel that
distance.
O Ex. A balloon traveled 50 m in 10 s. What is
the balloons speed?
O The SI unit for speed is meters per second
(m/s).
Determining Average Speed
O Most of the time, objects do not travel at a
constant speed – you probably do not walk
at a constant speed from one class to the
next.
O
Average speed= total distance
Total time
Average Speed
O Examples:
O An athlete swims a distance from one end of
a 50m pool to the other end in a time of 25s.
What is the athletes average speed?
O Jake jogs to a store 72 m away in a time of
36s. What is Jake’s average speed?
Recognizing Speed on a
Graph
Distance vs. Time
What is the average speed?
Distance (m)
Time (s)
Velocity: Direction Matters
O Imagine that 2 students leave the same
classroom at the same time. They both walk
at 10km/hr for 5 min, 12 km/hr for 8 min,
and 5 km/hr for 10 min. Why don’t they end
up at the same place?
Velocity
O Velocity- Speed of an object in a particular direction
O You must always include a direction in your answer
O Constant velocity only occurs if neither speed nor
direction changes.
O Velocities can be added or subtracted together to give a
resultant velocity.
O Add velocities that are in the same direction.
O Ex: Walking on a bus as the bus is moving forward.
O Subtract velocities that are in opposite directions.
O Ex. Walking to the back of the bus while it is moving
forward
Finding Resultant Velocities
O Same direction
2 km/min east
15 km/min east
O Resultant Velocity =
Finding Resultant Velocities
O Opposite directions
2 km/min west
15 km/min east
O Resultant Velocity=
Acceleration
O Acceleration is the rate at which velocity
changes.
O Velocity changes if:
speed changes
O direction changes
O both change
O
O Increase in velocity = positive acceleration
O Decrease in velocity = negative acceleration
(deceleration)
Acceleration
O Calculating Average Acceleration
O Avg Accl = final velocity-starting velocity
time it takes for velocity to change
O A = vf – vi = A = Δv
t
t
O Velocity is expressed in m/s and time is
expressed in s.
O Therefore, acceleration is expressed in
meters per second per second (m/s/s) or
m/s2
Calculating Acceleration
0:01
1 m/s
0:02
2 m/s
Equation:
Calculation:
0:03
3 m/s
0:04
4 m/s
0:05
5 m/s
Acceleration Problems
O A skater goes from a standstill to a speed of
6.7 m/s in 12 seconds. What is the
acceleration of the skater?
O A plane passes point A at a velocity of 240 m/s
north. Forty seconds later, it passes point B at a
velocity of 260 m/s north. What is the planes
average acceleration?
Centripetal Acceleration
O Centripetal Acceleration- the acceleration
that occurs in a circular motion.
O Ex: ferris wheel, the moon’s orbit
5.2 Forces
O A force is a push or pull
that causes a resting
object to move, or it can
accelerate a moving object
by changing its speed or
direction.
O Unit = Newton (N)
O 1 kg m/s2
O Combination of all forces
acting on an object is
called the net force.
Types of Forces
O Balanced Force
O When the forces on
an object are
balanced, the net
force on an object is
zero and there is no
change in the
object’s motion.
O Unbalanced force
O When the forces on
an object are
unbalanced, there is
a net force and the
object accelerates.
Calculating Net Force
O Forces in the same direction
O Add forces to determine net force
Calculating Net Force
O Forces in opposite directions
O Subtract forces to determine net force
Forces
O
Affect of force on moving object
O
Change in speed or direction
O
O
Ex. When soccer ball is passed to another
player and is kicked
Affect on force on stationary object
O
Cause a nonmoving object to start moving.
O
Ex. Stationary soccer ball kicked
5.3 Friction
O Friction - A force that opposes motion
between two surfaces that are in contact.
O Friction causes moving objects to slow down
and eventually stop.
O What causes friction?
O When the hills and valleys of one surface come
in contact with the hills and valleys of the other
surface
Friction
O Rough surfaces have more hills and valleys
than smooth surfaces do.
O The rougher the surface, the greater the
friction.
O Ex. A soccer ball rolling on grass, verses a
hockey puck on ice.
O The greater the weight
of the object, the greater
the friction will be
Types of Friction
O Static Friction is the
friction force that acts
on a stationary object.
It opposes the applied
force.
O Kinetic Friction is the
force that opposes the
direction of motion of
an object as it slides
over a surface.
O Solid over solid
Types of Kinetic Friction
O Rolling Friction is the
force of friction felt on
rolling objects
O Example: wheels
and balls.
O Fluid friction opposes
the motion of a moving
object in a fluid (gas or
liquid)
O Example: Air
resistance
Is friction helpful or harmful?
O Helpful
O Harmful
O Movement of tires
O Machine wear
O Walking
O Erosion
O Eraser
O Burns/blisters
Decreasing Friction
O Lubricants- Substances that are applied to
surfaces to reduce the friction between the
surfaces.
O Ex: motor oil, wax, grease
O Lubricants are usually liquids, but can be
solids or gases. (Air in air-hockey)
Decreasing Friction
O Switching from sliding friction to rolling
friction.
O EX. Ball bearings are placed between the
wheels and axles of skates
O Make surfaces that rub against each other
smoother.
Gravity
O Gravity is a force of attraction that acts
between objects with mass.
O Objects close to Earth accelerate at 9.8m/s2.
O All objects fall at the same rate.
Gravity
O If gravity affects all objects with mass – why
aren’t all objects stuck together?
O Masses of most objects are too small, you
can’t detect this force
O Earth has a huge mass, so the gravitational
force of Earth is large. It pulls everything
toward the center of the Earth. – Dropped
objects fall to the floor.
Law of Universal Gravitation
O All objects in the universe attract each other
through gravitational force.
O The size of the force depends on the masses
of the objects and the distance between the
objects.
O Greater the mass the greater the force.
O Smaller the distance, the greater the force.
Mass vs. Weight
O Mass is the amount of
O Weight is a measure of
matter in an object.
O Unit = kilogram
O Measuring device is a
balance.
gravitational force on
an object.
O Unit – Newton
O Measuring device is a
spring scale.
O Weight = mass x gravity
N = (kg) x (m/s2)