Forces and Motion Interactive Lesson

Different Reference Frames

Motion

Reference Frame

●Object moves if it changes position against a
background that stays the same

●Reference frame

●Stationary background

●Change in position in a reference frame

●Distance traveled from a fixed point

●Like a race

Different

Reference Frames

Speed

●How fast an object moves

●Involves measurements of distance and time

●Distance traveled

●Time it takes to travel that distance

●Units: m/s (meters/second)

speed = distance

v=d

time

t

Distance (m)

Time (s)

0

0

96

1

192

2

288

3

384

4

Speed

●Suppose a track star finished a 400 m dash in
58.6 s.

●What does the speed tell us?

●Every second the athlete ran

they covered 6.83 meters.

Practice

●Calculate the speed of the following objects:

●A mouse crosses a 10 m room in 3.6 s.

●A car drives 2500 m in 30 s.

●A bowling ball rolls down the 5.6 m long lane in 3.2
s.

Constant Speed

●When an object covers equal distance in equal
amount of time

●Race care moving 96 m/s

●Every second moves 96 meters

Velocity

●Velocity is a speed BUT with a direction

●Want to know how fast something is going in what
direction

●55 mph (miles per hour) NORTH

●19 m/s to the RIGHT

●The direction makes it a velocity

●A car chase

●A helicopter will track the car’s velocity

●Heading South on I-71 at 80 mph

●Without the direction we wouldn’t know which way the car
is going

Acceleration

●When you pedal your bike hard your velocity
changes

●Then it changes again when you stop pedaling

●Changes again when you go around a curve

●Change direction

●Acceleration

●Any change in velocity

●Change in speed or direction

●Unit: m/s2 meters per second squared

Acceleration

●Tells how the velocity is changing per second

●Small number = gradually changing

●Runner’s acceleration of 2 m/s2

●Big number = fast change

●Sports car that goes from 0 to 96 km/s (60 mph) in 3.7 s

●Acceleration of 7.2 m/s2

●Negative acceleration

●Object is slowing down

Acceleration

●Measure velocity at different times and find
amount of time between measurement

Acceleration = final velocity – initial velocity

time

a = vf – vi
or a = Δv

t

t

Practice

●A turtle swimming in a straight line toward
shore has a speed of .50 m/s. After 4.0 s, its
speed is .80 m/s. What is the turtle’s
acceleration?

●Find the acceleration of a northbound subway
train that slows down from 12 m/s to 9.6 m/s in
0.8 s.

Force

●When throwing or catching a ball

●Exert a force

●Force

●Any push or pull that changes an object's velocity

●Speed or direction

Net Force

●Net force

●Combination of all the forces acting on an object

●Objects accelerates in the direction of their net
forces

●Moves in direction of strongest force

●If object doesn’t accelerate

●Net force = 0

●No motion

Balanced Forces

●Balanced forces don’t change motion

●Completely cancel each other out

●Combined force equals 0

50 N left

50 N right

Cancel out, net force = 0 N

Unbalanced Forces

●Opposing forces that don’t have the same
strength don’t cancel each other out completely

●Some force left over

●Objects accelerate in direction of left over force

100 N left

50 N right

Friction

●A car rolls on a flat, freshly paved road

●If no forces act on it, it should keep moving

●Gradually slows down

●Friction

●A force that acts in the opposite direction of the
motion of the object

Friction

●Can also affect objects that aren’t moving

●Object sitting on an incline and not sliding

●Reason why athletic shoes have cleats or spikes

●Tennis shoes with rubber soles – create friction

●Smooth soles (dress shoes) and smooth floors

●Little friction, very slippery!

Air Resistance

●Interaction between the surface of a moving
object and air molecules

●Type of friction

●Size, shape of object and speed affect

●Larger objects

●Faster moving objects

●More air resistance

Air
Resistance

Gravity

●Reason why things
fall to the ground

●Every object exerts a
gravitational force on
every other object

●Different from other
forces

●Gravity acts even
when the objects
don’t touch

●Pulls objects to the
ground

●Force acts down

Gravity

●Force between two objects depends on their
masses and distance between the objects

●Larger mass = larger gravitational force

●Closer together the two objects are = larger
gravitational force

●Person sitting next to you…

●You are all exerting forces on each other

●Just can’t feel it

●Our forces are extremely small compared to the gravitational
force of the Earth

Newton’s Laws
of Motion

Every motion you observe
or experience is related to
force

AKA: Law of Inertia

Newton’s
First Law

●An object at rest will remain at rest, or an
object in motion with constant velocity will
remain in motion with constant velocity until
acted upon by an unbalanced force

●Inertia explains many common events

●Car suddenly coming to a stop

●You keep moving forward

●Unless you have your seatbelt on

●The force from the seatbelt prevents you from moving
forward

Inertia

●Inertia

●The tendency of an object to resist change in its
motion

●All objects have inertia

●All objects resist changes in motion

●The amount of inertia an object has depends on
how much mass it has

●Greater the mass, the greater the inertia

Newton’s Second Law

●The unbalanced force acting on an object
equals the object’s mass times acceleration

FORCE = MASS * ACCELERATION
F=MA

●Newton – the unit for force

●1 N = 1 kg*m/s2

Newton’s Second Law

●A 52 kg water-skier is being pulled by a
speedboat. The force causes her to accelerate
at 2 m/s2. Calculate the net force that causes
this acceleration.

●What net force is needed to accelerate a 55 kg
cart at 15 m/s2?

Changes in Force and Mass

●How can you increase the acceleration of the
wagon?

●Changing the force

●Acceleration and force change in the same way

●Increase of force increases the acceleration

●Change the mass

●Acceleration and mass change in the opposite way

●Decrease of mass increases the acceleration

Free Fall

●When gravity is the only force
acting on an object

●Free fall acceleration = 9.8 m/s2

●Results from the downward force of
gravity

●In the absence of air resistance

●All objects fall at the same rate

Weight

●Weight

●NOT THE SAME AS MASS!!!!!

●Amount of matter something has

●Measure of the force of gravity on an object

●Weight is a force

●Can be written as an equation (Newton’s 2nd Law)

Weight = mass * acceleration due to gravity (g = 9.8 m/s2)
W = mg

Newton’s Third Law of Motion

●Forces are not “one-sided”

●Whenever one object exerts a force on another

●The action force

●A force equal and opposite is exerted back

●The reaction force

●Newton’s third law of motion states that if one
object exerts a force on another object, then the
second object exerts a force of equal strength in
the opposite direction on the first object.

Equal but Opposite

●Two skaters pushing against each other

●They both move

●Their speed will depend on their mass

●Skater with more mass will have a

slower velocity

●Force is equal, but mass changes the acceleration

Action-Reaction in Action

●When walking you push with your feet

●The ground pushes back against you

●With equal and opposite force

●A squid exerts a force on the water that it repels
from its body cavity

●The water pushes equally and opposite in direction
back on the squid