Download Forces and Motion

Forces
Force: a Push or a Pull on an object.
SI Unit of Force: One Newton (N) is the force that causes a 1-kilogram
mass to accelerate at a rate of 1 meter per second each second (1 m/s2).
1 N = 1 kg•m/s2
Combining Forces
Representing Force
Arrows can represent a force. The lengths of the arrows show relative
amounts of force.
Net Force: the sum of all forces acting on an object. ∑F = sum of all
forces
• Forces in the same direction add together.
• Forces in opposite directions subtract from one another.
Balanced Forces: when two or more opposing forces on an object are
equal to each other
(Equal and Opposite)
The “net force” on the object = zero.
Net force = Zero = NO acceleration
Unbalanced Forces: when two or more opposing forces are unequal to
each other.
The “net force” on the object is greater than zero.
Net force > zero = Acceleration in the direction of the greatest force.
Forces in the same direction add together.
(Vector addition)
Forces in opposite directions subtract from
each other. (Vector subtraction)
Friction
Friction: force that opposes the motion of objects that touch as they
move past each other.

How would life be different if there was no friction?
Four main types of friction:
 static friction ( 2 stationary objects)
 sliding friction (1 stationary and 1 sliding)
 rolling friction (1 stationary and one rolling)
 fluid friction (objects immersed in gases or liquids)
http://www.youtube.com/watch?v=nmnzLW6YoF0 (Bill Nye Friction 0-3:05min)
Examples:
Fluid Friction
 Fluid friction acting on an object moving through the air is known as air resistance.
 Fluid friction increases as the speed of the object moving through the fluid increases.
Sugar Glider
http://www.youtube.com/watch?v=yqSGJzSiz5s
(coolwhip)
http://www.youtube.com/watch?v=QkKDHkwq59Y (colugo)
http://www.youtube.com/watch?v=3Cs-zZ0Qu5Y (wing suit flight)
Wing suit
http://www.youtube.com/watch?v=u_aesaJZYts (Speedo swim suit professor 2008- cut off
before end 2:20)
http://www.youtube.com/watch?v=dvMdqvO3R9g&feature=related (news article)
http://www.youtube.com/watch?v=0VmTuvps2TU (2012)
Newton’s Laws of Motion
1st Law:
Objects at rest and objects in motion maintain their velocity until acted
on by an outside unbalanced force.
Also called the law of Inertia
Inertia: the tendency of an object to resist any change in its motion
More mass = more inertia (An object that is harder to move has more
inertia.)
Inertia is not affected by velocity. Same mass = Same inertia
http://www.youtube.com/watch?v=e30roNZthw4 (Eureka inertia)
http://www.youtube.com/watch?v=d7iYZPp2zYY&feature=related (with/without seatbelt)
http://www.youtube.com/watch?v=Q0Wz5P0JdeU&feature=related (ISS Newton in space 1st)
http://www.youtube.com/watch?v=iG-d5n9ZetM
(Bill Nye, 2:50-6:45)
2nd Law:
Acceleration is proportional to the forces acting on it divided by mass
A= F/m. EX/ The more massive the object the harder it is to accelerate.
You already instinctively know Newton’s second law….
Push harder = accelerate more
More massive = harder to accelerate
This equation shows the relationship between Force, Mass and
Acceleration
Force = mass x acceleration
F= ma
The acceleration of an object is directly proportional to the force acting on it.
Force  (increases) = Acceleration  (increases)
The acceleration is inversely proportional to the objects mass.
Mass (increase) = Acceleration  (decreases)
Or
More force = more acceleration (and vise versa)
More mass = less acceleration (and vise versa)
Practice Problem:
Zookeepers lift a lion on a stretcher. The total mass lifted is 175 kg. The lion’s upward
acceleration is 0.66 m/s2. What force is needed to lift the lion at this rate of acceleration?
http://www.youtube.com/watch?v=WzvhuQ5RWJE&feature=related (ISS Newton in Space 2nd)
3rd Law
For every action there is an equal and opposite reaction.
Examples: swimming, skateboarding, skating, paddling a kayak, boat or canoe,
rockets, etc…
 So if they are equal and opposite….Why is
there acceleration of the objects?….Why do they
move?
Action/Reaction Forces are on different objects,
Example:
A swimmer applies force to the water pushing it backwards. (ACTION)
Water propels the swimmer forward. (REACTION)
http://www.youtube.com/watch?v=cP0Bb3WXJ_k&feature=related (ISS Newton in Space, 3rd)
http://www.youtube.com/watch?v=OZY8279b7BU&feature=relmfu (wonders of the universe- zero g)
Gravity
Gravity: mutual force of attraction between any two objects in the
universe
The amount of gravitational force depends on
 Mass of the objects
 Distance between the objects
How Mass Affects Gravitational Force (Fg)
 The more massive the object the more gravitational force it
exerts on other objects.
Mass  = Force 
Greater Mass = More Gravitational Force (and vise versa)
http://www.youtube.com/watch?v=0iDyhg0BPZA&feature=relmfu (extreme gravity)
How Distance Between Objects Affects Gravitational Force (Fg)
 As the distance between two objects increases, the Gravitational force
between them is decreased.
Distance  = Force 
Greater Distance = Less Gravitational Force (and vise versa)
Below is a diagram of two satellites A and B orbiting a planet.
R = the distance between the center of the planet and the satellite.
1. Which satellite is closest to the planet?
2. From the diagram do you know the mass
of either satellite?
3. Which satellite experiences a higher
gravitational force?
4. If the gravitational force on satellite A is
160,000 N, what is the gravitational force on
satellite B?
http://www.youtube.com/watch?v=XT261O7jXh4&feature=related
(Spin around Gravity)
Mass versus Weight
Mass: amount of matter in an object.
Weight: the gravitational force an object experiences due to its mass.
Weight is a force and is measure in Newtons, (N).
Mass is NOT determined by gravity. It does not change with location.
Weight is the product of an object’s mass and gravity acting on the object.
Weight changes when gravity changes.
Weight = mass x acceleration due to gravity.
W = mg
(Force = mass x acceleration, F =ma)
g = 9.8 m/s2 on earth.
Example problems:
1. Calculate the weight of an object on earth with a mass of 250kg.
2. An object has a weight of 300N. What is the mass of the object? ON Earth
m=W/g
3. The moon has a mass 1/6 the size of Earth’s mass. If identical objects are placed
on the surface of each which exerts more gravitational force on the objects, the
earth or the moon?
4. Why?
5. How much more?
6. If an object weight 600N on earth, how much does it weigh on the moon?
7. What does g = on the moon?
http://www.youtube.com/watch?v=5C5dOEyAfk (Nasa Footage)
http://www.stmary.ws/highschool/physics/home/videos/ForceMotion/massFreeFall.html (freefall animation)
Acceleration Due to Gravity
Free Fall: the motion of an object falling towards Earth due to only gravity pulling
it down.
Acceleration due to gravity (g) is a constant rate. On Earth, g = 9.8 m/s 2
(This rate is referred to as 1g or normal gravity.)
Acceleration due to gravity is
CONSTANT near the surface
of any planet or moon.
Each second an object is in free fall, its velocity increases downward by 9.8 m/s.
The change in the stone’s speed is 9.8 m/s2, the acceleration due to gravity.
t = 0s, v = 0m/s2
t = 1s, v = 9.8m/s2
t = 2s, v=19.6m/s2
t = 3s, v=29.4m/s2
If you know how long an object has been falling (and you ignore friction), you can
calculate its instantaneous velocity.
(Time of fall) (g) = Velocity of the object
tg = v
Examples: IGNORE FRICTION! Remember g= 9.8m/s2
1. How fast is an acorn moving if it fell from a branch five seconds ago?
v= tg
v= 49 m/s
2. A coin dropped from a building has a velocity of about 30m/s. Knowing its
velocity, estimate how long it has been falling.
t= v/g
t= 3.06s
Free fall can cause objects to experience “weightlessness”.
The objects have the “floor pulled out from under them” so to speak.
Examples of experiencing “weightlessness”:
 Astronaut orbiting earth in the space station or shuttle
The people and the shuttle are both”falling” toward earth at the same rate.
 Roller coaster rides
 Parabolic flight in an airplane (hypergravity and microgravity)
http://www.youtube.com/watch?v=YLaBZlWYStI (dog in airplane)
http://www.youtube.com/watch?v=iyRlZB3cvkg (teacher zero gravity flight)
http://www.youtube.com/watch?v=gTqLQO3L4Ko&feature=related (fluids in zero g)
http://dsc.discovery.com/videos/mythbusters-penny-drop-minimyth.html
Terminal Velocity: When the force of gravity pulling an object towards earth is
equal to the amount of air resistance the object experiences as it falls.
The forces become BALANCED instead of unbalanced forces acting on the object.
BALANCED forces = NO ACCELERATION.
The object stops accelerating and continues to fall at a constant maximum
“terminal” velocity.
Terminal velocity for humans typically ranges from 115-125 miles/hour.
http://www.youtube.com/watch?v=1ukf2vntU44 (falcon/human terminal velocity)
http://www.youtube.com/watch?v=ghPSl3j0gE4&feature=related (G R AV I T Y)