Motion and Forces

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Motion and Forces

• Physical Science
• Unit 6

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Motion

• Everyday life motion is so common, it seems to
  appear very simple.
• However, understanding motion requires some new
  and advanced ideas.
• How do we know when an object is moving?????

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Motion

• Motion change in position over a certain time
• Distance, time and direction
• Frame of reference

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Energy

• Potential energy stored energy
• Gravitational any object raised above the
  surface of the Earth
• such as the energy in a stretch or compressed
  spring
• such as stored in chemical bonds, food, fuel,
  batteries
• Kinetic energy energy of moving objects or
  energy in motion

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Speed and Velocity

• An object is moving if its position changes but
  the background stays the same.
• Horse galloping but the trees remain stationary
• Person walking but the road remains in place

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Speed and velocity cont.

• Stationary background reference frame
• This can be used to measure the change in
  position of the moving object

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Speed

• Speed describes how fast an object moves
• Which moves faster?
• Flying eagle or galloping horse
• Speeding race car or flying jet

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Speed cont.

• The speed depends on the distance traveled and
  the time it took to travel that distance.
• This is a scalar quantity in which only the
  magnitude is being measured.
• Quantities that can be described by a single
  number are called scalars.

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Speed

• To determine speed you must have
• Distance traveled by an object
• Time it takes to travel that distance
• SI unit for speed is meters per second (m/s)

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Speed cont.

• Speed equation
• Speed distance
• time
• Or
• s d
• t

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Units of speed

• Units
• Distance measured in meters, kilometers, or
  centimeters
• Time measured in seconds, minutes, or hours
• Therefore speed is in meters per second (m/s) or
  kilometers per hour km/hr

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Types of speed

1. Instantaneous speed at that exact moment
2. Constant does not change through the time
3. Average total distance over time

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Practice problems

• Wheelchair racer finishes a 132 m race in 18 s.
  What is the speed?
• s d 132 m 7.3 m/s
• t 18 s

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• A person jogs 500 meters in 360 seconds.
  Calculate the speed.

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• A person jogs 500 meters in 360 seconds.
  Calculate the speed.
• s d 500 m 1.39 m/s
• t 360 s

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• A person walks 4 miles in 2 hours, then stops for
  an hour for lunch. After lunch they walk 8 miles
  in 3 hours. Calculate the persons average speed.

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Distance Time
4 miles 2 hrs
0 miles 1 hr
8 miles 3 hrs
Total 12 miles 6 hrs

• A person walks 4 miles in 2 hours, then stops for
  an hour for lunch. After lunch they walk 8 miles
  in 3 hours. Calculate the persons average
  speed.

s d/t 12 miles/6 hrs 2 mph
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Velocity

• Velocity quantity describing both speed and
  direction.
• Speed in a given direction or displacement
  divided by time
• Needs a direction, third dimension, north, east,
  south, west
• Vector quantity uses both magnitude and
  direction

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Speed vs. velocity

• Speed is a scalar
• Velocity is a vector

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• Speed has no direction scalars have no direction
• Velocity has direction vectors have direction

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• Speed or velocity?
• Person walks 4 m/s
• Person walks 2 m/s north
• Car drives 60 mph toward Biloxi
• Car drives 30 mph
• Boat is pulled by a 53 newton force

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• Speed or velocity?
• Person walks 4 m/s - - - speed
• Person walks 2 m/s north - - velocity
• Car drives 60 mph toward Biloxi - - velocity
• Car drives 30 mph - - - speed
• Boat is pulled by a 53 newton force - - -
  velocity

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Acceleration

• Acceleration rate of change in velocity
• Means of acceleration
• Decrease speed negative acceleration or
  deceleration
• Increase speed
• Change the direction

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Acceleration formula

• a V final V initial ?v
• time t

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Acceleration examples

• A plane starts at rest and ends up going 200 m/s
  in 10 secs. Calculate the acceleration.
• A V final V initial 200 0 m/s 20 m/s2
• time 10 s
• Positive answer acceleration

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• A race car starts at 400 m/s and then stops in 20
  seconds. Calculate the cars acceleration.
• A V final V initial 0-400 m/s - 20 m/s2
• time 20 s
• Negative answer deceleration

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Force

• Force a push or a pull that one body applies to
  another
• A force can cause an objects motion to change
• When two or more forces combine they create a
  net force

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Force cont.

• Balanced forces are equal in size and opposite in
  direction. (They do not cause motion)
• Unbalanced forces are unequal in size and/or are
  in the same direction. (They will cause an
  object to move!)

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Newtons 1st Law of Motion

• An object moving at a constant velocity keeps
  moving at that velocity unless a net force acts
  on it.

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Newtons 1st Law of Motion

• What does that mean?????
• An object in motion stays in motion unless acted
  on by an outside force same for an object at
  rest

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Newtons 1st Law

• This is called inertia an objects resistance to
  change its motion.
• Inertia is a property of matter.
• The amount of inertia that an objects has depends
  on its mass. The more mass an object has, the
  harder it is to change its motion.
• Newtons First Law of Motion is also called the
  Law of Inertia.

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Example of Inertia

• In an automobile crash a person not wearing a
  seat belt will keep moving forward at the cars
  speed even after the car has stopped.

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Newtons 2nd Law of Motion

• A net force acting on an object will accelerate
  in the direction of the force.

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Newtons 2nd Law of Motion

• In other words, force and motion are related
• The rate of acceleration is determined by the
  size of the force and the mass of the object.
• An object will have greater acceleration if a
  greater force is applied to it.
• An object with less mass will accelerate faster.

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Example of Newtons 2nd Law

• A baseball and a bowling ball are both hit with
  the same bat and the same force.
• The baseball will have a greater acceleration
  because it has less mass.

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Fma

• All of this is represented mathematically in the
  equation Fma
• Where
• F force (N)
• m mass (kg)
• a acceleration (m/s2)
• Newton (N) is a unit of force.
• It is equal to 1 kg ? m/s2

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Fma example

• Zookeepers lift a stretcher with a sedated lion.
  To total mass of the stretcher and lion is 175
  kg, and the upward acceleration is 0.657 m/s2.
  What is the force needed to move the lion?
• F ma 175 kg x 0.657 m/s2
• 115 kg ? m/s2 115 N

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Newtons 3rd Law of Motion

• For every action there is an equal and opposite
  reaction.

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Newtons 3rd Law of Motion

• Example
• When you throw a basketball with a force of 3
  newtons, the basketball exerts and equal force
  back on you. You would see it if you were
  wearing roller skates!

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More information you need to know about forces!

• Friction A force between two objects that are
  touching each other.
• The amount of friction depends on two things
• The kind of surface the objects have how smooth
  or rough
• The force that is pressing the objects together
  the harder they are pressed together the more
  friction there will be.

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Air resistance

• Air resistance an upward force that air exerts
  on an object that is falling through it. A force
  that works against gravitys downward force. The
  amount of air resistance depends on the shape,
  size, and speed of the falling object.

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Bernoullis principle

• In a fluid
• High velocity low pressure

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Momentum

• Momentum a property of a moving object because
  of its mass and velocity. It decides how much
  force is needed to change an objects motion.

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Momentum cont.

• Equation pmv
• p momentum (kg ?m/s)
• m mass (kg)
• v velocity (m/s)

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Example

• A 75 kg speed skater moving forward at 16 m/s.
  Calculate the momentum.
• p mv
• P 75 kg ? 16 m/s
• 1200 kg?m/s

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• A 135 kg ostrich running north at 16.2 m/s.
  Calculate the momentum.

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• A 135 kg ostrich running north at 16.2 m/s.
  Calculate the momentum.
• P mv 135 kg ? 16.2 m/s
• 2187 k?m/s

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• A 0.8 kg kitten running to the left at 6.5 m/s.
  Calculate the momentum.

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• A 0.8 kg kitten running to the left at 6.5 m/s.
  Calculate the momentum.
• P mv 0.8 kg ? 6.5 m/s
• 5.2 kg?m/s

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Momentum cont.

• Simple terms
• The more mass an object has and the faster it is
  moving, the harder it is to change its motion.
  An object at rest has no momentum!
• Example A toy truck is easier to stop than a
  real truck because it has less mass, therefore
  less momentum.

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The Law of Conservation of Momentum

• Momentum can be transferred between objects
  momentum is neither created nor destroyed in the
  transfer.

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Gravity

• Gravity
• Is the force that every object exerts on every
  other object.
• The amount of gravitational force depends on 2
  things
• The mass of the object
• The distance between the objects.

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Gravity cont.

• Weight is a measure of the force of gravity on an
  object.
• W ma, where a is acceleration due to gravity
  (9.81 m/s2 on Earth)
• All objects fall towards Earth the same
  acceleration regardless of their mass 9.81 m/s2.
  This is why if there were no air resistance two
  falling objects would hit the ground at the exact
  same time.

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Free fall

• Free fall the motion of a body when only the
  force of gravity is acting on it.
• Free fall acceleration results from gravity, it
  is abbreviated as g.
• Near the Earths surface, g 9.8 m/s2

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Free Fall and Weight

• The force on an object due to gravity is called
  its weight.
• On Earth, your weight is the amount of
  gravitational force exerted on you by Earth.

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Free fall and Weight

• If you know the free-fall acceleration, g, acting
  on a body then you can use Fma to calculate the
  body weight.
• Weight equals mass times freefall acceleration.
• Weight mass x free-fall acceleration
• w mg

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Example

• A small apple weighs about 1 N.
• A 1.0 kg book has a weight of
• 1.0 kg x 9.8 m/s2 9.8 N

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Terminal velocity

• Terminal velocity the maximum velocity reached
  by a falling object that occurs when the
  resistance of the medium is equal to the force
  due to gravity.

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Khan academy clips
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