Download Unit: Forces in Motion

Unit: Forces in Motion
Sections: Friction
Straight Line Motion
Speed, Velocity & Acceleration
Newton’s Laws
Gravity
Magnets
Interaction of Force & Mass
Key Words: friction, motion, kinetic energy, inertia, acceleration, speed, velocity, mass, gravity, weight,
magnet, magnetic field, force, unbalanced force, balanced force, contact force, electrical force,
magnetic force, gravitational force, torque
Essential Questions (by section):
Friction
- What is friction?
- How does friction affect the motion of an object?
Straight Line Motion
- What is motion?
- What causes an object to speed up or slow down?
- How can we describe the motion of an object?
Speed, Velocity & Acceleration
- How can motion be described?
- How are speed, velocity, and acceleration related?
Newton’s Laws
- What will happen when object A applies a force to object B?
- What happens when an object at rest or in motion experiences an unbalanced force?
- What causes an object to accelerate?
Gravity
- What is gravity?
- How does the force of gravity affect objects on Earth and in our Solar System?
Magnets
- What is a magnet?
- What is a magnetic field?
- Where is a magnet’s magnetic field strongest?
Interaction of Force & Mass
- What happens when two unbalanced forces act on an object?
- What kinds of forces can act on an object?
Friction
-
the opposing force between two or more surfaces
contact force between 2 solids, liquids, or gases
o Never speeds an object up
o Converts energy of an object’s motion to heat energy
o Always slows an object down
o Causes objects to stop – called kinetic/dynamic friction
o Can reduce friction with lubricants
 Water
 Oil
 Soap
o Types of friction
 Static Friction



Holds objects in place
Helps us walk
Sliding Friction
Opposes force of something sliding
Brake pads on a car used to slow the wheel down
Rubbing hands
Rolling Friction
Pushes back on the wheel so that it can move forward
Opposes the motion of the wheel so it can move
Air Resistance
Opposes the direction of the object’s motion
Increases as speed of the object increases
Straight Line Motion
Motion – change in position over time
- Compared to a reference point or frame of reference
o Sitting at your desk – from your reference not moving much
o From sun’s reference – moving quickly in space on Earth’s rotation
Force – a push or pull
- Balanced
o When the effects of the forces cancel each other out so they do not cause change in an object’s
motion
 You sitting in your chair – gravity pulling on you and the floor pushing you up
 Inertia – an object’s mass determines it’s ability to resist movement
Directly proportionate – more mass, more inertia
- Unbalanced
o Causes an object to accelerate
o Object stays moving in its direction unless all forces become balanced
o Friction – outside force that slows objects down
- Ex. Throwing a ball – you exert a force on the ball, so it goes forward (push)
Gravity exerts a force on the ball (pull) so it falls towards the ground
The ground exerts a force on the ball (push) so it bounces or stops
Speed, Velocity, & Acceleration
Unbalanced Forces
- object will accelerate in the direction of the net force
- speeds up, slows down, or turns
o speeds up
 hitting a ball, kicking a ball, swinging a bat, etc.
 falling – gravity 9.8 m/s/s
o slows down
 catching a ball, trapping a ball, diving
o turns
 diving, hitting a baseball, etc.
Momentum
- Mass * Velocity
- The longer the net force acts upon the object, the greater the change of momentum
o P2 – P1 = difference of momentum before and after net force
o Force * time = change in momentum
 Ft = P2 – P1
 Ft = mv2 – mv1
Newton’s Laws
Newton’s First Law of Motion (Law of Inertia)
-
“An object at rest will stay at rest, an object in motion will stay in motion, in a straight line and at a constant
speed, unless acted upon by an outside force (net force).”
Newton’s Second Law of Motion
- “An object acted upon by a net force will accelerate in the direction of the force.”
- Equation: acceleration = Net force/mass
- Equation: Net force = mass/acceleration
- Force is measured in Newtons = 1 kg*m/s2
- The larger the mass, the slower the acceleration
Newton’s Third Law of Motion
- “Forces always act in equal but opposite pairs.”
- “For every action, there is an equal but opposite reaction.”
- Force applied (action) causes a return force (reaction)
- No delay between the two forces
o Walking – you push down as the ground pushes up
o Paddling, rockets, etc.
Gravity
Force – a push or pull
- Gravity – a pulling force
o Force of attraction that occurs between 2 objects that have mass
o Greater the mass, the greater the gravitational pull exerted
o Relatively weak force
 Objects will notice gravitational force when they are near an object with a greater mass
 Distance affects –
Smaller distance, greater pull
Larger distance, weaker pull
Magnets
Magnetism
– property of some materials that allows for attraction of opposite polar qualities
– Structure and arrangement of atoms creates the quality
o Motion of electrons gives the magnetic field
o When groups of atoms are aligned with magnetic fields, creates a magnetic domain
o Ferromagnetic materials
 Fe (iron)
 Co (cobalt)
 Ni (Nickel)
All have magnetic domains but pointing in different directions
Have to be “rubbed” with a magnet and that aligns the material
End result is it becomes a magnet
- Magnets
o Produces a magnetic field that extends in all directions from the magnet
o
http://hyperphysics.phy-astr.gsu.edu/%E2%80%8Chbase/magnetic/elemag.html
o
o
Within the field – attraction of ferromagnetic materials
Attracts or repels
 Determined by the orientation of the magnets to each other
North seeking pole
South seeking pole
Like poles repel, opposites attract
o
o
(http://www.physics4kids.com/files/elec_magneticfield.html)
Types
 Permanent
Retains the field for a long time
 Temporary
Lose their fields quickly
Force of the field decreases as the distance increases
Interaction of Force & Mass
Force – a push or pull
- Newton’s 3rd law
o When a force is applied to one object by a second, the first object will exert equal and opposite
force on the 2nd.
- Newton’s 2nd law
o If there is only one force applied, the object will move in the direction of the force applied
o If there are multiple forces from different directions
 The net movement will be in the direction indicated by the sum of the vectors
corresponding to each force
 Not necessarily in the direction of the strongest force
- Different kinds of forces
o
o
-
Contact
 Occur between two masses that are directly touching
 Ex. Friction
Oppose motion
Dependent on
o Composition of materials in contact
o Mass of the materials
o Other forces acting
Distance
 Electrical
 Magnetic
 Gravitational
 Tend to be stronger when the distance decreases
Torque
o measure of how much a force acting on an object causes that object to rotate
o The product of the magnitude of the force and the perpendicular distance from the force to the axis
of rotation (i.e. the pivot point)
o