PHYS 218 Chapter 4
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This chapter focuses on dynamics: the motion of objects and the forces that cause motion.
Forces
Interaction between two or more objects/bodies
Measured in Newtons (N) = kg m/s2
- Usually an interaction that one body exerts on another
- Vector quantity
Newton's Laws of Motion
All laws apply in Inertial Reference Frame (no acceleration acting on reference frame)
- An object with no forces acting on it maintains a constant state of motion
- Force = mass × acceleration
- For every action force, there is an equal and opposite reaction force.
Mass vs. Weight
Weight is a vector force that is acted upon by gravity (measured in Newtons)
Mass is a scalar and an intrinsic property of an object.
Monday, February 14, 2011 (PHYS 218 Exam 1 Review)
Normal Forces
contact force between 2 surfaces that is perpendicular to the surfaces
1 pair per surface (action/reaction) because of Newton's 3rd law
- origin on object being described pointing away from contact surface
Monday, February 21, 2011
Gravitational Force
(constant; on surface of a planet)
weight (downward force) is equal to the mass times the acceleration due to gravity: Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle w = mg}
- Origin is at the center of gravity
- If Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle g} is constant and the mass distribution of the object (i.e. density) is homogenous, center of gravity is the geometrical center of the object
Tension in a Rope
A means of applying a force to something else
If rope is massless, then force is transferred perfectly
- tension is uniform throughout rope
Pulleys
Change direction of tension; magnitude stays the same!
Frictional Forces
Contact force between two surfaces that is parallel to surface
Kinetic Friction Force
- Move with respect to each other
- Opposite direction of velocity
- Magnitude is proportional to Normal force: Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle |F_k| = \mu_k |N|\,\!}
- μk (coefficient of kinetic friction) is a constant that depends on the characteristics of the two surfaces.
Static Friction Force
- Does not move with respect to each other (i.e. objects are not moving)
- "Direction opposes movement you would have if friction force wasn't there to begin with"
- Magnitude grows to oppose applied force (up to a certain maximum given by Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle |F_s| \le \mu_s |N|\,\!}
)
- μs (coefficient of static friction; generally Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \mu_s \le \mu_k} )
Used in ABS to attempt to keep overall frictional forces at a maximum.
Example: Walking on earth
Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \frac{F_{earth\ on\ you}}{m_{you}}=a_{you}}
Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \frac{F_{you\ on\ earth}}{m_{earth}}= a_{earth}}
Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \frac{-F_{earth\ on\ you}}{m_{earth}}=a_{earth}}
Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \frac{a_{you}m_{you}}{m_{earth}}=a_{earth}}