Important Topics
- Newton's First Law
- Newton's Second Law
- Newton's Third Law
- Force Diagrams and System Schemas
- Force Calculations
- Relating Representations of Motion and Force Models
- Solving Problems with Forces and Motion
Introduction to Forces
Force - An interaction between two objects that results in a push or pull (change in motion)
System Schemas / Force Diagrams
System Schema- Used to map out the forces that are interacting between different objects
Force Diagram- Used to show the direction of the different forces
Force Table- Break forces into horizontal and vertical components (good way to keep track of information for force problems)
- Start with the desired object in the center
- Write down all of the other objects interacting with it
- Connect the circles with a solid line for a contact force and a dotted line for a non-contact force (like gravity)
- Label the force
Force Diagram- Used to show the direction of the different forces
- Draw a dot (represents your object)
- Draw arrows in the direction of the force
- Label each arrow by saying "(Force) by (object) on (central object)"
Force Table- Break forces into horizontal and vertical components (good way to keep track of information for force problems)
Newton's Laws Overview
- Newton's First Law- An object will continue to move at a constant velocity unless it feels an unbalanced push or pull
- Newton's Second Law- Acceleration is directly proportional to total force and inversely proportional to mass
- Newton's Third Law- Whenever two objects interact, each object exerts an equal force on the OTHER object. These forces are equal in magnitude but opposite in direction
Newton's First Law
- An object will continue to move at a constant velocity unless it feels an unbalanced push or pull
- Inertia- Object's tendency to resist change (we are referring to change in motion)
- Depends on the mass of the object
- A given force accelerates a less massive object MORE than it accelerates a more massive object
- A more massive object is harder to stop
- Balanced force = Constant velocity, acceleration of 0 m/s^2
- Constant unbalanced force = Constant acceleration
Think about guiding the bowling ball and basketball through the obstacle course
Newton's Second Law
- Acceleration is directly proportional to total force and inversely proportional to mass
- F = ma
- Connects forces and motion
- Allows you to use both force equations and kinematic equations to solve problems
- Allows you to use both force equations and kinematic equations to solve problems
Newton's Third Law
- Whenever two objects interact, each object exerts an equal force on the OTHER object. These forces are equal in magnitude but opposite in direction
- Newton's Third Law Paradox- Forces don't balance out because they act on different objects (don't show up on a single force diagram)
- The magnitude of the force the moon exerts on the Earth is equal to the force the Earth exerts on the moon. The reason the moon orbits the Earth is because of inertia (Earth is more resistant to a change in motion)
Force Calculations
Force of Gravity
Spring Force
Force of Friction
- Force of gravity = Weight
- Force of gravity = Mass * Gravitational Field Strength
- Fg = mg
Spring Force
- Spring force is proportional to strength
- Hooke's Law: Fs= -k * ∆x
- Spring Force = Spring Constant * Stretch
- Negative sign shows that the force vector and stretch vector are in opposite directions
Force of Friction
- Friction Force is proportional to normal force
- ƒ = µ * Fn
- Friction = Coefficient of Friction * Normal Force
- Static Friction- Friction when object is not moving, changes to keep the forces balanced but has a maximum value
- Kinetic Friction- Friction when object is sliding, stays constant, always less than the maximum value of the static friction
- When the maximum value of the static friction is overcome, the object starts to slide
Models of Motion
Constant velocity
Uniform acceleration
- Velocity = change in position / change in time
- Strobe and motion maps
- Position vs. time graphs (linear)
- Velocity vs. time graphs (flat line)
Uniform acceleration
- Acceleration = change in velocity / change in time
- Velocity vs. time graphs (linear)
- Acceleration vs. time graphs (flat line)
- Kinematic equations
Newton's Second Law connects these models of motion to models of forces (force diagrams, system schemas, force tables, adding vectors, etc)
Solving Force Problems
- Draw a system schema, force diagram, and force table
- Write out known values
- Write the equation you are going to use but without plugging in values
- Plug in and solve for desired variable
Advanced Force Problems
Follow the same steps as above, but these will require multiple equations
Elevator Physics
- You feel your weight based on the normal force from what you are on top of (apparent weight)
- Normal force changes in an elevator, not the force of gravity
- At rest in an elevator- Balanced forces
- Accelerating upwards- Normal force > Force of gravity - Feel heavier
- Accelerating downwards- Normal force < Force of gravity - Feel lighter
- Free fall- Only Force of Gravity - Feel weightless
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