Newton's First Law (or the Law of Inertia)
* 0m/s (at rest) is a constant velocity
*A given force accelerates a less massive object MORE than it accelerates a more massive object (since mass is the only thing that affects inertia)
Newton's Second Law
Acceleration is proportional to the net force acting on a system, and inversely related to the total mass of a system (net force = mass*acceleration)
Newton's Third Law
Whenever there is an interaction between two objects there is a force upon each of the objects. The two objects each feel the same magnitude force from the other, but in opposite direction
System schema & Free body diagrams & Force tables
System schema: visual way to keep track of the objects in a system and how they interact; each object is a circle, every force is a line connecting the two circles
Example)
- Inertia - an object's tendency to resist a change in motion / acceleration - solely depends on the mass of an object
- Force - a push/pull which attempts to accelerate an object
* 0m/s (at rest) is a constant velocity
*A given force accelerates a less massive object MORE than it accelerates a more massive object (since mass is the only thing that affects inertia)
Newton's Second Law
Acceleration is proportional to the net force acting on a system, and inversely related to the total mass of a system (net force = mass*acceleration)
Newton's Third Law
Whenever there is an interaction between two objects there is a force upon each of the objects. The two objects each feel the same magnitude force from the other, but in opposite direction
System schema & Free body diagrams & Force tables
System schema: visual way to keep track of the objects in a system and how they interact; each object is a circle, every force is a line connecting the two circles
- Label forces with the force type (using subscripts)
- Direction DOES NOT MATTER
- use dotted line for non-contact force like the force of gravity
- select object of interest (assume it to be the center of mass)
- draw an arrow for each force acting on the object & consider direction
- Label the force with 3 subscripts (type of force, BY, ON)
- Add up each column to calculate the net force
- Balanced Force: when net force is zero, Unbalanced Force: when net force is NOT zero
Example)
Force Calculations
Net Force Calculation: As explained above, construct a force table and add up the columns for horizontal & vertical components
Net Force Calculation: As explained above, construct a force table and add up the columns for horizontal & vertical components
http://mechanicsmap.psu.edu/websites/A1_vector_math/vectoraddition/vectoraddition.html
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Vector Additions:
"tip to tail" method - sum of vectors can be determined geometrically - put the tail of each vector at the tip of the last vector until you have added taken all vectors into account. Once the vectors are all drawn tip to tail, the sum of all the vectors will be the vector connecting the tail of the first vector to the tip of the last vector. |
Solving Force Problems
1) Before solving the problem, set your calculator to DEGREE mode
2) Read the question and draw a system schema & free body diagram
**If one of the forces are at an angle, redraw the free body diagram so that most of the forces align with the axes
3) Create a force table, filling in the information you know from reading the question
4) Fill in the blanks by calculating the forces
**For the forces that are at an angle, use trigonometry to separate the force into horizontal and vertical components
5) Solve for the thing asked in the problem (acceleration/net force/mass, etc.)
Helpful Equations...
1) Before solving the problem, set your calculator to DEGREE mode
2) Read the question and draw a system schema & free body diagram
**If one of the forces are at an angle, redraw the free body diagram so that most of the forces align with the axes
3) Create a force table, filling in the information you know from reading the question
4) Fill in the blanks by calculating the forces
**For the forces that are at an angle, use trigonometry to separate the force into horizontal and vertical components
5) Solve for the thing asked in the problem (acceleration/net force/mass, etc.)
Helpful Equations...
- Hooke's Law: Spring force & the displacement of stretch act in opposite directions (the spring always wants to go back to equilibrium), the constant k measures the spring's stiffness
- Static Friction: surfaces do not slide relative to one another, the coefficient of static friction can change to keep the net force zero.
- Kinetic Friction: surfaces do slide relative to one another, the coefficient of kinetic friction is constant and less than the maximum static friction
Motion & Force
(http://www.pedersenscience.com/uploads/5/6/3/8/56384743/how_read_position_velocity_acceleration_graphs.pdf)
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1. Stationary object (object at rest) - velocity & acceleration is zero - net force is ZERO - Newton's 1st law states that an object at rest will remain at rest unless it feels an unbalanced force 2. Object traveling in a constant velocity - velocity remains the same - acceleration is zero - net force is ZERO - Newton's 1st law states that an object will continue to travel at a constant velocity unless it feels an unbalanced force 3. Constant acceleration - velocity increases - acceleration is constant - net force is NOT zero - constant amount of force exerted on the object causes constant acceleration |
Solving motion - force problems
*use force calculations to figure out the acceleration
*then, use kinematic equations to solve for the question
EXAMPLE (from mastering physics 2.3 #7)
*use force calculations to figure out the acceleration
*then, use kinematic equations to solve for the question
EXAMPLE (from mastering physics 2.3 #7)