Unit 1 Summary

Equilibrium and Net Force

-Equilibrium occurs anytime a net force, also known as the total force, on an object adds up to zero N, or newtons.

-Equilibrium can occur at either constant velocity or at rest.

-Net force is a type of force, and a force is a push or a pull measured in newtons.

-A newton is about ¼ of a pound.

-All of this relates to inertia, which is an object’s resistance to change.

-Mass is a measure of inertia, so an object with a larger mass will be less resistant to change than an object with a smaller mass. Mass is not to be confused with weight, because weight is measured in newtons and mass in measured in kilograms.

-Since net force is the total force on an object, taking the difference in forces will allow you to solve for the net force of the object. For example, if someone is pushing on a box with a force of 5n to the right (5n -->) and someone else was pushing on the box with a force of 10n to the left (<--- 10n) the net force, or total force, of the box is 5n. The acceleration is 5n going in the left direction. (<-- 5n)

-If someone was pushing on the box with 5n to the right (5n -->) and someone else was pushing on the box with 5n to the left (<-- 5n) the net force of the box would be 0n so the box would be at equilibrium moving at constant velocity.

Speed and Velocity

-Velocity and speed are a little different from each other. For example…

Velocity can be changed in three ways: if the force accelerates

                                                                if the force decelerates

                                                                if the force changes direction (velocity requires a specific direction, and you can’t change the direction without changing the velocity)

-Speed on the other hand doesn’t have a specific direction. You could be moving at a constant speed, but be changing your velocity at the same time. For example, if a racecar was driving at a constant speed around a circular racetrack, the car is maintaining a constant speed, but the car is also changing direction, so the velocity is changing.

-To have constant velocity you must have constant speed, but constant speed does not always entail constant velocity.

-To solve for speed, you would take the distance that the object moved over how long (the time) that the object was in motion.     Speed = distance           

                                                                    time

-For example, if a car moved 3m every 5 seconds how fast was the car moving?

3/5=.6 so the car moved at .6m/s  (You must never forget to put the units!) Speed is                                                                    measured in m/s (meters per second)

-If the question had specified that the car was moving at constant speed, then the car would continue to move at 3m/s

-The formula to solve for speed with constant velocity is     v = d
 You would plug in your given distance and time, and then         t
 divide the distance by the time to solve for velocity. 
*notice that it is the same formula that we used for speed*

-The formula to solve for distance with constant velocity is     d = vt
You would plug in the given velocity and time, multiply them, and that would equal
your distance.

Acceleration

(change in v)  = m * 1 = m = m    the units for acceleration 

         t                 s     s    s*s   s^2

-Constant velocity and acceleration are sort of enemies, because you can’t have constant velocity with acceleration and you can’t have acceleration with constant velocity.

-If you have constant velocity you don’t have acceleration, because one of the three ways that velocity changes is by the force accelerating.

-If you have acceleration you can’t have constant velocity, because if the object is accelerating, the object is longer moving at a constant rate.  

-Anytime there is a net force, there is acceleration. Back to our box-pushing example, if someone is pushing on a box with a force of 5n to the right and someone else was pushing on the box with a force of 10n to the left, the net force, or total force, of the box is 5n. This means that the acceleration is 5n going in the left direction.

To solve for the acceleration, you divide the change in velocity by the time interval.
Acceleration = change in velocity

                             time interval

For example, if a car accelerated from 0mph to 30mph in 10 seconds what is the acceleration of the car? change in velocity  =  30-0   =  30 = 3m/s^2

            time interval            10         10

*Don’t forget your units! Acceleration is measured in meters per second squared       

                                                                                                                      (m/s^2)

-The formula to solve for speed with constant acceleration is --> v = at
You would plug in your given acceleration and time, multiply
them and that would be your velocity.

-The formula to solve for distance with constant acceleration is      d = 1/2at^2
First you would plug in your given acceleration and time. Then you would square your time, take one half of your acceleration, and lastly multiply your acceleration by your time to get the distance.

Ramps

You can determine if a ramp's acceleration and speed is increasing or decreasing. 

*Speed is always increasing, just at different rates*

This ramp is straight, so is has constant 

acceleration as well as increasing speed.

This ramp is getting less steep, so the acceleration

is decreasing, but the speed is still increasing just at a slower rate. 

                       

This ramp is getting more steep, so the acceleration is increasing and the speed is also increasing.

Using a Graph (equation of a line) to solve problems

Steps for solving for the slope
1) Turn line equation into words/symbols
2) Identify the physics formula that the line equation looks like
3) Line up the equation of the line with the "look-alike" physics formula to see what's missing
4) Solve for what is missing, which will be your slope.

Equation:   y = 4x

Looks like: d =vt

Line up the equations: y =4x

                                    d = vt

                                    v = 4

(Velocity was missing, so that is our slope)

If the time was squared on our x-axis,                      Equation:    y = 2x

then the equation would look a little                         Looks like: d = .5a(t^2)

different... -->                                                            Line up the equations: y = 2x

                                                                                                                       d = .5a(t^2)

                                                                                                                      .5a = 2

                                                                                                                      (2).5a = 2(2)

                                                                                                                       a = 4
                                                                                 (Acceleration was missing, so that is our slope)

Inertia/Newton's First Law

Newton's First Law states that "things in motion will stay in motion, and things at rest will stay at rest unless acted upon by an outside force". This video demonstrates that law. For example, in the ball throwing experiment, the ball was thrown up into the air will an initial force, the ball wanted to keep moving because things in motion like to stay in motion, but the outside force (in this case gravity) pulled the ball back down. Another example of Newton's First Law is the cup and paper experiment. The force (a pull) was on the paper and not the cup, so the cup stayed in place, because things at rest like to stay at rest.