Newtons 2nd Law October 29, 2012

“Do Now” How did the egg drop demonstration explain Newton’s First Law of Motion? (3-5 sentences)

Newton’s 1st Law of Motion An object at rest will stay at rest and an object in motion will stay in motion unless an unbalanced force acts on it Inertia: An object’s resistance to changing it’s motion Think about the book and the piece of paper. The book had more mass, so it had more inertia. Since it had more inertia it was less likely to become an object in motion when I removed the tablecloth. However, the piece of paper had less mass so it had less inertia. When I pulled the table cloth out, the piece of paper was less resistant to changing its motion so it came off the table with the table cloth. **Watch this video if you are still unclear about Newton’s 1st Law of Motion** http://www.youtube.com/watch?v=sabH4bJsxWA Or this table cloth demonstration: http://www.youtube.com/watch?v=-cM9S2AzU28 *more mass = more inertia

Transition from Newton’s First Law into Newton’s Second Law of Motion Newton’s first law tells us that an object’s velocity will not change unless an unbalanced force acts on it Newton’s second law tells us how much velocity changes when there is an unbalanced force! The first law told us the logic behind Newton’s Laws of Motion. The second law gets specific by giving us a number and calculation for how much an object’s acceleration, mass and force are affected.

Write down a list of at least 5 things that will change their motion when you apply an unbalanced force to them (30 seconds) Example: A table

Forces and Acceleration If you apply an unbalanced force, an object will accelerate For each object in your list, would you expect the acceleration of an object to be more or less when you increase the force you apply to it? (write this answer, under your examples, in a complete sentence) Do this in the table on your guided notes.

If you apply the same net force to different objects, which ones will change their velocities the most? (write this answer down in a complete sentence) Rank each object (1-5) you wrote down based on how much you think it will accelerate. 1= will accelerate the least 5=will accelerate the most Do this in the table as well

(write this on your paper in 2-4 complete sentences) What trends do you see based on what you’ve thought about so far? (write this on your paper in 2-4 complete sentences) Write this in the space provided below your table in your guided notes

Force, Acceleration, and Mass A bigger force will make an object accelerate more and a smaller force will make an object accelerate less If there is a bigger mass, an object will accelerate less and if there is a smaller mass, an object will accelerate more There is a mathematical equation that explains this… As mass goes up, acceleration goes down As mass goes down, acceleration goes up. This is what is known as an inverse relationship. One goes up, the other goes down. **Think about it logically. If you push a heavy person with the same mass as a light person, the lighter person will accelerate less than the heavier person.

Acceleration = Net Force / Mass This means that if… The net force increases, acceleration will increase If there is the same mass and the force doubles, acceleration will double The mass increases, acceleration will increase If there is the same force and mass doubles, acceleration drops by half

Newton’s Second Law Fnet = ma We rewrite the formula as Net Force=Mass x Acceleration to get rid of the division Fnet = ma Fnet=Net Force= Total force acting on an object m=mass a=acceleration (Put a big star next to this part in your notes! This equation is very important) Fnet simply means the sum (add/subtract them all up) of all forces acting on an object. Can be represented with the Greek letter Sigma (Σ)

Fnet = m a To use the formula properly, we need to make sure we have the correct units We measure: Force in Newtons (N) mass in kilograms (kg) acceleration in meters/second2 (m/s2)

Law #2: The acceleration of an object depends on the object’s mass and the strength of the outside force acting on it. (write this down in your notes) Write Newton’s Second Law of Motion down in your notes.

Law #2: The acceleration of an object depends on the object’s mass and the strength of the outside force acting on it. If applying the same force… Small mass=big acceleration Big mass=small acceleration

Example Problems Fnet=ma What is the net force required to make an object with a mass of 30 kg accelerate at 2 m/s2 to the right? *Hint: Use your 4 steps to solve!* What if we want to accelerate it to 6 m/s2 to the right? What is the net force required to make an object with a mass of 10 kg accelerate at 2 m/s2 to the right? Simply plug the variables into the equation where they belong and solve for the 1 unknown variable left. A good tip to know which goes where is to look at the units. Mass is measured in units of kg, acceleration is measured in units of m/s2, force is measured in N. Example 1: Fnet=(30kg)(2m/s2) Fnet=60kgm/s2  kgm/s2= N So… Fnet=60N

Check YOUR Understanding Answer the questions on your guided notes sheet. If you are having trouble, look back at your notes, ask a classmate, or ask Ms. K.