Newton’s Second Law of Motion How fast does it go?

Acceleration An unbalanced force causes something to accelerate.

Acceleration Acceleration is directly related to the size of the force and the direction of the force. It accelerates in the direction you push or pull it.

In other words…. Large Force = Large Acceleration F a

In other words…. Small Force = Small Acceleration F a So….if you push twice as hard, it accelerates twice as much.

But there is a twist…. Acceleration is INVERSELY related to the mass of the object.

In other words…..using the same amount of force…. F Large Mass a Small acceleration F Small Mass Large acceleration a

Newton’s Second Law Newton, that brilliant genius, observed those “rules” of acceleration and came up with his second law of motion. It is both a formula & a law.

Newton’s Second Law The acceleration of an object is directly proportional to the net force & inversely proportional to it’s mass. F = ma Force = Mass x Acceleration

Calculating Acceleration Acceleration Equation Acceleration = (final velocity(in m/s) – initial velocity (in m/s)) Time (in seconds) A= (v f -v) t

Okay then… First, you need to know the units of Force, Mass & Acceleration. –The units used for force are Newtons (N) –The units used for mass are kilograms (kg) –The acceleration units are meters per second squared (m/sec2).

Next, remember what speed is?

Average Speed Speed Comparison of time and distance –Distance traveled per unit time

Acceleration A change in velocity –Speeding up Positive acceleration –Slowing down Negative acceleration –Changing direction

Calculating Speed Given Distance & Time Velocity = Distance ÷ Time D VT Velocity = Distance Time

Calculating Acceleration Acceleration Equation Acceleration = (final velocity (in m/s) – initial velocity(in m/s)) Time (in seconds) A= (v f -v i ) t Oh… I get it now!

Let’s practice… Rob is really bored one Saturday night and goes outside to study the nocturnal habits of mice in the hayfield. He sees a mouse sniffing along at 0.1 m/s. but it hears and starts to scurry for safety. In just 3.7 s it accelerates to 0.9 m/s. Find its acceleration.

Let’s Practice Acceleration = (final velocity (in m/s) – initial velocity(in m/s)) Time (in seconds) A= (v f -v i ) t A=0.9m/s – 0.1 m/s 3.7s A=0.8m/s 3.7s A= 0.22 m/s 2

Let’s Practice A roach moves down the hall at 1.2 m/s. When he sees the janitor coming down the hall, he begins to run. After 3.2 s, he is moving at 3.6 m/s. What is his acceleration?

Free Fall When air resistance doesn’t affect the motion of a falling object, we say the object is in free fall. Free falling objects gain speed at the rate of 10 m/s m/s 2 is acceleration due to gravity or g. Downward falling objects GAIN speed because of the force of gravity

Free Fall Objects thrown upward LOSE speed because they work against gravity. At the highest point have an instantaneous speed of 0 m/s.

Friction is a force that affects motion Friction occurs when one objects rubs against something else. When an object falls towards the Earth in the presence of air, the force of friction or air drag acts upward. The amount of air drag depends upon: The speed of the object The area of the object The amount of friction between two surfaces depends on: The kinds of materials the surfaces are made. The area of contact.

Friction is a force that affects motion Four types of friction are: –static friction friction between two surfaces that aren't moving relatively to each other. –rolling friction friction between a rolling object and the surface that it is rolling on. – sliding friction friction where an object slides or rubs against another surface. – fluid friction friction of something going through any type of liquid or air.