Newton’s Second Law of Motion Force & Acceleration Introduction to Science A. King EHS

Acceleration Galileo define rate of change of velocity as acceleration. The term acceleration is typically used when the velocity increases. The term deceleration is typically used when the velocity decreases – another way of saying this is that “deceleration” is when negative acceleration is occurring. In both cases, “acceleration” is the process that’s actually occurring.

Calculating Acceleration ΔV Acceleration = t Where ΔV is the change in velocity and t is the time it takes for the velocity to change. ΔV acc t :: x

Units for Acceleration Speed and velocity are easily measured in distance/time units, such as miles/hr, m/s, etc. Units of acceleration are more complicated, and have to take other stuff into account. Sample problem: What is the acceleration if we speed up from 10 km/h to 30 km/h in 10 seconds? Acceleration = Δv/t (30 km/h – 10 km/h)/ 10 sec = 2 km/h/s

Acceleration depends on the net force. Acceleration ~ net force.

Mass Mass is a measure of an object’s inertia and a measure of how much material an object contains. (It depends on the number and kind of atoms making the object up.) Mass is measured in grams: grams, kilograms, milligrams…

Weight Weight depends on gravity. Weight is the force due to gravity that acts on an object’s mass. Although weight and mass are different from each other, they are directly proportional to each other. 1 kilogram weighs 9.8 newtons.

Volume Volume is a measure of space.

Newton’s Second Law states: The acceleration produced by a net force on an object –is directly to the net force. –Is in the same direction as the net force. –Is inversely proportional to the mass of the object. –In formula: F = m x a

By using consistent units, such as newtons (N) for force, kilograms (kg) for mass, and meters per second squared (m/s 2 ) for acceleration, we get the exact equation: Δ v v f – v i Acceleration = = t t

Acceleration Problems While drag racing out of our school parking lot, I time myself at a speed of 40 meters per second seven seconds after starting. What was my acceleration during this time? A = v f – v i / t = (40 m/s – 0 m/s )/7s = (40 m/s)/7s = 5.7 m/s/s

Friction Occurs when one object rubs against something else. Occurs for solids, liquids and gases. Always acts in a direction opposite to motion. The amount of friction between two surfaces depends on the kinds of material and how much they are pressed together.

Free Fall When an object is dropped in a place with gravity (k.e. not deepspace), gravity causes the object to fall downward. When this occurs, the object is said to be in “free fall.” For problems like this, we’ll imagine that gravity is the only force that’s acting on the object – wind resistance and other stuff will be ignored.

How fast is an object moving in free fall? Let’s consider the following data table: The increase in speed per second is the acceleration which in this case is 10 m/s/s. The instantaneous speed of an object that is accelerating is equal to the acceleration multiplied by the amount of time the object is falling. In equation form: v = at Elapsed time (sec) Instantaneous speed (m/sec) t10t

Formulas Wt m g :: x d ½ g t2t2 :: x Δv acc t :: x v a t :: x v f - v i acc t :: x F m a :: x