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Chapter 3: Newton’s Second Law of Motion
Force and Acceleration

Galileo Developed the Concept of Acceleration
Galileo used inclined planes (ramps) He found that balls rolling down inclines rolled faster and faster Acceleration = change in velocity/ time interval

Acceleration can be negative
Example: a car is moving in a certain direction. The driver steps on the accelerator. The car speeds up. The driver lets up on the accelerator and the car starts to slow down. The car decelerated (negative acceleration) Deceleration is indicated by an arrow in the opposite direction of motion

Question: If a car makes a turn, but is going the same speed, is it accelerating?
Speed doesn’t change, but direction does Remember that acceleration is change in velocity/time Velocity is speed with direction So, in the scientific sense the car is accelerating because the car’s direction is changing as it is going around the turn.

Explanation of Acceleration
Look at Figure 3.4 on page 36 What would be the reading of the speedometer at: Time - 3 sec = ______ m/sec2 Time - 4 sec = ______ m/sec2 Time - 5 sec = ______ m/sec2

Force causes Acceleration
Acceleration depends on net force. Larger force, larger acceleration and vice versa. Note: with mass constant. Example: Suppose you pull a wagon with a net force of 20N. Friction and gravity are working to slow down the wagon - Lets say 10N of force The net force on the wagon is 10N and the wagon moves in the direction of the larger force which is the pull that you generated

Acceleration ~ net force
~ stands for directly proportional As one increases, the other also increases As net force increases so does Acceleration There is a general rule: if the net force is X2 then accel. Is doubled If net force is X3, acel. Is tripled

Mass of an object corresponds to or relates to inertia
The greater an object’s mass the greater it’s inertia Mass is also a measure of how much material (# and kinds of atoms) an object contains

Don’t Confuse Mass and Volume
Volume is the amount of space an object takes up Example: 2 bags of equal size (1) contains cotton the other contains rocks Equal volumes but not equal masses

Mass resists Acceleration
The more massive an object is the more difficult to accelerate it Acceleration ~ 1/mass - When Force is constant Inverse relationship = mass and acceleration change in opposite ways As one goes up the other goes down and vice versa

Mass resists Acceleration
The more massive an object the more difficult to accelerate Twice as much mass has 1/2 the acceleration X3 as much mass results in 1/3 the acceleration

Newton’s Second Law Force = mass x acceleration Units Force = (N)
Mass = kilograms Acceleration = meters/sec2

Newton’s 2nd Law

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

Friction

Mass vs. Weight Mass is the amount of matter in an object
Weight is the force due to gravity that acts on an objects’ mass

Mass vs. Weight

Objects in Free Fall have equal Acceleration
Gravity m/sec2 Neglecting air drag or air resistance

Acceleration is less when air drag acts
For Free Fall the downward force is weight The upward force is air drag (air resistance) Net force = weight (N) - air drag (N) When air drag = weight we have ∑F = 0

Does that mean the object in free fall comes to a stop?
No, the object no longer picks up speed Example: Parachutist The diver reached terminal speed (velocity)

Forces on a Parachutist

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