Newton’s Laws

Compare/Contrast these situations Does the length of rope matter?

Newton’s First Law (of Inertia) DON’T COPY THIS: “An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by a force” If the forces acting upon an object are balanced the object will continue to do what it is doing. Inertia: the resistance to change in the state of a motion Objects maintain a constant velocity unless acted upon by another force, therefore acceleration requires unbalanced forces. What is inertia? Define law in your own word: Objects tend to keep doing what they are doing Imagine you are in your car with a cup of coffee if you accelerate forwards what happens to the coffee? What about if you abruptly stop? Quiz Page 10

Inertia & Mass Remember inertia is the resistance an object has to a change in it’s state of motion. Forces aren’t need to keep an object in motion, rather forces are needed to bring an object to rest. The tendency of an object to resist a change in motion depends on it’s mass; the more mass something has the more inertia it has.

State of Motion Redefined inertia is the tendency of an object to resist changes in it’s velocity According to Newton’s First Law: An object at rest will remain at rest with a velocity of zero unless acted upon by an unbalanced force An object traveling at a speed will remain at that speed unless acted upon by an unbalanced force Therefore inertia can be redefined as the tendency of an object to resist acceleration

Remember a force is a push or a pull on an object Balanced Forces Unbalanced Forces In order for two forces to balance they must be equal in magnitude. What does a force diagram look like for a book at rest on my table? There is no equivalent force opposing Cause acceleration What would the force diagram look like for a book on the table pushed leftwards? Sitting on desk: What forces are acting upon the book? Are they balanced? Being pushed

Newton’s First Law “An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by a force” Define Newton’s First Law in your own words.

Forces Contact Force Field Force When two interacting forces are perceived as physically touching. Ex: friction, tension, normal, air resistance, applied, & spring force. A force which is able to exert a push or pull without physical contact. “action at a distance force” Ex: gravity, electric, & magnetic force. Forces are vector quantities (magnitude & direction) Forces are measured in Newtons 1 N= the force required to five a 1kg object an acceleration of 1m/s2

Forces

Force Research Create a power point slide (emailed to me) or poster to “present” a force Make sure to: Describe/define the force Identify any specific equations associated with it Show an example of the force in a force diagram You will briefly share your findings tomorrow in class Resources

Mass vs. Weight Mass: the amount of “stuff” making up an object Weight: the force of gravity interacting with the amount of “stuff” making up an object On earth weight is equivalent to 9.8Newtons per a kilogram of “stuff” or 9.8N/kg On the moon your weight is equivalent to 1.7N/kg If there are .45kg in 1 pound what is your weight? What is your mass? What is your mass on the moon? What is your weight on the moon? My weight on earth 135lb or 60.75kg My mass on earth 6.20 kg or 13.7lb My mass on the moon is the SAME! My weight on the moon 10.5 kg or 23.1 lb

Free Body Diagrams Free body diagrams show the relationship between relative magnitude and direction of all forces acting upon an object. The size of the arrow represents the magnitude of the force Steps for drawing a diagram Identify what forces are acting upon the object Identify the direction and magnitude of these forces Draw the diagram representing: The object (or a box if you are not artistic) Arrows representing forces

Practice Free Body Diagrams A book falls of the desk and is free-falling to the ground A painter hangs on a rope secured to the ceiling. A force is applied to drag a sled with a rightwards acceleration across snow (ignore air resistance)

Net Force Unbalanced force: when vertical forces (up/down) do not cancel each other out and/or horizontal forces (right/left) do not cancel each other out. Unbalanced forces have a net force ≠ 0 Net force is a vector sum of all forces Observe in the following examples that downward vectors provide a partial or full cancellation of an upward vector. What are the net forces for each example? If balanced forces result in no acceleration, unbalanced forces, or the presence of a net forces causes an acceleration. What are the net force of the three examples? Quiz 45

Newton’s Second Law The acceleration of an object directly depends upon the net force acting upon the object and is inversely related to the mass of the object Fnet=ma or a=Fnet/m 1 Newton= 1kg * 1m/s2 Acceleration is in the same direction as the net force.

Practice Calculations 5 m/s/s 10 m/s/s 5 m/s/s What happens when mass is doubled? Acceleration is halved What happens when mass is halved? Acceleration is doubled 20 N 1 kg