Regents Physics n Agenda n Introduction to Forces

Regents Physics n Agenda n. Introduction to Forces n. Intro to Newton’s three Laws of Motion n. HW: Read p. 117 -125

What’s a Force? n We’ve learned that acceleration is the change in an object’s velocity. . And n Velocity is the change in an objects position. . n By what causes the change in acceleration? A Force!

What’s a Force? n Forces can be described as a push or a pull that is applied to an object by something else. . n Forces are vectors – magnitude and direction n The ability to understand how forces affect us is crucial to success in many fields n Ex: building of homes and bridges

Kinds of Forces n Contact force – acts on an object only by touching it n Ex: book on table, friction n Long-range force – is exerted without contact n Ex: magnetic force, force of gravity

Forces have agents. . n Each force has a specific, identifiable, immediate cause called the agent n The agent can be animate – such as a person or n inanimate – such as a desk, floor or magnet What’s the agent for the pull of gravity?

Solving Force Problems - intro n First step is to draw a pictorial model, called a free body diagram, and identify the contact and long range forces n Draw the vectors n Example F desk on book Book on a table F book on desk

Solving Force Problems - intro n Examples Ball on a rope F rope on ball F ball on rope Skydiver F air on diver F gravity on diver

Practice Problems n Draw a free body diagram for each of the following: n Book held in your hand n Book pushed across the desk by your hand n Book pulled across the desk by a string n Book on a desk with you hand pushing down n Ball just after the string that was holding it broke

The man…Sir Isaac Newton n 300 hundred years ago an apple fell on his head…and he wondered why? n Explained the way forces influence motion n Summed it up in three famous laws

2 nd Law of Motion n The force exerted on an object is equal to the objects mass times its acceleration, or F = ma n Expressed in newtons = kg x m/s 2 n Example: Mr. O n Mass = 95 kg n Acceleration = gravity = 9. 80 m/s 2 F = ma = (95 kg)(9. 80 m/s 2) = 931 N

2 nd law continued n F = ma can be rearranged to be a= F m n we say that the force exerted on an object is proportional to its acceleration (since the objects mass doesn’t change) n The larger the force. . the greater the acceleration n We have a linear relationship!

2 nd law continued n Multiple forces can combine and act on a system n They could act in the same direction or in different directions n Because forces are vectors, the total force on an object is the vector sum of all forces on the object n This vector sum is called the net force

Finding a net force n Two horizontal forces act, 225 N and 165 N, are exerted in the same direction on a crate (assume no friction). Find the net horizontal force on the crate. n Step 1 – draw a free body diagram F = +225 N F = +165 N Fnet = Facting on the crate Fnet = 225 N + 165 N = +390 N worksheet

Regents Physics n Agenda n Newton’s first law of motion - Intro n Newton’s second law practice probs n HW Chap Problems: n RC #1, 3, 5, 6, 8, 9 n AC # 10, 15, 16 n Probs # 20, 22, 25, 27, 30, 32, 36, 38, 40

1 st Law of Motion n An object that is at rest will remain at rest or an object that is moving will continue to move in a straight line with a constant speed, if and only if the net force acting on that object is zero. n Also called the Law of Inertial Mass n What does that mean to us?

1 st Law of Motion n What is inertia? Inertia is the tendency of an object to resist change Anything that has mass has inertia! We resist what tries to change us! Examples Sitting still / moving at a constant velocity Object in space Turning in a car

1 st Law of Motion n What is equilibrium? If an object is at rest or it is moving at a constant velocity, the net force is zero in both cases We are happy just chillin. .

Some Types of Forces n See table 6. 2 pg. 123

Common Misconceptions n When we throw a ball, the force from our hand stays on it n A force is needed to keep an object moving n Inertia, itself, is a force n Air does not exert a force

Calculations with Newton’s 2 nd Law n Using F = ma n Mass and weight. . what’s the difference? n Ex: a bathroom scale n Draw a freebody diagram for this and label the forces

A system of solving. . n Read the problem and visualize! n Choose a coordinate system n Write your known and unknowns n Use f = ma to link acceleration and net force n Rearrange, plug in numbers and solve n Check your answer to see if it’s reasonable

Practice Problem n On Earth, a scale shows that you weigh 585 N n a) What is your mass? n B) What would the scale read on the moon? (g = 1. 60 m/s 2)

Regents Physics n Agenda n More on Newton’s Second Law n Review Intro to Forces Worksheet n Drawing Free-body Diagrams Worksheet n HW: More Advanced Newton’s Second Law Problems

Practice Problem - elevator n You still weight 585 N. Find the reading on a scale in an elevator when: n n n a) the elevator moves up at a constant speed b) it slows at 2. 0 m/s 2, while moving upward c) It speeds up while moving 2 m/s 2 downward d) it moves downward at a constant speed e) it slows to a stop at a constant magnitude of acceleration End

Working with friction. . . n Friction is a force that generally opposes the direction of motion n Different surfaces have different amounts of friction that affect an objects movement n There are types of friction: n Kinetic Friction = uk n Static Friction = us Ffr = Fnu

Solving Problems with Friction n Find the net force on an object that has a mass of 20. 0 kg, acceleration of 2. 3 m/s 2 on an surface with a friction coefficient of Known: 0. 21 Fn Ffr Fn=Fg=mg=(20. 0 kg)(9. 8 m/s 2) = 196 N Fa = ma = (20. 0 kg)(2. 3 m/s 2) = 46 N Ffr = Fnuk = (196 N)(0. 21) = 41. 2 N Fa Fg Worksheet Fnet = Fa - Ffr Fnet = Fa - Fnuk Fnet = 46 N - 41. 2 N = 5 N

3 rd Law of Motion n Every force has an equal and opposite force, or FA on B = -FB on A Fyou on wall = Fwall on you