Unit 3: Forces

Objectives Level A: Level C: Students will be able to determine net force acting on an object and whether or not an object is accelerating according to Newton’s 1st Law As well as the following: Level C: Can state Newton’s 1st law Can apply Newton’s 1st Law for a stationary or moving object Can define equilibrium

Newton’s 1st Law and Forces An object in motion will stay in motion and an object at rest will stay at rest unless acted upon by an unbalanced force.

Newton’s Laws and Forces Newton's first law – main idea: -the motion of an object does not change as long as the net force acting on the object is zero. (Whatever an object is doing, it will continue to do unless an outside force acts upon it) Balanced forces = no acceleration So the object will either: -stay at rest OR -stay at constant speed Unbalanced forces = acceleration Speed up, slow down, or change direction  

Newton’s 1st Law AKA Inertia Newton's First law is also called the law of inertia Inertia- is the tendency of an object to resist a change in its motion So how do we measure inertia? By the amount of mass the object has. For next slide: Why do we need to wear seatbelts?

http://www.physicsclassroom.com/mmedia/newtlaws/cci.gif

Force Force- a push or a pull on an object. A force can cause an object to accelerate- (slow down, speed up, or change direction) Objects can but do not have to touch to apply a force Ex: Contact Forces Distance Forces

https://www.youtube.com/watch?v=gA6EL0FPZHA

Magnet video https://www.youtube.com/watch?v=2QEvfQtngKE

Force 1. The metric unit of Force is measured in Newtons (N) 2. The American unit for Force is pounds (lbs) (We will only use #1 in this class!!!!)

Combining Forces Net force- the result of all individual forces on an object If you push on an object to right with a force 10 Newtons, and your friend pushes on it to the left with a force of 15 Newtons, there is a net force of 5 Newtons to the left, or you could say a net force of -5 Newtons Forces are added when moving in the same direction. Forces are subtracted when moving in opposite directions (Forces are vectors) (vector- an arrow showing magnitude and direction)

Net Forces When the forces on an object are balanced, the net force is 0 and there is no change (stay at rest or stay at constant speed) in the object's motion. When the forces on an object are unbalanced, the net force is something other than 0, the object will accelerate (speed up, slow down, or change direction) in the direction of the net force.

If the left side pulls with 100 N, and the right side pulls with 100 N, what happens?

https://www.youtube.com/watch?v=qD5hhErLcqw

Newton's first law of motion declares that a force is not needed to keep an object in motion. If I slide a book it will eventually stop because of a force acting on it. But if no force than the book will continue sliding with a constant speed. If I apply a force it will accelerate!!! https://www.youtube.com/watch?v=--MDILG7Znk

Check your understanding 1. Imagine a place in the cosmos far from all gravitational and frictional influences. Suppose that you visit that place (just suppose) and throw a rock. The rock will: gradually stop. b. continue in motion in the same direction at constant speed.

A 2-kg object is moving horizontally with a speed of 4 m/s  A 2-kg object is moving horizontally with a speed of 4 m/s. How much net force is required to keep the object moving at this speed and in this direction? 0N

Think about it- Write it down- Discuss   Ben Tooclose is being chased through the woods by a bull moose that he was attempting to photograph. The enormous mass of the bull moose is extremely intimidating. Yet, if Ben makes a zigzag pattern through the woods, he will be able to use the large mass of the moose to his own advantage. Explain this in terms of inertia and Newton's first law of motion.

The End The rest of these notes will be covered at a later time in this unit, but you can certainly read about them to get a heads up. www.rakersscw.weebly.com  

Friction Friction- a force that opposes the motion In a frictionless world, every surface would be more slippery than a sheet of ice.

http://www.youtube.com/watch?v=hQVdw0ZCRGI

Fluid Friction Fluid friction is a force that opposes the motion of an object in a fluid. Both Liquids and Gases are fluids You feel fluid friction when stirring thick cake batter Fluid friction acting on an object moving through the air is known as air resistance

Gravity Newton stated, Gravity is an attractive force that acts between any two objects with mass Gravity pulls objects together

Falling Objects Falling Objects are affected by both gravity and air resistance Gravity causes objects to accelerate downward, whereas air resistance acts in the direction opposite to the motion and reduces acceleration.

Falling Objects If there was no air, all objects would fall at -22 mph/s; however, air resistance reduces the acceleration As objects move faster, air resistance becomes stronger When an objects begins to fall, it is going slow. So gravity is much stronger than air resistance, so the object accelerates downward But as the object falls faster and faster, the air resistance gets stronger and stronger if the object falls long enough, it will reach a point where the force of air resistance becomes equal to the force of gravity. So when these forces become balanced, the objects stops accelerating. it doesn't stop moving, it stops accelerating. This point is called Terminal Velocity

Feather and Hammer

Terminal Velocity Terminal velocity is when the force of air resistance equals the force of gravity. Balanced forces result in no acceleration

Static Friction (not moving) Static friction is the friction force that acts on objects that are not moving. Static friction always acts in the direction opposite to that of the applied force.

Sliding Friction Sliding friction is a force that opposes the direction of motion of an object as it slides over a surface. Because sliding friction is less than static friction, less force is needed to keep an object moving than to start it moving.

Rolling Friction Rolling friction the friction force that acts on rolling objects. The force of rolling friction is about 100 to 1000 times less than the force of static or sliding friction. Any object with wheels would be subject to rolling friction