* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Forces
Survey
Document related concepts
Modified Newtonian dynamics wikipedia , lookup
Newton's theorem of revolving orbits wikipedia , lookup
Electromagnetism wikipedia , lookup
Rigid body dynamics wikipedia , lookup
Nuclear force wikipedia , lookup
Fictitious force wikipedia , lookup
Fundamental interaction wikipedia , lookup
Centrifugal force wikipedia , lookup
Classical central-force problem wikipedia , lookup
Newton's laws of motion wikipedia , lookup
Transcript
Review of Newton’s Laws Newton’s First Law Objects at rest tend to stay at rest and objects in motion tend to stay in motion at the same velocity unless acted upon by an outside force. Inertia Newton’s 1st Law is also called the law of inertia. Inertia An object’s resistance to change in motion. Where does inertia come from? ○ Mass Which of the objects in each pair has more inertia? Feather or bird Which of the objects in each pair has more inertia? cat or bird Which of the objects in each pair has more inertia? Cat or dog Which of the objects in each pair has more inertia? Dog or bear Which of the objects in each pair has more inertia? Bear or Bill Nye If Bill Nye was being chased by a bear in the woods, which path should Bill choose? Why? Newton’s 3rd Law (we’ll get back to the 2nd Law later) For every action force, there is an equal and opposite reaction force. Forces come in action-reaction pairs. Name the force pair: Person walking off of a boat. What is the action force? Reaction force? Name the force pair: Air coming out of a balloon. What is the action force? The reaction force? If the action force is 3 Newtons, how big is the reaction force? Newton’s Second Law Relates force to mass and acceleration. Commonly written as F=ma Force equals mass times acceleration F=force in Newtons m=mass in kilograms a=acceleration in m/s/s Newton is the SI unit of force and is shortened to just “N”. Before we do any math… F=ma If you push your pencil across the desk with more force, what happens to mass? what happens to acceleration? If you sharpen your pencil and then want it to have the same acceleration as before, do you have to use more or less force than before? Rearranging the equation: F=ma • force equals mass times acceleration m=F/a • mass equals force divided by acceleration a=F/m • acceleration equals force divided by mass Number your notes from 1 to 5. Leave space for each problem. Problem 1: An object with a mass of 10 kg is accelerated across the table at 2 m/s/s. What force is required? Looking for: Force equation: F=ma m=10 kg and a=2 m/s/s F= 10kg*2m/s/s F=20 N Problem 2: A force of 20 N acts upon a 5 kg block. What is the acceleration of the object? Looking for: Acceleration Equation: A=F/m F=20 N and m=5 kg A=20 N/5 kg A=4 m/s/s Problem 3 A net force of 15 N is exerted on a book to cause it to accelerate at a rate of 5 m/s/s. Determine the mass of the book. Looking for: Mass Equation: M=F/a F=15 N and a=5 m/s/s M= 15 N/5 m/s/s M= 3 kg Problem 4 A 10 kg bowling ball would require what force to accelerate it down an alley at a rate of 3 m/s/s? Looking for: Force Equation: F=ma m=10 kg and a=3 m/s/s F=10 kg* 3 m/s/s F=30 N Problem 5 What is the mass of a truck if it produces a force of 14,000 N while accelerating at a rate of 5 m/s/s? Looking for: mass Equation: m=F/a F=14,000 N and a=5 m/s/s m=14,000 N/5 m/s/s m=2,800 kg Exit Slip: Turn in when finished! Answer the following questions on your own. 1. The SI unit of force is __________. 2. The equation used to solve for force is F=_______. 3. If I increase the mass of an object and want to keep acceleration the same, force must ____________. (increase or decrease) 4. A boy pushes a box with a mass of 30 kg and it moves at an acceleration of 5 m/s/s. With what force does the boy push on the box? Force Equation Sheet Solving For… Force Mass Acceleration Givens… Equation Unit for Answer… Quick Review on Forces: A force is a push, pull or any action that can change motion. Force is measured in the unit of Newtons. We will look at types of forces later. Mass Every object is made up of ________. The ______ (more/less) matter an object has, the ______ (bigger/smaller) the object is, and the _______ (more/less) mass it has. Mass Every object is made up of matter. The more matter an object has, the bigger the object is, and the more mass it has. Mass is measured in kilograms (kg) or grams (g). Mass is how heavy something is without gravity. this means that an object has the same mass here on earth or in space. Weight Weight is a force caused by gravity. Also measured in Newtons. (because it’s a force) Example: You can solve for weight (w) by multiplying mass (m) and acceleration due to gravity (g). W=mg but what is acceleration due to gravity??? Acceleration due to gravity The acceleration due to gravity on earth is 9.8 m/s/s. For this class we will round. We will write g=______ m/s/s Acceleration due to gravity The acceleration due to gravity on earth is 9.8 m/s/s. For this class we will round. We will write g=10 m/s/s Example Problem If you have a mass of 60 kg, how much do you weigh on earth? Looking for: Weight Equation: w=mg Given: m= 60 kg and g=10 m/s/s W=60 kg*10 m/s/s W=600 N More about Weight Weight problems are almost identical to force problems. (Weight is a force, so it makes sense) The only difference is that acceleration for weight problems will always be 10 m/s/s. Types of Forces Forces occur from interactions. Contact forces are from physical interactions. Examples include: Frictional Force Normal force Applied force Air resistance Frictional Force: A force that opposes motion More on Friction Without frictional forces, a moving object may continue moving for a longer period. Frictional forces are usually greater on rough surfaces than on smooth surfaces. Normal Force: Support force exerted upon an object in contact with another stable object. Always at a 90° angle from the surface. Applied Force: force exerted on an object by another person or object. Example: Pushing Air Resistance: a force that acts against the force of gravity on an object in free fall. Action-at-a-distance forces AKA non-contact forces Result from non-physical interactions. Examples: Electrical forces Magnetic forces gravity Gravity Gravity is an attractive force between two objects. Anything with mass has gravity. More massive objects have more gravity. Electrical Force Attractive or repulsive force between two charged objects. Magnetic Force Attractive or repulsive force based on a magnetic pole or field. All magnets have a north and south pole. What types of forces are working? For a book on a table? What types of forces are working? On a parachute? What types of forces are working? When you push the grocery cart? Foldable Gravity Contact Forces Normal Electrical Force Applied Air resistance Magnetic Force Non-contact forces Frictional Force Diagrams Used to show force on an object. An arrow, with a name, length and direction is used to represent a force. Example: In a force diagram, the bigger the arrow, the bigger the force. Notice how the 5 N force arrow is larger than the 2 N force arrow. How to draw force diagrams The object is drawn as a box. The arrow for each force is drawn from the box outward. All forces for a situation must be drawn Steps for Force Diagrams Read the situation and figure out which forces are present (friction, gravity, normal, etc.) 2. Determine the direction each force acts in. (For example, gravity is always down) 3. Draw the box (for the object) and the arrows. 4. label each arrow with the type of force, and if available, the strength of the force. 1. Example Problems Sketch a force diagram for a book being held up by a person. Example 2 Sketch a force diagram of a laptop sitting on a table. Example 3 You are trying to push a heavy box across the floor. It is not very easy because the floor is not very smooth. Sketch a force diagram of the box. Example 4 A sled is falling down a sloped surface. Sketch a force diagram of the sled. Balanced Forces Forces that are acting in the opposite direction and are the same magnitude (meaning they’re the same size) Anytime there is a balanced force on an object, the object stays still or continues moving at the same speed and in the same direction. Examples of Balanced Forces Unbalanced Forces When two forces are not equal in size. Unbalanced forces cause acceleration, which is change in velocity. Unbalanced forces causes can cause: a still object to move speed moving object to __________ up or ________ slow down moving object to stop moving object to change direction Examples of Unbalanced forces Balanced or Unbalanced? Riding a bike at a constant speed. Balanced Balanced or Unbalanced? Lifting weights off the ground Unbalanced Balanced or Unbalanced? A skydiver in the air Unbalanced Balanced or Unbalanced? Person (or cat) Sleeping Balanced Adding Forces What is net force? Sum of all forces acting on an object. What is the net force on this object? 3 N to the right. Finding net force If no force is acting on an object, then the net force is zero. Although this happens in physics problems, it is unlikely to happen in real life. Force Diagram: Finding net force If there is just one force acting on an object, then that force is the net force. In free fall, the net force acting on an object is its weight (mass times gravity (10 m/s/s) What is the net force on this object? 5N Finding Net Force If 2 forces push or pull in opposite directions, and they cancel each other exactly, the net force is zero. 5N 5N Finding Net Force If 2 forces push or pull in opposite directions, and they don’t cancel each other exactly, the net force is what is left over (the difference). 3N 5N What is the net force? Finding Net Force If 2 or more forces are acting in the same direction, the net force is the sum of the forces. 5N 5N What is the net force? Net Force Any time a stationary object stays still, its resultant force (or net force) is zero. As soon as force is applied, acceleration begins. The rate of acceleration will force applied and depend on the ________ mass of the object. the _______ Examples: Practice Problem 1 During 2nd period, Ladoris was a beast. She resisted the forces applied by 5 people in her class all at once. They applied a total force of 95 N to the right, and Ladoris still did not move. 1. Draw a force diagram 2. What was the net force? 95 N 95 N If Ladoris didn’t move, how should we finish the force diagram? What is the net force? Practice Problem 2 During 7th period, we put Vincent in a box because he was talking too much. We still heard his voice through the box, so we decided to push him outside. The force of friction of the ground on the box was 68 N to the left. Mrs. Nipper can apply a force of 25 N to the right and every other 9th grade student can apply a force of 6 N to the right. Draw a force diagram. How many students would Mrs. Nipper need to make the box start moving and go outside? Applied by Mrs. Nipper= 25 N Friction = 68 N Applied by Student = 6 N How many students do we need to make the box move? (remember to include Mrs. Nipper in the calculation!)