Introduction to Physics

Chapter One: Motion Introduction to Physics

Section 1.1 An object in motion changes position Position describes the location of an object. The position, or location, of an object is described relative to a reference point. The choice of reference point affects how the position is described. For example, a city can be located by measuring its direction and distance from another city, or by using a grid system, such as the longitude-latitude system.

Grid vs. Direction & Location • Peoria is located at 40N 90W (grid) • Peoria is about 85.5 miles NW of Decatur. (direction & location)

There are two ways to measure the distance an object has traveled. One is to measure the length of the path the object followed. Another is to measure the straight-line distance of an object from its starting point. This straight-line distance is called the displacement of an object.

Distance vs. Displacement Displacement Distance

Reviewing Concepts- If relatives living in different cities wanted directions to your house, would you give them the same or different directions? Why? If you travel by car from Decatur to Chicago, would measuring the path or displacement be a better estimate for figuring distance? If you traveled by plane from Decatur to Chicago, would measuring the path or displacement be a better estimate for figuring distance?

In the following activity, I would like you to describe the position of the ice cream cone. in two different ways: Using the grid Using a another object for a reference point. A B C D E F 2 3 4

In the following activity, I would like you to measure the distance in two different ways: Using the path traveled from the point A to the Bank. Using displacement. A

Section 1.2 Speed measures how fast position changes • Position can change at different rates. • Speed is a measure of how fast something moves a particular distance over a given amount of time. • *Speed=distance/time or S=d/t • Average speed is the average of several instantaneous speeds whose measurements are taken over a specific period of time. (This is what you are familiar with )

A distance-time graph shows how both distance and speed change with time. • You can use these graphs to determine the speed of an object by calculating the slope of the line. A positive slope means the object is moving away from its starting point. A negative slope means the object is moving back towards its starting point. • *Slope= change in distance/change in time=speed

Distance-Time Graph Distance (mi) Time (hrs)

Let’s pretend that graph shows you my travel from Decatur to Woodstock. The train traveled at a different speeds between cities. Here is a table of information that will help you calculate speed and average speed of the whole trip: To calculate speed from the starting point to the first stop, you divide the change in distance by the change in time. So (35-0)/(.5-0) = speed. Speed= (35)/(.5)= 70 mph To calculate speed from the first stop to the second, divide change in distance by change in time. So (68-35)/(1-.5) = speed. Speed= (33)/(.5) = 66 mph To calculate the train’s average speed for the whole trip, divide the total change in distance by the total change in time: (207-0)/(3-0) = 69 mph

Velocity is speed in a specific direction • Velocity is represented by a vector. • A vector is a quantity that has both size and direction. • Vectors are shown with arrows. • The longer the arrow, the faster the speed. • The direction of the arrow indicated the direction of motion.

Speed and velocity are not the same. • If two runners run at the same speed in opposite directions, they will have identical speed but different velocities. • p. 22 in textbook These two arrow have different speeds but same direction…therefore, their velocities are different These arrows have the same speed in different directions….therefore their velocities are different

Section 1.3 Acceleration measures how fast velocity changes. • Speed and direction can change with time. • Acceleration is the rate at which velocity changes with time. • Contrary to a popular misconception, acceleration is not limited to increases in velocity but includes any change in velocity. (It isn’t just speeding up, but also slowing down and/or changing direction)

The following are examples of acceleration: • Speed increases • Speed decreases • direction changes (regardless of speed)

Acceleration can be calculated from velocity and time. • You determine acceleration from the change in velocity and how long the change took. (The saying “from zero to 60 in 20 seconds” is an expression describing acceleration of a car. It is like the rate at which you are speeding up or slowing down) All of these checkpoints have varying velocities: 0-1= 70 mph 1-2= 66 mph 2-3= 74 mph …..and so on

This is the information you saw on the last slide: 0-1 (half-hour of travel time)= 70 m/h 1-2 (half hour of travel time)= 66 m/h 2-3 (half hour of travel time)= 74 m/h To calculate Acceleration from the first checkpoint to the second, subtract the initial velocity from the final velocity and divide by time. A= (66-70)/1 = -4 m/h2 • The formula for calculating acceleration is: • A=(Vfinal-Vinitial)/t • Turn to page 28 and do the example with me. • Negative acceleration is a decrease in velocity during a specific period of time. • The acceleration formula yields a negative result when the final velocity is less than the initial velocity. (When you slow down)

A velocity-time graph shows how both velocity and acceleration change with time. • Turn to page 30 in your textbook. The graph on the top is a velocity-time graph; compare it with the distance-time graph for the same set of data, which is shown below. The graphs show a boy (1) starting and speeding up on his scooter (2) coasting (velocity is constant), and (3) slowing to a stop.

Section 2.1 Forces Change Motion • A force is a push or a pull. • Force in physics is defined as “a push or a pull”. Some forces: • require contact between objects, such as friction and • act at a distance, such as gravity and electromagnetic forces.

Net force is the total force that affects an object when multiple forces are combined. • The net force depends on both the direction and the size of the individual forces. • Ex: You can use the information provided by net force to predict who would win an arm wrestling match. Imagine a Anderson Silva (MMA fighter) arm wrestling Justin Bieber. Forces act in opposite direction, but Silva’s will be much greater. Therefore the net force acts against Bieber’s and Silva will likely win.

To show you visually how to measure net force, use the following:

Newton’s First Law • Newton’s first law relates to force and motion. • They key points of Newton’s first law are: • Objects with no net fore acting on them either have constantor zero velocity • force is needed to start or change motion • Inertia is the resistance of an object to a change in its motion; it is directly proportional to the object’s mass.

Section 2.2 Force and mass determine acceleration • Newton’s second law relates force, mass, and acceleration. • The key points of Newton’s second law are that the acceleration of an object is • Directly proportional to the force acting on the object • Inversely proportional to the mass of the object • In the same direction as the net force acting on the object

What does Newton’s second law mean exactly? • Imagine you are trying to move a large bookshelf into another room. • Since force and acceleration have a direct relationship, this means that the more force you apply to the bookshelf the quicker it will accelerate. • Since acceleration and mass have an inverse relationship, if you were to fill the bookshelf and apply the same amount of force, it would not accelerate as quickly.

Imagine the same amount of force is applied to each bookshelf. The top bookshelf is the “full” shelf, the bottom one is the empty one.

Newton’s second law is summed up by the equation • Force=mass x acceleration or F=ma • Force is measured in Newtons (N). • Since Force=mass times acceleration (kg x m/s2), you can use the units of each of these to determine the value of a Newton. • 1 N is equal to 1kg x 1m/s2 • You can rearrange this equation to find different values: • F=ma m=F/a a=F/m

Let’s review this relationship a little bit more: • F=ma • If I increase acceleration, force must increase • F=ma • If I increase mass, force must increase • F= ma

If I increase mass and want to apply the same amount of force, acceleration must decrease • F= ma • If I increase acceleration and want to apply the same amount of force, mass must decrease • F= ma

Here is an example problem: • Kayla is helping her mom buy groceries for her and her little brother. The shopping cart is accelerating at 5 m/s2. The force on it is 75 N. What is the mass of the shopping cart & its content? • What do you know? • What do you want to find out? • Choose formula • Substitute known values • Calculate and simplify

Here is an example problem: • Feynman, Alex, and Andrew are pulling a trampoline. If their combined force is 1000 N and the trampoline is 250 kg, what is the acceleration of the trampoline? • What do you know? • What do you want to find out? • Choose formula • Substitute known values • Calculate and simplify

Forces can change the direction of motion. • Force can change the direction of an object without changing its speed if the force acts at right angles to the motion. • A force that continuously acts at right angles to an object’s motion will pull the object into circular motion. • Any force that keeps an object moving in a circle at a constant speed is called centripetal force. • The centripetal force needed to keep an object moving in a circle depends on the mass of the object, the speed of the object, and the radius of the circle. • Centripetal force = (mass x speed2)/radius

Here is an example problem: • Jordan is pushing his bike up a hill. The bike is accelerating at 15 m/s2. The force on it is 300 N. What is the mass of the bike? • What do you know? • What do you want to find out? • Choose formula • Substitute known values • Calculate and simplify

Here is an example problem: • Jorge is pushing a bookshelf across a room. If the combined force is 300 N and the table is 60 kg, what is the acceleration of the bookshelf?

Section 2.3 Forces Act in Pairs • Newton’s third law relates action and reaction forces. • They key points to this law are that when objects A and B interact: • The force of A on B equals the force of B on A • The forces are in opposite direction

In action/reaction pairs either force can be considered the action force or the reaction force. (p. 58 for more examples) • Imagine you are pinching shut a balloon filled with air. You let go of the balloon. Think about what usually happens. • What is the action force? • What is the reaction force?

The two forces occur simultaneously-at the same time. • Example: when you push down on a table, the force from the table’s resistance increases instantly to match your force. • Action/reaction force pairs occur when any two objects interact, not just through contact forces. • Example: the pull of Earth on a falling baseball is exactly that of the baseball pulling on Earth. Earth is so much more massive; however, that Earth’s acceleration from the pull is nearly nothing. The acceleration of the baseball is very noticeable.

Action and reaction forces are different from balanced forces. • Action and reaction forces are equal and opposite. • Two forces acting in opposite directions (you go to pick up your dog-you act against gravity, it acts against you) • Balanced forces act on a single object. • Two siblings arguing over a remote (both applying a balanced force on a single object)

Newton’s three laws describe and predict motion • Newton’s laws work together to explain changes in the motion of objects, such as a squid moving forward when squirting water backward, or a bird flying higher or changing direction. • Newton’s laws are also useful in calculating how objects move under conditions found in everyday life. • Scientists such as Albert Einstein have added to our understanding of motion since Newton’s time. • Under certain conditions, such as extreme speed or gravity, Newton’s laws need to be adjusted.

2.4 Forces Transfer Momentum • Before we begin learning about a new concept, let’s review what we already know about forces. • http://science.discovery.com/interactives/literacy/newton/newton.html • If there were parts in this interactive you did not understand, please arrange a time to come meet with me. This is review and we will not be going over it again before the pop quiz and test.

Objects in motion have momentum. • Momentum can be though of as inertia for moving objects. • It is the tendency of a moving object to keep moving at a constant velocity, and it depends on the mass and velocity of the object. • What do you predict would happen to an object’s momentum if you increase the mass of the object? • Imagine you are throwing a tennis ball against a wall. Now imagine you are throwing a bowling ball against a wall. • What happens when you increase the mass of an object?

What do you predict happens to momentum when you increase the velocity of an object? • Imagine you gently drop a grape on a piece of paper towel held horizontal by another person. Now imagine you increase the grape’s velocity by throwing it against the paper towel. • What happens when you increase the velocity of an object? • Momentum=mass x velocity, or p= mv • What do you predict the units of momentum are? • Determine the units for mass and velocity. • Combine the units to calculate momentum.

Momentum, like velocity, is a vector, so it has both size and direction. • Adding the momentum of two objects is similar to adding net forces. • Let’s try some example problems using momentum: • What is the momentum of a 1.5 kg ball moving at 2 m/s?

What is the momentum of a 3 kg ball moving with a velocity of 1 m/s? • What is the momentum of a .5 kg ball moving at .5 m/s?

A force on an object changes the object’s momentum. • The change in momentum is equal to the force on the object multiplied by the time over which the force is acting. • Momentum can be transferred from one object to another. • Momentum is transferred during a collision. • Colliding objects exert equal and opposite forces on each other while they are in contact. • The forces in the collision will change the velocity of each object involved. • What do you predict will happen in a situation where a tennis ball and a bowling ball traveling toward each other collide? • In a collision, the object with less mass has a greater change in velocity.

Momentum is conserved. • In any case where no outside forces are acting on a system, the total momentum of the system will not change, even if the momentum of individual parts of the system changes. • This conservation of momentum is most easily seen in a collision. • The forces acting are equal and opposite, and they act over the same time period. • Therefore, the change in momentum for two colliding objects is equal and opposite, and the total change in momentum is zero.

Section 3.1 Gravity is a force exerted by masses. • Masses attract each other. • Gravity is the force objects exert on each other because of their mass. • It attracts any two masses anywhere in the universe. • Greater mass results in greater force. • Greater distance results in smaller force. • The strength of the gravitational force is proportional to the product of their masses divided by the distance between them squared. • Gravitational acceleration is symbolized by g and equals 9.8 m/s2 at Earth’s surface. • Any object falling in a vacuum, no matter how massive, has this acceleration. • The force of gravity, F, equals mg at Earth’s surface. • This is because you can substitute g in for a according to Newton’s first law, F=ma

Mass and Weight are not synonymous. • Mass is the amount of matter something contains. • Weight is the effect of gravity on the object. BrainPop Gravity Video • http://www.brainpop.com/science/motionsforcesandtime/

Gravity keeps objects in orbit. • An orbit is an elliptical path that one object takes around another object. • An orbital path is the result of the speed of the orbiting body and the gravitational pull between the two objects. • The speed an object must have to escape the gravitational pull of another body, such as a spacecraft leaving a planet, is called escape velocity. • Speeds lower than the escape velocity will result in an orbit. • A spacecraft and its contents in orbit are in free fall. • The environment is such that an astronaut can’t feel gravity.

Section 4.1 Work is the use of force to move an object. • Force is necessary to do work. • To do work on an object, a force must be applied to the object, and the object must move in the direction of the force. • For example, if the direction of force is away from a person (you are pushing an object), the object will move away. • If the direction of force is moving toward a person (you are pulling an object), the object will move toward him/her. • Work is done only by the component of the force that acts in the same direction as the movement of the object. • Work is done by force tht acts in the same direction of motion as the object.

Introduction to Physics