Forces and Motion 5.1 – Use measurement & time- distance graphs to represent the motion of an object in terms of its position, direction or speed.

Motion Motion is a change in position of an object with respect to time. The following terms are used to describe and measure motion. –Position –Direction –Speed

Position Position – the location of an object. Change in position is measured by distance (length of the path) moved from one position (reference point) to another. –What is a reference point? Class demonstration: One volunteer and the rest of the class is split into two groups. This activity will illustrate the idea of a reference point.

Reference Point – is a place or object used for comparison to determine if something is in motion. An object is in motion if it changes position relative to a reference point.

Direction Direction – is the relationship of the position of a moving object to another position.

Speed Speed – is the rate of change in position or “rate of motion”. How long it takes to move from point A to B. – Speed does not necessarily mean that something is moving fast.

Average Speed The average speed of an object tells you the (average) time at which it covers a given distance. –Average speed is the result of the total distance divided by the total time taken.

Calculating Average Speed (v = d/t) v = d/t where: v is the average speed of an object d is the distance or length of the path of the object t is the time taken to cover the path Speed is measured using the following types of units: –meters per second (m/s) –kilometers per hour (km/h) –miles per hour (mph) –Triangle "How To"Triangle "How To"

What is the difference between speed and velocity? Velocity is speed with direction

V=d/t v d t d=vt t=d/v

TO HELP SOLVE EQUATIONS, HERE IS A STEP BY STEP GUIDE TO FOLLOW. G U E S S G – find your Givens U – what is your Unknown (trying to find) E – what is the Equation you need? S – Substitute your givens S - Solve

Examples: Calculate the speed of an object using the following information: t = 35 s and d = 50m Step 1 – GUESS Givens are t = 35 s and d = 50 m Unknown: v Equation: v = d / t Substitute: v = 50 / 35 Solve: v = 50 divided by 35

Examples Suppose a cyclist travels 32 kilometers during the first two hours of riding, and 13 kilometers during the next hour. Calculate the average speed: -total distance = 32km + 13km -total time = 2h + 1h Average speed: 45km/3h = 15km/h or 15 kilometers per hour

Examples Use the speed equation to determine how far an object traveled. v=7m/s t=3s How far did the object travel? Rearrange v=d/t d=vt d=7m/s(3s) d=21m

1.How long will it take to get to Atlanta, GA for a Falcons game from Greenville, SC (145 miles or 233 km). You know you can travel about 70 mph? 2. If a bamboo plant grows 28 centimeters in 11 days, what was the average speed the plant grew per day? 3. A soccer player was practicing on the soccer field last week. He ran back and forth on the field for a total of 750 yards. His average speed was 12 yards per second. How long did it take him to finish?

Graphing Speed Do you think it is possible to represent speed using a graph? If so, what type of graph would you use? –Line graph, bar graph, circle graph????

When graphing speed you would use a line graph. A line graph shows the relationship between distance and time and how it relates to speed. The slope of the line gives us information about the speed and direction of an object.

You graph speed by taking measurements of time and distance. For instance: The following measurements were taken on how fast a turtle walks along a path. Time (h)Distance (m)

At position A, is the object moving away from or towards the reference point? At position C, is the object moving away from or towards the reference point? A B C

The steeper the slope, the faster the speed. The flatter the slope, the slower the speed.

When the slope of the line is horizontal to the x-axis, the speed is zero (the object is not moving)

Examples: Lets graph the motion of a drag racer. Time (s)Distance (m) Have to allow for a #25 and #35 on your X axis!

First, we need to label the data x and y. Time (s)Distance (m) X axisY axis

Next, draw and label the graph.

Now, you need to plot the points. Time (s) Distance (m)

Practice Problem: Graph the speed of an object as it travels down hill. Time (s)Distance (m) Time (s)Distance (m) Graph the speed of an object as it travels up hill.

Forces Friction and gravity are two types of forces we encounter everyday.

Gravity Gravity – is the force that acts to pull objects straight towards the center of the Earth. –An objects speed increases as it falls. Free Fall – When the only force acting on a falling object is gravity, the object is said to be in free fall.

Gravity An object in free fall accelerates as it falls. In free fall, gravity is the only force acting on the object so it accelerates. –The rate at which objects accelerate is 9.8m/s/s. –This means that for every second an object is falling, its velocity increases by 9.8m/s. ALL objects in free fall accelerate at the same rate.

Despite the fact that all objects are supposed to fall at the same rate, we know that this is not always the case. For example, let’s think about the following example: –An maple leaf and an acorn falling. Do they fall at the same speed? What causes the speed to be different?

Friction Friction is the force that one surface exerts on another when the two rub against each other. This force opposes motion The type of friction shown in the previous example is air resistance. The greater the surface area of an object, the greater the air resistance.

Gravity pulling the acorn down. Air resistance working against gravity is greater for the leaf. Air resistance working against the acorn is less than the leaf Gravity pulling the leaf down. In this situation, the acorn falls faster because it has less surface area and therefore, less air resistance.

Friction Friction acts in a direction opposite to the object’s direction of motion. Without friction an object would continue to move at a constant speed forever. Example: –Let’s push a book across the desk. –In your notes, predict what will cause the book to stop.

Friction Friction is increased by the amount of surface area the object has in contact with the surface. –For example, tires with more surface area have better traction on the road. Friction causes objects to slow down or stop.

Is Friction Useful? Friction allows you to walk across the floor. –Without friction your shoes would simply slide across the floor. Friction allows you to strike a match. –It is the friction that cause the flame to ignite.

How could friction not be useful? Tires wear out over time because of the friction between the tire and the road. Gears inside machines wear down because of the friction between parts.

WHAT IS FORCE? A force is a push or a pull! When one object pushes or pulls another object, you say that the first object is exerting force on the second object. Examples: **You exert a force on a pen when you write ** On a book when you lift it ** On a ball when you throw it ** On a nail when you hammer it into a piece of wood

When two people exert a force on the object, in the same direction, the total force is the SUM of both forces exerted. + =

When two people exert a force on the object, in the opposite direction, the total force is the DIFFERENCE of both forces exerted. If the force is the same, then they can cancel out each other. - = 0 Net Force: The overall force on an object after all the forces are added together.

When there is a net force acting on an object, the force is said to be unbalanced. An unbalanced force can cause an object to start moving, stop moving, or change direction. An unbalanced force acting on an object will change the object’s motion. + = An unbalanced force will cause an object to accelerate. The object will accelerate in the direction of the GREATER force.

Equal forces acting on one object in opposite directions are called Balanced forces. Balanced forces (equal) acting on an object will not change the object’s motion. + = 0 The net force is zero

CHECK FOR UNDERSTANDING What makes an arrow fly through the air to its target? What makes a long jumper thud to a stop? What will make a soccer ball change direction? If two kids are playing tug of war but the center is not moving they have ___________ force. If one of the kids in the tug of war game pulls harder than the other then you have an ____________ force. Which force will cause change in motion – balanced or unbalanced?

Example #1 Two tugboats are moving a barge. Tugboat A exerts a force of 3000 newtons on the barge. Tugboat B exerts a force of 5000 newtons in the same direction. What is the combined force on the barge? Barge Tugboat B Tugboat A 3000N 5000N 8000N = Total Force

Example #2 Now suppose that Tugboat A exerts a force of 2000 newtons on the barge and Tugboat B exerts a force of 4000 newtons in the opposite direction. What is the combined force on the barge? Barge Tugboat B Tugboat A 2000N 4000N 2000N = Total Force

Newton’s First Law of Motion (“the law of inertia”) An object at rest will remain at rest unless acted on by an unbalanced force. An object in motion continues in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

WHAT IS INERTIA? The tendency of an object to resist change in its motion. Whether an object is moving or at rest, every object resists any change to its motion. Examples: **Air hockey puck will ride on air once you push it ** Tennis ball flies through the air once you hit it with a racket In both of these cases, the object continues to move even after you remove the force.

Inertia explains common events like: When you are in a car and stop suddenly, you will continue to move forward. (WHY YOU NEED TO WEAR SEAT BELTS!!) Passengers in a moving car have inertia. Therefore a force is required to change their motion. That force is exerted by the safety belt. If the safety belt is not worn, that force is exerted by the windshield instead.

What is more difficult to move? A 1 quart jar of pennies or a 1 quart jar of feathers? Obviously the pennies…. Why? What is the difference between a quart of pennies and a quart of feathers? Both jars occupy the same amount of space and have the same amount of volume.

The difference is the amount of MASS. What is Mass? Mass is the amount of matter in an object. The amount of inertia an object has depends on how much mass it has. The greater the mass of an object, the greater its inertia, therefore an increase in friction Mass can also be defined as a measure of the inertia of an object.

Newton’s 2 nd Law of Motion The net force on an object is equal to the product of it’s acceleration and mass. Force = Mass x Acceleration –Remember, the unit of force is a Newton. Acceleration is the rate at which velocity changes.

Newton’s Second Law of Motion Acceleration will increase when you increase the FORCE on the object. Acceleration will increase when you decrease the MASS of an object.

Examples: You are pulling a wagon with two dogs in the back. If one dog jumps out, will the acceleration increase or decrease? Suppose you know the acceleration of a shopping cart as it rolls down a super market isle. You want to find the force with which it was pushed. What other information do you need in order to find the force?

Newton’s Third Law of Motion Newton’s third law states that if one object exerts a force on another object, then the second object exerts a force of equal strength in the opposite direction on the first object. In other words, every action has an equal, but opposite reaction.

Newton’s Third Law of Motion A squid applies Newton’s third law of motion to move itself through the water. –When a squid pushes water out, the expelled water pushes back and forces the squid to move ahead. Water pushed out is the action. Squid being propelled forward is the reaction.