NAME: date: Pd: WEIGHT

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1 NAME: date: Pd: Warm UP: read the information then fill in the chart below. WEIGHT Gravity is a force of attraction between objects. Gravity is not exerted by any one body in any one direction. Gravity is always exerted between two objects. All objects that have mass exert a gravitational force on all other objects, but the magnitude of their gravitational force can vary. Sometimes the gravitational force exerted by objects is so weak that it is not even measureable. For example, you and all of the objects around you (or even miles away), down to the tiniest molecules of oxygen you are breathing in right now, are exerting gravitational forces on all of the other objects around you. Be that as it may, most of these things do not exert a strong enough pull to be noticeable or even measurable. One object in particular, however, definitely exerts a noticeable pull on you though, and for this reason, we are not floating in the air, but are constantly pulled toward the ground. Weight is a measure of the gravitational force on an object. When you see or hear the word weight, it usually refers to Earth s gravitational force on an object. But weight can also be a measure of the gravitational force exerted on objects by the moon or other planets. Weight is related to mass, but they are not the same. Weight changes depending on how far away objects are from each others centers of gravity and how much mass each object contains. You actually weigh less as you increase in altitude away from earth because you are increasing the distance between the your center of gravity and the earth s center of gravity. For example, in Colorado s Rocky Mountain National Park, where you are sometimes 10,000 ft above sea level, you weigh about 0.1% less than you do at sea level). Mass is the amount of matter in an object. An object s mass does not change. You would still have the same amount of mass on top of any mountain, even though your weight would be different. And even if you moved to the surface of a different planet, like Jupiter, which exerts a much stronger gravitational force than earth does, your mass would still be the same. Your weight on the surface of Jupiter would not be the same because Jupiter s gravitational force is much stronger than earth s. So, although mass never changes, your WEIGHT depends on the distance between two objects and the mass of the objects. Use the chart below to compare/contrast mass and weight: MASS WEIGHT Basic definition Units Measurement Tool Type of quantity Varies based on location? Other:

2 NAME ETC. CONCEPT EXTENSION / challenge Exploring the Law of UNIVERSAL GRAVITATION with NEWTON S UNIVERSAL GRAVITATIONAL CONSTANT PART 1: ACTIVATING BACKGROUND KNOWLEDGE: What is the law of universal gravitation? (look-up or use your notes): What are the two things that affect the strength of something s gravitational pull? Forces are not just one way actions, they act in pairs (action/reaction force pairs). Fill in the blank in the diagram to the right. The earth is exerting a force on you, and you are exerting an equal force on the earth. Any time you remove the support force (like, by jumping in the air), you start accelerating towards the center of the earth on your way down. You actually also cause the earth to start accelerating towards you as well. Can you feel this acceleration of the earth moving towards you? Why/why not? For more help in your explanation, Google does earth accelerate towards you or check out PART 2: Calculating the attraction! TASK: What is the Gravitational Force that exists between you and your pencil? READ and BUILD KNOWLEDGE (highlight/underline): Newton formulated a law relating the force between two objects to their masses and the distance between them where m1 and m2 are the masses of the two objects and r the distance between them as shown below. In this formula, G is a constant known as the universal gravitational constant *. Its value, determined experimentally, is 6.67x10-11 Nm 2 /kg 2. It is used in Sir Isaac Newton's law of universal gravitation*, and in Albert Einstein's theory of general relativity*. G (often called big G ) is always capitalized to avoid confusion with g ( little g ), which signifies the acceleration of all objects on earth towards the center of the earth due to gravity (in the equation F=mg or F=ma) Research these at home or come in after school if you d like to know more about them! The figure below shows how to calculate the force of gravity between two objects of masses m1 and m2, separated by distance r (measured in METERS) m1 =mass of first object in KILOGRAMS m2=mass of second object in KILOGRAMS G= universal gravitational constant 6.67x10-11 Nm 2 /kg 2 r=distance between objects centers of gravities in METERS

3 IDENTIFYING THE VARIABLES: Follow the steps below 1) Find m 1 : your mass in KILOGRAMS. To find your mass convert your weight from pounds to Kg. At/around sea level on earth*, your mass in Kg can be found by using the equation to the right. Your approx..weight in lbs: 2) Find m 2 : The mass of the second object (your pencil,this time) in KILOGRAMS (kg) Measure its mass using a balance. Make sure to convert its mass from grams to kilograms. KHDBDCM (move the decimal places over to the ). kg= m 2 lbs x.454 Kg = kg = m lb 3) Find r : This is the distance between you and the center of your pencil - Place a pencil some distance away from you (this is up to you ) Measure its distance away from you (use the center of both objects) in METERS!! (convert from cm to m using KHDBDCM. (remember meters are the BASE). (move the decimal places over to the ). meters = r *This activity requires a graphing calculator!! 4) Plug and chug (Fill in the formula with the identified variables below). (formula and universal gravitational constant are on previous page) **Try different things, like, keep the same masses, but change the distance between the objects, OR change one or both of the masses and keep the distance the same between the objects. The force that exists between myself and my pencil, m away is. Fgrav Force of gravity between you and your pencil =

4 Now try it on your own with different objects. Try the same object at different distances, or the same distance but with different mass objects. 1) The force that exists between and, which are located meters away: 2) The force that exists between and, which are located meters away: 3) The force that exists between and, which are located meters away:

5 4) The force that exists between and, which are located meters away: 5) The force that exists between and, which are located meters away: SUMMARIZING THE CONCEPT: 1) How did distance affect the magnitude (size) of the gravitational force between objects? When objects were farther apart, the gravitational force between them was/would be When objects were closer together, the gravitational force between them was/would be 2) How did changing the masses affect the amount of gravitational force between objects? The more mass the objects had, the the gravitational force. between them. 3) Although gravitational force exists between all objects, is it noticeable? Explain: (for example, we notice the force of gravity on us exerted by the earth but does the earth notice our gravitational pull on it?)

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7 Net Force Review/Practice: Name/date: The force that results from all the combined forces acting on the object is called the net force. Calculate the net force acting on the box in the following problems. Be sure to include the direction of the net force! (left or right)! 11. What is net force? Show your work on the next problems by drawing a free-body diagram to represent the forces present in the situations below. 12. A boy pulls a boat on the beach with a force of 60 N east as another boy pushes it with a force of 40 N east. The force of friction under the boat is 30 N. What is the net force? 13. A pug takes a nap on the couch. The pug exerts a downward force of 70 N. on the couch. The pug is not moving. What force must the couch be exerting on the pug? What is the net force in this situation? 14. In a tug of war game, Mrs. Warden s class pulls on a rope with a force of 200 N to the left. Ms. Hall s class pulls with a force of 300 N to the right. Then Mrs. Lawrence s class comes along to help Mrs. Warden s class and pulls with an additional 200 N. Ms. Japec s class comes to help Ms. Hall s class, and pulls with a force of 150 N. What is the net force? Which way will the rope accelerate? 15. What happens to an object when all forces acting on it are balanced? An object in motion will An object at rest will 16. Draw a free body diagram that represents an object experiencing balanced forces. 17. What happens to an object when it experiences an unbalanced force? 18. Draw a free body diagram that represents an object experiencing unbalanced forces.

8 12. Get a pair of headphones and take out your ipad (or share with a neighbor). Go to (username-williamsburg, password-computer) search for and watch the clip on FORCES As you watch the clip, pause it and take notes in the space below. Take the quiz at the end and write your score down. FORCES NOTES: Brainpop FORCES quiz score: / 10 13) Stay on the brainpop site/app: Now, search for and watch the clip on GRAVITY As you watch the clip, pause it and take notes in the space below. Take the quiz at the end and write your score down. GRAVITY NOTES: Brainpop GRAVITY quiz score: / 10

9 BITESIZE BBC UK, FORCES (can t do unless have computers) We measure force with a unit called the ( ) But what is gravity anyway? Gravity is a that exists between and them together. But we really only see its effects with really massive objects. like planets Why? Gravitational force is stronger the more the objects have, and the they are. The moon is smaller (has less mass) than earth, so it exerts a gravitational force. Fill-in the astronaut s weight on earth and the moon: There s a large force pushing it forward and a small friction force pushing it back..unbalanced forces like this always cause in the way things move. Balanced forces don t mean an object doesn t move, Now, when we ease off on the accelerator, we are back to forces. The force from the engine is equal to the and air resistance pushing backwards. When forces are balanced, it doesn t mean that those things won t move Balanced forces means there are no in speed (or velocity). So the van moves along at a steady pace until you hit the accelerator and unbalance the forces again.