What is a Force? (No lab write-up) • Goal: Determine more than one way to create a force reading using the force sensor. • Results: What were the ways you used to create a force? How did or would you create a weak force? A stronger force? Can forces be negative? How did you get a negative force? • Conclusions: Based upon these ways, what is a force? Does a force depend on direction? How do you know from the experiment? How else is a force described outside of direction?

- The Nature of Force Combining Forces (Real World Examples + Tug-of-War Challenge) • Do forces usually act alone? The combination of all forces acting on an object is called the net force.

- The Nature of Force Unbalanced Forces • Unbalanced forces acting on an object result in a net force and cause a change in the object’s motion.

- The Nature of Force Balanced Forces • Balanced forces acting on an object do not change the object’s motion.

A force is… • Always balanced. • A push or a pull. • A pull that pushes. • What Jedi Knights possess.

A force is measured by its • Power only. • Strength or magnitude only. • Strength or magnitude and direction. • Direction and its pull.

A sky diver experiences a 900 N downward force and a 200 N upward force. What is the net force (in the correct direction)? • 700 N up • 700 N down • 1100 N up • 1100 N down

Which direction will the cart below move? • The cart will not because the forces are balanced. • Left because the net force is -90 N to the left. • Right because the net force is 90 N to the right. • Up because the cart is being pulled to the left and to the right.

The two dogs in the picture below are pulling on a rope. The dog on the left is pulling at 200 N while the dog on the right is pulling at 200 N. What direction will the rope move? • South because the forces are balanced. • West because the dog on the left is pulling harder. • East because the dog on the right is stronger. • The rope will not move because the forces are balanced.

- The Nature of Force Asking Questions • Before you read, preview the red headings. In a graphic organizer like the one below, ask a what or how question for each heading. As you read, write answers to your questions. Question Answer What is a force? A force is a push or pull. What happens when forces combine? Forces combine to produce a net force.

- The Nature of Force Links on Force • Click the SciLinks button for links on force.

End of Section:The Nature of Force

Noggin Knockers/Homework (8 points total- 2 points each) • 1- (A) Balanced forcesdo not affect an object’s motion because they cancel out. (B) Unbalanced forces will cause the object to move or change it’s motion. • 2- The purple arrow because it is longer. • 3- 120 N + 150 N = 270 Ndownward (strength & direction) • 4- 200 N – 100 N = 100 N to the right

Learning Objective • Describe friction and identify the factors that determine the frictional force between 2 objects.

Friction Experiments • Goal: Determine what 2 factors affect the frictional force between two objects. Also, determine how friction can be reduced. • Hypothesis (x3): For Part 1, determine which surface (floor or rug) will produce the greatest frictional force. For Part 2, determine how the added weight will affect the frictional force. For Part 3, determine how adding wheels will affect the frictional force (w/o weights). • Procedure (Use the Digits function, not the Graph function): • Part 1- Pull a force sensor attached to the block of wood to determine the frictional force that needs to be OVERCOME so the block of wood can START to move. Make sure you ZERO the sensor before you start pulling. • Part 2- Add weight to the blocks of wood and repeat Part 1. • Part 3- After removing the weights, add wheels to the blocks of wood and repeat Part 1. • Results: Organize your results for each part of the procedure.

Friction Experiment • Conclusions (in complete sentences): • Which surface produced a greater frictional force? Explain why this is the case. • What was the effect of extra weight on the frictional force? What was the effect of wheels on the frictional force? • Make your claim/conclusion (make sure it relates to the goal below), and back it up with evidence from the experiment. Use your conclusions from #1 and #2 above for guidance. • Goal: Determine what 2 factors affect the frictional force between two objects. Also, determine how friction can be reduced.

Friction Notes • For friction notes, examine your conclusions from the Friction Experiment. • Friction is a force that resists or inhibits the motion of objects when they move against each other. • Also, you should know that the type ofsurface of an object and that how hard surfaces push together affect friction (you increased the 2nd variable in bold by adding more weight). • Lastly, know how wheels affect friction as well as any other ways to reduce friction.

Markie and Suzy are moving a safe across the floor. Which of the following describes the frictional force? • Markie and Suzy both push the safe the same direction. • The weight of the safe exerts a downward force. • The floor pushes upward on the safe. • There is some resistance going against the direction the safe is moving.

Which change would require more force to pull the wooden block up the ramp? • Glue sandpaper to the surface of the ramp. • Reduce the mass of the wooden block. • Restack the books so the thinnest book is on the bottom. • Have the student use two hands.

Which of the following would produce the greatest frictional force between 2 objects? • A 100 kg wooden block sliding across a smooth table. • A 200 kg wooden block sliding across a smooth table. • A 100 kg wooden block sliding across a rug. • A 200 kg wooden block sliding across a rug.

The diagrams below show a person moving a 50-kilogram object up a ramp. In which diagram is there the least amount of friction on the object? • Diagram 1 • Diagram 2 • Diagram 3 • Diagram 4

The wheels and gears of machines are greased in order to decrease • Potential energy. • Output. • Efficiency. • Friction.

Sneaker Traction Friction Experiment (Just record your group’s data) • Goal: Determine which sneakers have the best traction (by examining the frictional force). • Background: What will you be measuring that indicates good traction? • Procedure: Same as Part 1 from the previous Friction Experiment. Record the frictional force on the SMART Board for the group member who had the shoes with the most traction. • Results: Organize your results into a table for your group. Add the other groups’ data into your table. • Conclusion: Make your claim (related to the goal) & back it up with evidence and relate it to what affects the frictional force between 2 objects from the previous experiment.

Motion Test: Commonly Missed Problems

Learning Objectives • Identify the factors that affect the gravitational force between two objects, and describe how they affect this force. • Explain why objects accelerate during free fall (by describing how gravity affects falling objects).

Gravitational Pull Experiment- Part 1 (write conclusions in your notes) • Goal- Determine what affects the gravitational force or attraction between 2 objects by using magnets & Planet Data. • Background: This brief experiment is only a MODEL of the gravitational pull between 2 objects. • Results: Determine what happens to the force of attraction between the magnets as they get closer together. • Conclusion: What affects the gravitational pull between 2 objects?

- Friction and Gravity Gravity • The force of gravity on a person or object at the surface of a planet is known as weight. • Right now you are experiencing 1G, which is the amount of force due to the gravity on Earth. • When you ride on a rollercoaster, you will experience more than 1G. Sometimes, you are feeling 3, 4, or 5 G’s due to the acceleration of the rollercoaster.

Gravitational Pull Experiment- Part 2:Mass & Gravitational Pull of the Planets • Name Mass (X 1024 kg) Gravitational force on the surface (relative to Earth) • Pluto 0.0013 0.1 • Moon 0.07 0.2 • Mars 0.64 0.4 • Earth 5.98 1.0 • Jupiter 1900 2.4 • Sun 1,989,000 7.1 • Conclusion: So, what else affects the gravitational pull between 2 objects?

- Friction and Gravity Gravity • Two factors affect the gravitational attraction between objects: mass and distance.

Which two factors determine the gravitational attraction between two objects? • time and temperature • shape and orbital speed • color and hardness • mass and distance apart

The gravitational force between the Moon and Earth depends on • Their masses only. • Their diameters only. • Their masses and how far apart they are. • Their diameters and how far apart they are.

Suppose three new planetary bodies are discovered near Earth. They all have the same mass. Planet A is 9 million miles away, Planet B is 7 million miles away, and Planet C is 30 million miles away. Which planet would Earth have the greatest gravitational attraction for? • Planet A • Planet B • Planet C • Planet D

On which planet in our solar system would you feel the most gravitational pull? • Earth • Saturn • Jupiter • Mars Name Mass (X 1024 kg) Gravitational force on the surface (relative to Earth) Earth 5.98 1.0 Mars 0.64 0.4 Jupiter 1900 2.4 Pluto 0.0013 0.1 Moon 0.07 0.2 Sun 1,989,000 7.1

Why does the Sun have the greatest gravitational pull? • It has the lowest mass. • It has the most mass. • It’s the closest to the Earth. • It’s gravitational force is 7.1 times that of Earth. Name Mass (X 1024 kg) Gravitational force on the surface (relative to Earth) Earth 5.98 1.0 Mars 0.64 0.4 Jupiter 1900 2.4 Pluto 0.0013 0.1 Moon 0.07 0.2 Sun 1,989,000 7.1

Suppose a new planet was discovered that had a mass of 4.00 (x 1024 kg). What would be a possible gravitational force for this new planet? • 0.85 • 0.3 • 1.4 • 8.0 Name Mass (X 1024 kg) Gravitational force on the surface (relative to Earth) Pluto 0.0013 0.1 Moon 0.07 0.2 Mars 0.64 0.4 Earth 5.98 1.0 Jupiter 1900 2.4 Sun 1,989,000 7.1

Learning Objectives • Explain why objects accelerate during free fall (by describing how gravity affects falling objects).

Acceleration Due to Gravity (No lab write-up) • Goal: Determine the acceleration due to gravity for objects in free fall. • Pre-Lab Demos (Dropping Paper and a Ball): Determine if objects of different masses accelerate slower or faster by observing when they hit the ground. Does the mass of an object change how fast it free falls? Why or why not? So, does the force due to gravity affect all objects on Earth the same? • Hypothesis: We will do the hypothesis as a class verbally. • Procedure: • Use a motion sensor from a high height (on the Digits and Acceleration settings) & drop a ball (AFTER HITTING PLAY) to determine the Acc. due to Gravity. • Repeat 3-5 times and RECORD the acceleration right before the ball hits the ground (hit play to pause the data collection). • Results/Conclusions (Make sure you can answer these questions): • What is the acceleration due to gravity from the experiment? • The actual value is m/s/s. How close were you to the actual value? • What may cause the ball to not accelerate as quickly (what could slow it down)?

- Friction and Gravity Free Fall = When gravity is the only force acting on the object • Use the graph to answer the following questions.

The slope is 9.8. The speed increases by 9.8 m/s each second. This is the actual acceleration due to gravity on Earth without any air resistance. Calculating: Calculate the slope of the graph. What does the slope tell you about the object’s motion? - Friction and Gravity Free Fall

58.8 m/s Predicting: What will the speed of the object be at 6 seconds? - Friction and Gravity Free Fall

The speed values would not change. Drawing Conclusions: Suppose another object of the same size but with a greater mass was dropped instead. How would the speed values change? - Friction and Gravity Free Fall

- Friction and Gravity Air Resistance • Falling objects with a greater surface area experience more air resistance.

Two objects are dropped from the top of a tall building and there is no wind. One object is a 16 lb. bowling ball and the other is a basketball. What will be each object’s approximate acceleration when they are dropped? • 9.8 m/s/s for the basketball and 8.0 m/s/s for the bowling ball. • 0.0 m/s/s for both objects. • 9.8 m/s/s for both objects. • 9.8 m/s/s for the bowling ball and 4.9 m/s/s for the basketball

When gravity is the only force acting on an object, then that object is • Moving at a constant velocity. • Not accelerating. • In free fall. • Moving quickly.

Suppose a large rock is dropped straight down from a high cliff while the other is pushed out from the top of the cliff. Which one will land first and WHY? • The rock that is pushed out from the cliff will hit first because it had an extra force to make it fall faster. • The rock that’s dropped straight down will hit first because it has a shorter path to travel. • They will both hit the ground at the same time because gravity acts the same on all free-falling objects. • Neither will hit the ground.

Why will a leaf fall slowly to the ground while other objects fall quickly? • It’s lighter. • Air resistance • Gravity • They just do.

Air resistance is a type of • band. • gravity. • friction. • Law.

- Friction and Gravity Comparing and Contrasting • As you read, compare and contrast friction and gravity by completing a table like the one below. Friction Gravity Pulls objects toward one another Opposes motion Effect on motion Types of surfaces involved, how hard the surfaces push together Depends on Mass and distance Measured in Newtons Newtons

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