Physics Classroom Website Webquest Lisa Peck http://www.physicsclassroom.com/class/newtlaws/newtltoc.html Lesson 1: Newton s 1st Law 1. There are many applications of Newton's first law of motion. Several applications are listed below try to provide explanations for each application. a blood rushes from your head to your feet when riding on a descending elevator which suddenly stops. b the head of a hammer can be tightened onto the wooden handle by banging the bottom of the handle against a hard surface. c a brick is painlessly broken over the hand of a physics teacher by slamming the brick with a hammer. (CAUTION: Do not attempt this at home!) d to dislodge ketchup from the bottom of a ketchup bottle, the bottle is often turned upside down, thrust downward at a high speed and then abruptly halted.
e headrests are placed in cars to prevent whiplash injuries during 2 rear-end collisions. f while riding a skateboard (or wagon or bicycle), you fly forward off the board when hitting a curb, a rock or another object which abruptly halts the motion of the skateboard. 2. Draw and label the diagram of forces are balanced Lesson 1: Inertia and Mass Define Inertia: Check Your Understanding Depress the mouse on the pop-up menu to check your answer. 1. Imagine a place in the cosmos far from all gravitational and frictional influences. Suppose an astronaut in that place throws a rock. The rock will: 2. An 2-kg object is moving horizontally with a speed of 4 m/s. How much net force is required to keep the object moving with the same speed and in the same direction?
3. Mac and Tosh are arguing in the cafeteria. Mac says that if he throws his jello with a greater speed it will have a greater inertia. Tosh argues that inertia does not depend upon speed, but rather upon mass. With whom do you agree? Why? 3 Lesson 1: State of Motion Define inertia: 1. Several physics teachers are taking some time off to play a little putt-putt golf. The 15th hole at the Hole-In-One Putt-Putt Golf Course has a large metal rim which putters must use to guide their ball towards the hole. Mr. Schmidgall guides his golf ball around the metal rim. When the ball leaves the rim, which path (1, 2, or 3) will the golf ball follow? Draw the diagram too. 2. A 4.0 kg object is moving across a frictionless surface with a constant velocity of 2 m/s. Which one of the following horizontal forces is necessary to maintain this state of motion? A) 0 N B) 0.5 N C) 2.0 N D) 8.0 N E) depends on the speed.
Lesson 1: Balanced and Unbalanced Forces pg 4 4. If the forces acting upon an object are balanced, then the object A. must not be moving. B. must be moving with a constant velocity. C. must not be accelerating. D. none of the above. Lesson 2:Types of Forces 1. Define the different types of contact forces. F f Friction Force F t Tension Force F n Normal Force F air Air Resistance Force F app Applied Force F spring Spring Force 2. List the 3 types of Action-at-a-distance forces and their symbols. Define Fg 3. Does a person diet to lose mass or to lose weight? Explain
Lesson 2: Free Body Diagram pg 5 Free-Body Diagrams: show all forces acting on an object. Use the answers at the bottom of the webpage to check your work. 1. A book is at rest on a table top. Diagram the forces acting on the book. 2. A girl is suspended motionless from a bar which hangs from the ceiling by two ropes. Diagram the forces acting on the girl. 3. An egg is free-falling from a nest in a tree. Neglect air resistance. Diagram the forces acting on the egg as it falls. 4. A flying squirrel is gliding (no wing flaps) from a tree to the ground at constant velocity. Consider air resistance. Diagram the forces acting on the squirrel.
pg 6 5. A rightward force is applied to a book in order to move it across a desk with a rightward acceleration. Consider frictional forces. Neglect air resistance. Diagram the forces acting on the book. 6. A rightward force is applied to a book in order to move it across a desk at constant velocity. Consider frictional forces. Neglect air resistance. Diagram the forces acting on the book. 7. A college student rests a backpack upon his shoulder. The pack is suspended motionless by one strap from one shoulder. Diagram the vertical forces acting on the backpack. 8. A skydiver is descending with a constant velocity. Consider air resistance. Diagram the forces acting upon the skydiver.
pg 7 9. A force is applied to the right to drag a sled across loosely-packed snow with a rightward acceleration. Diagram the forces acting upon the sled. See answer. 10. A football is moving upwards towards its peak after having been booted by the punter. Neglect air resistance. Diagram the forces acting upon the football as it rises upward towards its peak. 11. A car is coasting to the right and slowing down. Diagram the forces acting upon the car. See answer.
Lesson 2: Determining the Net Force pg 8 1 Free-body diagrams for four situations are shown below. For each situation, determine the net force acting upon the object. Lesson 3: Newton s 2nd Law 1. What acceleration will result when a 12-N net force is applied to a 3-kg object? A 6-kg object?
2. A net force of 16 N causes a mass to accelerate at the rate of 5 m/s 2. Determine the mass. pg 9 3. An object is accelerating at 2 m/s 2. If the net force is tripled and the mass of the object is doubled, what is the new acceleration? 4. An object is accelerating at 2 m/s 2. If the net force is tripled and the mass of the object is halved, what is the new acceleration? Lesson 3 : Finding Acceleration As you learned earlier in Lesson 3 (as well as in Lesson 2), the net force is the vector sum of all the individual forces. In Lesson 2, you also learned how to determine the net force if the magnitudes of all the individual forces are known. In this lesson, you will learn how to determine the acceleration if the magnitudes of all the individual forces are known. The three major equations which will be useful are: the equation for net force (Fnet = m. a) the equation for gravitational force (Fg = m. g) the equation for frictional force (Ff = µ. Fn)
pg 10 The process of determining the acceleration of an object demands that the mass and the net force are known. If mass (m) and net force (Fnet) are known, then the acceleration is determined by the equation: Thus, the task involves using the above equations, the given information, and your understanding of Newton's laws to determine the acceleration. Practice Problems To gain a feel for how this method is applied, try the following practice problems. Once you have solved the problems, use the pop-up menus to check your answers. Practice #1 An applied force of 50 N is used to accelerate an object to the right across a frictional surface. The object encounters 10 N of friction. Use the diagram to determine the normal force, the net force, the mass, and the acceleration of the object. (Neglect air resistance.)
pg 11 Practice #2 An applied force of 20 N is used to accelerate an object to the right across a frictional surface. The object encounters 10 N of friction. Use the diagram to determine the normal force, the net force, the coefficient of friction (µ) between the object and the surface, the mass, and the acceleration of the object. (Neglect air resistance.) Lesson 3: Finding Individual Forces Practice #2 A rightward force is applied to a 6-kg object to move it across a rough surface at constant velocity. The object encounters 15 N of frictional force. Use the diagram to determine the gravitational force, normal force, net force, and applied force. (Neglect air resistance.)
Practice #3 pg 12 A rightward force is applied to a 10-kg object to move it across a rough surface at constant velocity. The coefficient of friction, µ, between the object and the surface is 0.2. Use the diagram to determine the gravitational force, normal force, applied force, frictional force, and net force. (Neglect air resistance.) Practice #4 A rightward force is applied to a 5-kg object to move it across a rough surface with a rightward acceleration of 2 m/s2. The coefficient of friction, µ, between the object and the surface is 0.1. Use the diagram to determine the gravitational force, normal force, applied force, frictional force, and net force. (Neglect air resistance.)
Practice #5 pg13 A rightward force of 25 N is applied to a 4-kg object to move it across a rough surface with a rightward acceleration of 2.5 m/s2. Use the diagram to determine the gravitational force, normal force, frictional force, net force, and the coefficient of friction between the object and the surface. (Neglect air resistance.) Lesson 4: Newton s 3rd Law Check Your Understanding 1. While driving, Anna Litical observed a bug striking the windshield of her car. Obviously, a case of Newton's third law of motion. The bug hit the windshield and the windshield hit the bug. Which of the two forces is greater: the force on the bug or the force on the windshield? 2. Rockets are unable to accelerate in space because... a there is no air in space for the rockets to push off of. b there is no gravity is in space. c there is no air resistance in space. d... nonsense! Rockets do accelerate in space.
pg 14 3. A gun recoils when it is fired. The recoil is the result of action-reaction force pairs. As the gases from the gunpowder explosion expand, the gun pushes the bullet forwards and the bullet pushes the gun backwards. The acceleration of the recoiling gun is... a b c greater than the acceleration of the bullet. smaller than the acceleration of the bullet. the same size as the acceleration of the bullet. 4. In the top picture, a physics student is pulling upon a rope which is attached to a wall. In the bottom picture, the physics student is pulling upon a rope which is held by the Strongman. In each case, the force scale reads 500 Newtons. The physics student is pulling a b c with more force when the rope is attached to the wall. with more force when the rope is attached to the Strongman. the same force in each case.
Lesson 4: Identifying Action-Reaction Force Pairs pg 15 Consider the following three examples. The action force is stated; determine the reaction force. Use the pop-up menu to view the answer. 1. Athlete pushes bar upwards. 2. Bowling ball pushes pin rightwards. 3. Compressed air pushes balloon wall outwards. 4. Identify at least five pairs of action-reaction forces in the diagram.