Skating Skating grasses. Would the blade still cut the grasses if - PDF document

Skating 1 Skating 2 Introductory Question Introductory Question A rotary lawn mower spins its sharp blade A rotary lawn mower spins its sharp blade � rapidly over the lawn and cuts the tops off the rapidly over the lawn and cuts the tops off the Skating Skating grasses. Would the blade still cut the grasses if grasses. Would the blade still cut the grasses if they weren’t attached to the ground? they weren’t attached to the ground? they weren t attached to the ground? they weren t attached to the ground? Yes Yes A. A. No No B. B. Skating 3 Skating 4 Observations about Skating Observations about Skating 4 Questions about Skating 4 Questions about Skating � When you’re When you’re at rest at rest on a level surface, on a level surface, � Why does a stationary skater remain stationary? Why does a stationary skater remain stationary? � � without a push, you remain stationary without a push, you remain stationary � Why does a moving skater continue moving? Why does a moving skater continue moving? � with a push, you start moving that direction with a push, you start moving that direction � Why does a skater need ice or wheels to skate? Why does a skater need ice or wheels to skate? � When you’re Wh Wh When you’re moving ’ ’ moving on a level surface, i on a level surface, l l l l f f � How does a skater start or stop moving? How does a skater start or stop moving? � without a push, you coast steady & straight without a push, you coast steady & straight � with a push, you change direction or speed with a push, you change direction or speed Skating 5 Skating 6 Question 1 Question 1 Physics Concept Physics Concept � Why does a stationary skater remain stationary? Why does a stationary skater remain stationary? � Inertia Inertia (just the first part) � (just the first part) � What keeps the dishes in place on a table? What keeps the dishes in place on a table? � A body at rest tends to remain at rest A body at rest tends to remain at rest � If I pull the tablecloth, what will happen? If I pull the tablecloth, what will happen? � Does the speed at which I pull matter? � D D Does the speed at which I pull matter? th th p p d t d t hi h I p ll m tt r? hi h I p ll m tt r? 1

Skating 7 Skating 8 Question 2 Question 2 Physics Concept Physics Concept � Why does a moving skater continue moving? Why does a moving skater continue moving? � Inertia Inertia (the whole thing) (the whole thing) � What keeps a moving banana moving? What keeps a moving banana moving? � A body at rest tends to remain at rest A body at rest tends to remain at rest � Can I slice a moving banana in midair? Can I slice a moving banana in midair? � A body in motion tends to remain in motion A body in motion tends to remain in motion Skating 9 Skating 10 Newton’s First Law Newton’s First Law (Version 1) Question 3 Question 3 (Version 1) � An object that is free of external influences An object that is free of external influences � Why does a skater need ice or wheels to skate? Why does a skater need ice or wheels to skate? moves in a straight line and covers equal moves in a straight line and covers equal � Why does a hovercraft need an air cushion? Why does a hovercraft need an air cushion? distances in equal times. distances in equal times. Skating 11 Skating 12 Keeping Keeping It Simple It Simple Physical Quantities Physical Quantities � Real Real- -world complications mask simple physics world complications mask simple physics � Position Position – – an object’s location an object’s location � Solution: minimize or overwhelm complications Solution: minimize or overwhelm complications � Velocity Velocity – – its change in position with time its change in position with time � � To demonstrate inertia: To demonstrate inertia: � � work on level ground (goodbye gravity) work on level ground (goodbye gravity) � use wheels, ice, or air support (goodbye friction) use wheels, ice, or air support (goodbye friction) � work fast (overwhelm friction and air resistance) work fast (overwhelm friction and air resistance) 2

Skating 13 Skating 14 Newton’s First Law Newton’s First Law (Version 2) Physical Quantities Physical Quantities (Version 2) � An object that is free of external influences An object that is free of external influences � Position Position – – an object’s location an object’s location moves at a constant velocity. moves at a constant velocity. � Velocity Velocity – – its change in position with time its change in position with time � Force Force – – a push or a pull a push or a pull Skating 15 Skating 16 Newton’s First Law Newton’s First Law Introductory Question Introductory Question (revisited) (revisited) � An object that is not subject to any outside An object that is not subject to any outside A rotary lawn mower spins its sharp blade A rotary lawn mower spins its sharp blade � forces moves at a constant velocity. forces moves at a constant velocity. rapidly over the lawn and cuts the tops off the rapidly over the lawn and cuts the tops off the grasses. Would the blade still cut the grasses if grasses. Would the blade still cut the grasses if they weren’t attached to the ground? they weren’t attached to the ground? they weren t attached to the ground? they weren t attached to the ground? Yes Yes A. A. No No B. B. Skating 17 Skating 18 Question 4 Question 4 Physical Quantities Physical Quantities � How does a skater start or stop moving? How does a skater start or stop moving? � Position Position – – an object’s location an object’s location � What does a push do? What does a push do? � Velocity Velocity – – change in position with time change in position with time � What does a skater respond to a push? What does a skater respond to a push? � Force Force – – a push or a pull a push or a pull � Do all skaters respond equally to equal pushes? � D Do all skaters respond equally to equal pushes? D ll k t r r p nd q ll k t r r p nd q ll t ll t q q l p l p h ? h ? � Acceleration Acceleration – – change in velocity with time change in velocity with time � Mass Mass – – measure of object’s inertia measure of object’s inertia 3

Skating 19 Skating 20 Newton’s Second Law Newton’s Second Law Clicker Question Clicker Question � An object’s acceleration is equal to the net force An object’s acceleration is equal to the net force To find the full bottle of water, without lifting To find the full bottle of water, without lifting � exert on it divided by its mass. That acceleration exert on it divided by its mass. That acceleration anything, you should anything, you should is in the same direction as the net force. is in the same direction as the net force. compare the velocities of the two bottles. compare the velocities of the two bottles. h h l l i i i i f h f h b b l l A. A. acceleration = net force/mass acceleration = net force/mass compare the positions of the two bottles. compare the positions of the two bottles. B. B. push each bottle and observe its acceleration. push each bottle and observe its acceleration. C. C. net force = mass ⋅ acceleration net force = mass acceleration consult a Ouija board. consult a Ouija board. D. D. Skating 21 Summary about Skating Summary about Skating � Skates can free you from external forces Skates can free you from external forces � When you experience no external forces, When you experience no external forces, � You coast You coast – – you move at constant velocity you move at constant velocity � If you’re at rest, you remain at rest If If If you’re at rest, you remain at rest ’ ’ i i � If you’re moving, you move steadily and straight If you’re moving, you move steadily and straight � When you experience external forces When you experience external forces � You accelerate You accelerate – – you move at a changing velocity you move at a changing velocity � Acceleration depends on force and mass Acceleration depends on force and mass 4