1. Chapter 7 Motion & Forces

2. 7.1 Motion Speed & Velocity Speed & Velocity An object is moving if it changes position against a background that stays the same. An object is moving if it changes position against a background that stays the same. The stationary background is the frame of reference. The stationary background is the frame of reference. Speed: describes how fast an object moves. Speed: describes how fast an object moves. How do we determine speed? How do we determine speed?

3. Speed measurements involve time & distance Speed: distance traveled per unit of time. Speed: distance traveled per unit of time. The SI unit for speed is meters per second or m/sec or sometimes km/h (kilometers per hour) or in United States mph (miles per hour) The SI unit for speed is meters per second or m/sec or sometimes km/h (kilometers per hour) or in United States mph (miles per hour)

4. Constant speed is the simplest type of motion When an object travels equal distances in equal amounts of time, it has constant speed. When an object travels equal distances in equal amounts of time, it has constant speed. Ex.: Cruise control in a car. Ex.: Cruise control in a car. 96 m/s means that you travel 96 meters every second of time, just like 60 mph means that you travel 60 miles each hour of time. 96 m/s means that you travel 96 meters every second of time, just like 60 mph means that you travel 60 miles each hour of time.

5. Distance–Time Graph Distance plotted on vertical axis (y-axis). Distance plotted on vertical axis (y-axis). Time plotted on horizontal (x-axis). Time plotted on horizontal (x-axis). Constant speed results in a straight line on distance-time graph. The slope is the speed. Constant speed results in a straight line on distance-time graph. The slope is the speed. The steeper the slope – the faster the speed. See graph p. 219 The steeper the slope – the faster the speed. See graph p. 219 An object at rest has a slope of zero, which is graphed as a horizontal line. An object at rest has a slope of zero, which is graphed as a horizontal line.

6. Speed = distance/time Speed = d / t[distance divided by time] Speed = d / t[distance divided by time] Most objects do not have constant speed. Most objects do not have constant speed. Use the above formula to calculate average speed. Use the above formula to calculate average speed. Average speed is the total distance / total time. Average speed is the total distance / total time. Avg. speed does NOT tell you the speed at any given time. Avg. speed does NOT tell you the speed at any given time.

7. Time (s) Distance (m) faster fast Not moving (i.e. no change in distance) Slope = 0

8. Velocity denotes both speed & direction 4.5 m/s north or 4.5 m/s toward the highway 4.5 m/s north or 4.5 m/s toward the highway Velocity changes if either speed or direction changes. Velocity changes if either speed or direction changes. See Math Skills p. 221 See Math Skills p. 221 Law of Conservation of Momentum: the total amount of momentum in a system is conserved. Law of Conservation of Momentum: the total amount of momentum in a system is conserved. The total momentum of 2 cars that crash is the same after the crash as it was before the crash. The total momentum of 2 cars that crash is the same after the crash as it was before the crash. The car with greater momentum will determine which way the cars will move after the collision. The car with greater momentum will determine which way the cars will move after the collision.

9. Momentum Momentum: a quantity defined as the product of mass & velocity. [Mass times Velocity] Momentum: a quantity defined as the product of mass & velocity. [Mass times Velocity] A larger object moving the same speed as a smaller object has more momentum. A larger object moving the same speed as a smaller object has more momentum. Momentum = mass x velocity Momentum = mass x velocity P = m v P = m v Momentum includes direction (velocity). Its momentum is in the same direction as its velocity. Momentum includes direction (velocity). Its momentum is in the same direction as its velocity. No movement = no momentum. See Math Skills p. 223 No movement = no momentum. See Math Skills p. 223

10. 7.2 Acceleration & Force Acceleration: change in velocity divided by the time interval in which the change occurred. Acceleration: change in velocity divided by the time interval in which the change occurred. Any change in velocity is acceleration…i.e. any change in speed or direction. Any change in velocity is acceleration…i.e. any change in speed or direction. Acceleration occurs if you speed up, slow down or change direction. Acceleration occurs if you speed up, slow down or change direction. Acceleration is ALWAYS in the direction of the acting force. Acceleration is ALWAYS in the direction of the acting force.

11. Acceleration Acceleration = final velocity – initial velocity time interval The SI unit for acceleration is meters per second per second or m/s/s or m/s 2 The SI unit for acceleration is meters per second per second or m/s/s or m/s 2 If acceleration is a small number, velocity is changing gradually. If acceleration is a large number, the velocity is changing more rapidly. If acceleration is a small number, velocity is changing gradually. If acceleration is a large number, the velocity is changing more rapidly. A decrease in speed is negative acceleration and will result in your answer being a negative number. See Math Skills p. 226 A decrease in speed is negative acceleration and will result in your answer being a negative number. See Math Skills p. 226

12. Acceleration can be determined from a velocity-time graph On a velocity-time graph, velocity goes on the vertical axis and the time is on the horizontal axis. On a velocity-time graph, velocity goes on the vertical axis and the time is on the horizontal axis. A slanted straight line indicates constant acceleration. i.e. the velocity changes the same amount each time. (slope of line = acceleration) A slanted straight line indicates constant acceleration. i.e. the velocity changes the same amount each time. (slope of line = acceleration) Acceleration is zero if velocity is constant (because there is no change in speed or direction) See graphs p. 227 Acceleration is zero if velocity is constant (because there is no change in speed or direction) See graphs p. 227 Movement w/constant speed in a straight line is constant velocity. Movement w/constant speed in a straight line is constant velocity.

13. Time (s) Velocity (m/s) Acceleration Speeding up quickly Acceleration Speeding up slowly No acceleration i.e. (constant velocity) Slope = 0

14. Force: Push Push Pull Pull or

15. Force Force: the cause of acceleration, or change in an objects velocity. Force: the cause of acceleration, or change in an objects velocity. When you throw or catch a ball you exert a force to change the ball’s velocity. When you throw or catch a ball you exert a force to change the ball’s velocity. Balanced Forces: forces that act on an object that combine to produce a net force of zero. Balanced Forces: forces that act on an object that combine to produce a net force of zero. Balanced forces are equal and opposite i.e. A tug of war ( >>) or an offensive lineman pushing a defensive lineman, but there is no acceleration. (>>> >>) or an offensive lineman pushing a defensive lineman, but there is no acceleration. (>>> <<<)

16. Unbalanced forces Unbalanced Forces result in a net force! Unbalanced Forces result in a net force! Someone “wins” in the tug of war & football linemen examples. Someone “wins” in the tug of war & football linemen examples. Balanced forces never change velocity Balanced forces never change velocity (NO ACCELERATION) Unbalanced forces always change velocity Unbalanced forces always change velocity(ACCELERATION)

17. Friction & Air resistance Friction is a force between 2 objects in contact with each other that opposes their motion. Friction is a force between 2 objects in contact with each other that opposes their motion. Ex.: a shopping cart rolling along a sidewalk will eventually come to a stop. It stops because of friction. Ex.: a shopping cart rolling along a sidewalk will eventually come to a stop. It stops because of friction. Friction also acts on objects that are not moving. A car parked on hill with its parking brake applied or someone sitting in a chair. Friction also acts on objects that are not moving. A car parked on hill with its parking brake applied or someone sitting in a chair.

18. Frictional force varies w/the surfaces that are in contact Frictional forces are great when the surfaces are rough. Frictional forces are great when the surfaces are rough. Smooth surfaces produce less friction. Smooth surfaces produce less friction.

19. Air Resistance is a form of friction Air resistance is the result of interaction between the surface of a moving object & air molecules. Air resistance is the result of interaction between the surface of a moving object & air molecules. The amount of air resistance depends on size & shape of the object as well as the speed of the object. The amount of air resistance depends on size & shape of the object as well as the speed of the object. Larger objects create more air resistance as does greater speed. Larger objects create more air resistance as does greater speed. Design of cars, planes etc. take into account air resistance. Design of cars, planes etc. take into account air resistance.

20. Gravity Gravity: the force of attraction between 2 particles of matter due to their mass. Gravity: the force of attraction between 2 particles of matter due to their mass. Newton’s Law of Universal Gravitation states that all objects in the universe are attracted to all other objects in the universe. Newton’s Law of Universal Gravitation states that all objects in the universe are attracted to all other objects in the universe. The amount of the force depends on 2 factors: 1. the mass of the objects & The amount of the force depends on 2 factors: 1. the mass of the objects & 2. the distance between the 2 objects. 2. the distance between the 2 objects. The greater the mass… the greater gravity is. The greater the mass… the greater gravity is. Gravity rapidly decreases as distance is increased. Gravity rapidly decreases as distance is increased.

21. 7.3 Newton’s Laws of Motion Newton’s First Law of Motion Newton’s First Law of Motion An object at rest will remain at rest and an object in motion will remain in motion at a constant velocity unless acted upon by an unbalanced force. Ex.: sitting in a car that is stopping. You are pushed backward by your seatbelt. Also, when the car turns, you really aren’t pushed against the car. You are really trying to continue going straight. Ex.: sitting in a car that is stopping. You are pushed backward by your seatbelt. Also, when the car turns, you really aren’t pushed against the car. You are really trying to continue going straight.

22. Inertia Inertia really is another name for Newton’s 1 st Law of Motion. Inertia really is another name for Newton’s 1 st Law of Motion. Inertia: the tendency of an object to remain at rest or, if in motion, to remain in motion at a constant velocity. Inertia: the tendency of an object to remain at rest or, if in motion, to remain in motion at a constant velocity. A small mass object has low inertia – it takes only a small force to be accelerated. A small mass object has low inertia – it takes only a small force to be accelerated. A large mass object (such as a car) has high inertia – it takes a large force to be accelerated. A large mass object (such as a car) has high inertia – it takes a large force to be accelerated.

23. Newton’s Second Law of Motion The unbalanced Force acting on an object equals the mass times the acceleration. F = m a The unbalanced Force acting on an object equals the mass times the acceleration. F = m a F = m a Consider pushing a full grocery compared to pushing an empty grocery cart. You must push the full cart with greater force for it to accelerate at the same rate. Consider pushing a full grocery compared to pushing an empty grocery cart. You must push the full cart with greater force for it to accelerate at the same rate. If you & a friend both push empty carts but you push w/greater force, your cart will have greater acceleration. See Math Skills p. 236. If you & a friend both push empty carts but you push w/greater force, your cart will have greater acceleration. See Math Skills p. 236. Force is measured in Newtons. 1N = 1kg x 1 m/s 2 Force is measured in Newtons. 1N = 1kg x 1 m/s 2

24. Free fall & Weight Free fall: the motion of a body when only the force of gravity is acting on it. Free fall: the motion of a body when only the force of gravity is acting on it. Free fall is due to gravity, g. g = 9/8 m/s 2 Free fall is due to gravity, g. g = 9/8 m/s 2 If there is no air resistance all objects fall at the same rate – 9.8 m/s 2 If there is no air resistance all objects fall at the same rate – 9.8 m/s 2 A penny will hit the ground at the same time as a cannon ball if dropped at same time from the same height. A penny will hit the ground at the same time as a cannon ball if dropped at same time from the same height. A heavier object has a greater gravitational force, but because it has more mass it needs a greater force to accelerate. On Earth, the ratio of weight divided by mass of ANY object is always 9.8 m/s 2. A heavier object has a greater gravitational force, but because it has more mass it needs a greater force to accelerate. On Earth, the ratio of weight divided by mass of ANY object is always 9.8 m/s 2.

25. Weight = mass x free-fall acceleration w = m g w = m g The force on a body due to gravity is called its weight. Weight is measured in Newtons The force on a body due to gravity is called its weight. Weight is measured in Newtons If I have a mass of 82 kg & gravity is 9.8 m/s 2. Then I weigh 82 kg x 9.8 m/s 2 = 804 N If I have a mass of 82 kg & gravity is 9.8 m/s 2. Then I weigh 82 kg x 9.8 m/s 2 = 804 N Mass & weight are different Mass & weight are different Mass = amount of matter in an object. Mass = amount of matter in an object. Weight = force of gravity acting on your mass. Weight = force of gravity acting on your mass. Force & weight are the same Force & weight are the same

26. Velocity is constant when air resistance balances weight When air resistance equals the force of gravity, objects stop accelerating. (i.e. balanced force) When air resistance equals the force of gravity, objects stop accelerating. (i.e. balanced force) The objects continue to fall at a constant velocity (i.e. no acceleration). This is called terminal velocity. The objects continue to fall at a constant velocity (i.e. no acceleration). This is called terminal velocity. Terminal velocity is approx. 200 mi/h or Terminal velocity is approx. 200 mi/h or or about 320 km/h.

27. Newton’s Third law of Motion For every action, there is an equal & opposite reaction. For every action, there is an equal & opposite reaction. Ex.: 1. kick a soccer ball – the ball experiences a change in motion and the ball is exerting an equal & opposite force on your foot. 2. while swimming in a pool you push off the side of the pool. How far you go into the pool is determined by how hard you pushed off the pool. 3. Jet planes & the space shuttles utilize Newtons 3 rd law to propel themselves. See fig. 7-17 on p. 240 Ex.: 1. kick a soccer ball – the ball experiences a change in motion and the ball is exerting an equal & opposite force on your foot. 2. while swimming in a pool you push off the side of the pool. How far you go into the pool is determined by how hard you pushed off the pool. 3. Jet planes & the space shuttles utilize Newtons 3 rd law to propel themselves. See fig. 7-17 on p. 240