Chapter 7 Motion & Forces

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

Speed measurements involve time & distance 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 U.S. mi/h (miles per hour)

Constant speed is the simplest type of motion When an object covers = distances in = amounts of time it has constant speed. Ex.: Cruise control in a car. 96 m/s means you traveled 96 meters in a second, every second.

Distance – time graph Distance on vertical axis. Time on horizontal axis. Constant speed results in a straight line on distance-time graph. The steeper the slope – the faster the speed. See graph p. 219 An object at rest has a slope of zero.

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

Velocity includes both speed & direction 4.5 m/s north or 4.5 m/s toward the highway Velocity change if either speed or direction changes. See Math Skills p. 221 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 is was before the crash. The car with greater momentum will determine which way the cars will move after the collision.

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

7.2 Acceleration & Force Acceleration: change in velocity divided by the time interval in which the change occurred. Any change in velocity is acceleration… speed or direction. Velocity changes if you speed up, slow down or change direction.

Acceleration = final velocity – initial velocity time The SI unit for acceleration is meters per second per second or m/sec/sec or m/sec2 If the answer is a small number, speed is increasing gradually. A larger value for acceleration means the speed up is more rapid. A decrease in speed is negative acceleration and will result in your answer being a negative number. See Math Skills p. 226

Acceleration can 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. A straight line indicates constant acceleration and the velocity changes the same amount each time. 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.

Force 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. Balanced Forces: forces that act on an object that combine to produce a net force of zero. Balanced forces are opposite and equal. ex. 1. A tug of war (<<< >>>) & 2. an offensive lineman pushing a defensive lineman but there is no movement (>>> <<<)

Unbalanced forces Unbalanced Forces: Forces that are not opposite and/or equal. The net forces DO NOT = zero. Someone “wins” in the tug of war & football linemen examples. Balanced forces can not change motion. Unbalanced forces must change motion.

Friction & Air resistance Friction: the force between 2 objects in contact that opposes motion of either object. Ex.: a car rolling along on a flat road will eventually come to a stop unless a force is applied to the car. It stops because of friction. Friction also acts on objects that are not moving. A car parked on hill w/it’s brake on & you sitting in a chair.

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

Air Resistance is a form of friction 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. Larger objects create more air resistance as does greater speed. Design of cars, planes etc. take into account air resistance.

Gravity 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. The amount of the force depends on 2 factors: 1. the size of the objects & 2. the distance between the 2 objects. The greater the mass… the larger gravity is. Gravity rapidly decreases as distance is increased.

7.3 Newton’s Laws of Motion Newton’s First Law of Motion an object @ rest will remain @ rest & an object in motion remain in motion @ the same velocity unless an unbalanced force acts on it. Ex.: sitting in a car that is stopping. You are pushed forward against you seatbelt. Also, when the car turns, you really aren’t pushed against the car. You are really trying to continue going straight.

Inertia Inertia really is another name for Newton’s 1st Law of Motion. Inertia: the tendency of an object to remain at rest or in motion at a constant velocity. A small object has only a little inertia – it takes only a small force to be accelerated. A large object such as a car needs a large force to overcome its inertia.

Newton’s Second Law of Motion The unbalanced force acting on an object equals the mass times the acceleration. F = ma Consider pushing an empty grocery vs a full grocery cart. If you push both w/the same force the empty one will have greater acceleration. 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/s2

Free fall & Weight 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/s2 If there is no air resistance all objecst fall at the same rate – 9.8 m/s2 A penny will hit the ground at the same time as a shot put if dropped at same time from the same height. Heavier object has greater gravitational force, but because it has more mass it is harder to accelerate.

Weight = mass x free-fall acceleration w = mg The force on a body due to gravity is called its weight. Weight is measured in Newtons I have mass of 82 kg & gravity is 9.8 m/s2. So I weigh 82 x 9.8 = 804 N Mass & weight are different Mass = amount of matter in an object. Weight = gravity acting on your mass.

Velocity is constant when air resistance balances weight When air resistance equals the force of gravity, objects stop accelerating. They continue their speed but stop going faster. This is called terminal velocity. Terminal velocity is app. 200 mph.

Newton’s Third law of Motion 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 3rd law to propel themselves. See fig. 7-17 on p. 240