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Speed Speed describes how fast something goes A cheetah moves faster than a horse, but how can we determine this? Speed= Distance Travelled Time it took to travel • You will see this written as • Speed = d t Examples: A wheelchair racer finishes a 132 meter race in 18 seconds, what is his speed? 132meters 18 seconds 7.3 meters/sec What is velocity and how is it different from speed? Velocity: describes both speed AND direction For example a lions speed maybe reported as 4.5 m/s But in order to report its velocity we must know which direction it is travelling For example a possible velocity could be 4.5 m/s North, or 4.5 m/s towards the lake Find the velocity in meters per second of a swimmer who swims exactly 110 meters towards the shore in 72 seconds 110 m/ 72 s towards the shore Calculate the distance in meters that a cyclist would travel in 5 hours at a speed of 1200 meters per hour S= d/t 1200 m/h = d/5hours Solve for d: D= 1200 m/h x 5 h = 6000 m If a car and a train are driving along the same path at the exact same speed, which will be harder to stop? Why? Train it has more momentum Momentum: a quantity defined by both an objects velocity and it’s mass Equation: Momentum= mass x velocity Calculate the momentum of a 6.00 kg bowling ball moving at 10 m/s down the alley. P= M x V 6.00 kg x 10 m/s =60 kgm/s Acceleration: change in velocity divided by the time interval in which the change occurs a= final velocity – initial velocity time Practice Problems: A flowerpot falls off a 2nd story windowsill. It starts from a rest and hits the sidewalk 1.5 s later with a velocity of 14.7 m/s. Find the average acceleration 2 (14.7m/s – 0m/s) / 1.5 s = m/s John is riding his bike, and he accelerates from a speed of 2 m/s to 4 m/s in 3 seconds. What is his average acceleration (4 m/s– 2 m/s) / 1.5 s = m/s2 Lydia is driving on I-95, when she hits a traffic jam. She slows down from a speed of 60 mph to a complete stop in 10 seconds. What is her average acceleration? Have to convert 10 s hours 10 s x (1 min/60 secs) x (1 hr/60 mins) = .0027 hrs Classwork(:0- 60 m/h)/.0027 hr = -22,000 m/h page 226 15 copy the question Newtons Laws Newtons 1st Law “An object at rest remains at rest and an object in motion remains in motion, unless it experiences an unbalanced force” Examples: When you are in a car accident… INERTIA: the tendancy of an object to remain at rest or in motion with constant velocity All object have inertia because they resist changes in motion nd Newtons 2 Law The unbalanced force acting on an object always equals the objects mass time its acceleration F= ma Force= mass x acceleration Example: Pushing an empty shopping cart vs. pushing a full shopping cart If you push both carts with the same amount of force which will go faster? Newtons! What does a “newton” represent? Force is measured in “newtons” 1Newton (N)= 1 kg m/s2 1 Newton= .225 lbs Example: Zookeepers lift a stretcher holding a sick lion. The mass of the lion and the stretcher is 175 kg. The acceleration upwards is .657 m/s2. What force are the zookeepers using to lift the lion? 175 kg x .657 m/s2=. What is the necessary force for a 1600 kg car to accelerate forward at 2m/s2 1600 kg x 2m/s2= A baseball accelerates downward at 9.8 m/s2. If the force acting on the baseball is 1.4 N, what is the baseballs mass? X kg x 9.8m/s2= 1.4 N 1.4 N/9.8m/s2= Xkg X= A sailboat and its crew have a combined mass of 655 kg. If it has a 895 N force acting on it, what is its acceleration? 655kg x Xm/s2= 895 N 895 N/ 655kg = Xm/s2 X= Free Fall Free fall: the motion of a body/object when the only force acting on it is gravity r The acceleration of gravity is always 9.8 m/s2 What’s the difference between mass and weight? Weight Weight= mass x gravity -Weight is measured in newtons Newtons 3rd Law “For every action, there is an equal and opposite reaction” For example: When you kick a soccer ball, do you feel anything in your foot? The force exerted on the ball by your foot is the action force, while the force exerted on your foot by the ball is the reaction force CW page 240 #16