Force and Motion Notes
Unit 4 Force and Motion Learning Goals (TEKS): Force, motion, and energy. The student knows that there is a relationship between force, motion, and energy. The student is expected to: 8.6B differentiate between speed, velocity, and acceleration 8.6A demonstrate and calculate how unbalanced forces change the speed or direction of an object s motion 8.6C investigate and describe applications of Newton s law of inertia, law of force and acceleration, and law of actionreaction such as in vehicle restraints, sports activities, amusement park rides, Earth s tectonic activities, and rocket launches
Sir Isaac Newton (1643 1727) One of the world s greatest scientists Developed the 3 Laws of Motion His ideas are still correct and very much in use today!
How is motion described? What is your current position? Position is the location of an object If you decide to move around, your position changes each time you move. Motion is an object s change in position relative to reference point Whenever your position changes you are in motion A reference point is used to determine if an object has moved or not
A point about Reference Points A place or object used for comparison to determine if something is in motion Ideally, that reference point should itself not be moving. But that is impossible, because the ground we walk on is part of a moving tectonic plate. Our planet Earth spins on its axis, and orbits around our Sun. Our solar system in turn orbits around the center of the Milky Way Galaxy. Our galaxy moves in relation to other galaxies. In fact, no reference point that we could choose is in fact fixed. But we can assume that a reference point, such as a starting line on a running track, is fixed for purposes of measuring motion.
What is a Force? A force is a push or pull When one object pushes or pulls another object, the first object is exerting a force on the second object Forces are described by strength and direction And are drawn with arrows
Forces are measured in Newtons SI unit of force Symbol: N Measured by using a spring scale
Forces occur in pairs Balanced Forces (No change in motion) No Acceleration! Unbalanced Forces (Change in Motion) Acceleration!
Net Force We calculate forces using net force. The overall force on an object after all the forces acting on it are determined is called the net force. Net Force = 3N
Balanced Forces and Motion Are equal forces acting on one object in opposite directions. Equal forces acting on an object will not change the object s motion. The net force is 0 (zero). Net force = 0 (push) Net force = 0 (pull)
Unbalanced Forces The forces acting on an object are not equal When a net force greater than 0 acts on an object the forces are UNBALANCED. One or more force is stronger than others, the object moves in the direction of the greatest force Change in Motion occurs
Unbalanced Forces and Motion Can cause an object to: start moving stop moving change direction or speed. All of these are considered change in motion When two forces act in the same direction, they are added together. When forces act in opposite directions they are subtracted. If one force is greater than the other force, the overall force is in the direction of the greater force.
Unbalanced Forces - Calculation Practice Unbalanced 225 N Unbalanced 13 N Bill Nye Balanced 0 N =
Calculating Force Force = mass x acceleration Don t forget the units!
Force problems practice Get out calculators and try these 3 problems: 1. The mass of a large car is 1000 kg. How much force would be required to accelerate the car at a rate of 3 m/sec2? 2. A 50 kg skater pushed by a friend accelerates 5 m/sec2. How much force did the friend apply? 3. What is the force on a 1000 kg elevator that is falling freely at 9.8 m/sec2?
Force problems answers Get out calculators and try these 3 problems: 1. The mass of a large car is 1000 kg. How much force would be required to accelerate the car at a rate of 3 m/sec 2? (1000g x 3 m/sec 2 = 3,oooN) 2. A 50 kg skater pushed by a friend accelerates 5 m/sec 2. How much force did the friend apply? (50kg x 5m/sec 2 = 250N) 3. What is the force on a 1000 kg elevator that is falling freely at 9.8 m/sec 2? (1000kg x 9.8 m/sec 2 = 9,800N)
A few motion terms: Speed - "how fast an object is moving." Speed can be thought of as the rate at which an object covers a certain distance. Velocity It is speed in a specific direction. Also known as: the rate at which an object changes its position. Acceleration a measure of how quickly speed changes Means a change in velocity It doesn t always mean to speed up!!