Motion and Forces
Unit 3 Force and Motion Student understandings for 8.6A Learning Goals (TEKS): Force, motion, and energy. The student knows that there is a relationship between force, motion, and energy. 8.6A demonstrate and calculate how unbalanced forces change the speed or direction of an object s motion A force is a push or a pull. Forces act in a particular direction and are measured in Newtons. Forces occur in pairs (acting with equal force on different objects in opposite directions). More than one force can act on an object at a time. These forces can be either balanced or unbalanced. Determine net force and direction of net force Force = Mass x Acceleration change of motion
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!
What is Motion? Motion is one of those things that scientists measure. They can measure on the atomic scale (like solid molecules vibrating in place) or on the larger scale (people and cars). In either situation motion is when an object changes position compared to a reference point. Common examples of the idea of motion: pushing a block is an example of linear motion, opening a door is an example of rotation, rotating ball with attached thread is an example of a circular motion.
How is motion described? Tell me, what is your current position? Position is the location of an object (or person) If you decide to move around, your position changes each time you move. When you move, you are in motion. 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
Reference Point Most motion we can see with our eyes. How do we know that something is moving? It has to do with comparing it to other objects. Motion is hard to detect if you have your eyes closed, because you cannot tell if something is moving by seeing go past something else. Think about a car that has whizzed past you. You could tell that they were moving by comparing the car s movement your mailbox, driveway, your bike that is laying in the yard, or your friend s house across the street.
Reference Point The object that appears to not be moving (mailboxes, bikes, driveways, etc) is what is known as a reference point. When we were in the hallways today, how did you know that you were moving? Well, you compared yourself to the walls, tiles, and even other classmates. Those were all reference points. They helped you to determine that you were in fact moving.
Reference Point Keep in mind, a reference point does not have to be completely still, but it has to be moving at a different speed so you can use it to tell another object is moving. An example of a reference point that is moving is when you look out the window of a car and notice that you are moving faster than the car next to you. The car next to you is moving, it is just moving slower than your car. You use that other car as a reference point to determine that your car is moving.
So, just to clarify Reference point: A place or object used for comparison to determine if something is in motion Got it? But wait everything I just told you about reference points is a lie, sort of -
A point about Reference Points Ideally, that reference point should itself not be moving (or at least moving slower than the second object). 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.
Ok, back to basics Objects move because there are forces at work on them. Let s review the concept of forces
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 They are drawn with arrows. The size of the arrow indicates magnitude (size) of the force
Forces are measured in Newtons SI unit of force (SI is the abbreviation for System Internationale. SI is the name given to the standard metric system of units) 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 by mathematically looking at all of the forces involved. 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 N (zero Newtons). Net force = 0 (push) Net force = 0 (pull)
Unbalanced Forces This means the forces acting on an object are not equal (one is greater than the other) When a net force greater than 0 N 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 (Newton s Second Law) 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)
Video:
HW Force Practice Problems:
Let s work the first problem together: