Forces and Newton s Laws of Motion. UCVTS AIT Physics

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2 Newton s First Law of Motion - Inertia Aristotle ( BC) on Motion (4 th century BC) Natural Motion It was thought to be either straight up or straight down a rock would fall, smoke would rise. Circular motion was natural for planets, stars, etc.thus planets and stars moved in perfect circular motion around the earth These motions were NOT thought to be caused by FORCES Violent Motion Imposed motion was the result of forces that pushed or pulled A cart moved because it was pulled by a horse, a ship was pushed by the force of the wind Objects in their natural resting places could NOT move by themselves

3 Newton s First Law of Motion - Inertia Copernicus ( ) on Motion Theory of the moving earth Interpreted astronomical observations by assuming that the earth and other planets move around the sun This idea was extremely controversial people preferred to think that the earth was the center of the universe. Copernicus worked on his ideas (De Revolutionibus, 1543) in secret to avoid persecution» Can you think of similar controversial scientific ideas current to our time??

4 Newton s First Law of Motion - Inertia Galileo ( ) on Motion Galileo was outspoken in his support of Copernicus And was put to trial and house arrest because of this Great contribution to physics Demolishing the notion that a force is necessary to keep an object moving. A force is any push or pull Friction is the force that acts between materials that touch as they move past each other Examples..??? Galileo argued that only when friction is present (as it usually is) is a force needed to keep an object moving Galileo was concerned with HOW things move rather than WHY

5 Issac Newton ( )..WHY things move First Law of Motion or Law of Inertia Every object continues in a state of rest, or of motion in a straight line at constant speed, unless it is compelled to change that state by forces exerted upon it Objects in motion tend to stay in motion (planet earth, hockey puck on ice, etc..objects at rest tend to stay at rest..(this room, tables, you, etc.)

6 Newton s second law of motion When a net force acts on an object of mass m, the acceleration a of the object can be obtained from F ma what is this symbol F F x y ma ma x y?

7 Force and Mass Forces and Newton s Laws of Motion Force is a push OR pull Force is a Vector quantity F Mass is a property of matter that determines how difficult it is to accelerate or decelerate an object MASS-a measure of inertia Mass is not volume Bag of cotton.bag of nails Same volume but if you get hit by one one hurts and the other doesn t Mass is not weight Mass is a measure of the amount of material (number of and kind of atoms) in an object Mass is measured in kilograms (kg) Mass is a measure of the inertia

8 Weight Weight=mass X acceleration due to gravity (weight=mg) W=mg (g = -9.8m/s/s) Weight=mass X acceleration due to gravity (weight=mg) W = mg Weight is a measure of the gravitational force acting on an object (depends on the objects location) In space an astronaut has the same mass as she/he does on earth, but much different weight!» Weight and force are measured in newtons (slightly less than a quarter of a pound) What is the weight (W) of the objects at the right?

9 NET FORCE Forces and Newton s Laws of Motion In the absence of a force, objects at rest stay at rest and objects in motion continue in motion Specifically in the absence of a net force Forces can be added and subtracted mathematically depending on the direction in which they are applied Net Force is the vector sum of all forces acting on an object Equilibrium When the net force equals zero, a=0 When an object is at rest with the net force on it being zero, the object is in static equilibrium

10 Free Body Diagrams Draw a box to symbolize each object in the system. Each object of interest should have its own free-body diagram. Pretend the object is by itself in empty space. (It is a "free body".) Include all the forces acting on the body. (Make sure you include only those forces acting on the body of interest. Don't include the forces the body causes on other objects. The net force is the sum of other forces not a force itself. Remember that accelerations are not forces!) Draw an arrow from the box representing each force. The direction of the arrow should indicate the direction of the force. Label each arrow clearly. Picture of Situation Free Body Diagram

11 M car =1000 kg F ma F F x y ma ma x y Let's calculate the acceleration for the car

12 Normal Force Forces and Newton s Laws of Motion The force supporting the block (since the block is not accelerating downward) is the normal force It s the force pushing back on the block by the table normal to the surface This is why it is called the Normal Force

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14 Static and Kinetic Frictional Forces The normal force is one component of the contact force between two objects, acting perpendicular to their interface. The frictional force is the other component; it is in a direction parallel to the plane of the interface between objects. Friction always acts to oppose any relative motion between surfaces and is parallel to the surface Static Frictional Force The magnitude of the static frictional force depends on the magnitude of the applied force and can assume any value up to a maximum of f f MAX s s N s s N s or F F is coeffient of static friction

15 Some Coefficients of Friction

16 Static and Kinetic Frictional Forces Kinetic Frictional Force The magnitude of the static frictional force depends on the magnitude of the applied force and can assume any value up to a maximum of f k k N k F is coeffient of kinetic friction Now we re moving We have a kinetic frictional force f k

17 Newton s Third law of motion Action-Reaction Law Whenever one objects exerts a force on a second object the second object exerts an equal and opposite directed force on the first object. Example 4, note the difference in acceleration of the astronaut and the spacecraft..why? Action? Reaction?

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19 Tension Force Forces and Newton s Laws of Motion A special description for a force acting through a rope, cable or beam Because of tension, a rope or cable transmits a force from one end to the other T=-T. T is the reaction force to T (equal and opposite direction)

20 Equilibrium Revisited Zero acceleration therefore F ma if a 0 then F 0 Static equilibrium is when a=0 and v=0 This is a special branch of engineering called Statics F F x y 0 0 Static Equilibrium can be used to analyze the forces on a bridge truss design

21 Non-Equilibrium Revisited there is acceleration therefore F ma a m F Atwood s Machine is a good example of equilibrium or non-equilibrium. Why?

22 Newton s Law of Universal Gravitation The force between any 2 masses is represented by the equation: F G Gm m d Nm kg 2 2 con s equ en ce s It s why you are being pulled towards the ground Because the earth is so massive Is there also a force between you and the person sitting next to you? Is the earth also accelerating towards you? Why is g=9.8 m/s/s for any size mass????

23 The Real World of Newton s Laws Newton's first law states, an object stays in motion unless is is acted upon by an unbalanced force. Another reason why we do not notice it is because there are other forces which act against it to lessen the effects. One way in which we could see the effects of Newton's first law would be if we were moving at a high rate of speed then hits a solid object and comes to a stop abruptly. Even though the car comes to a stop, the passenger inside the vehicle is still moving at the same speed that car was originally traveling. To help counteract these immediate effects of Newton's first law, things like the air bag and crumple zone were invented. By slowing down the time it takes for the passengers in the car to come to a stop, it will lessen the injuries that would normally be fatal or serious to people. Newton's second law can describe the exact effects of what happens when things like the air bag and crumple zone are put to use in an automobile. Newton's second law can be easily expressed by one equation alone. Acceleration - Force / Mass. This law governs acceleration and is simple to understand. You can lower the acceleration which is produced by your body in a collision by using safety features like crumple zones and air bags. "The acceleration a of a mass m by an unbalanced force F is directly proportional to the force and inversely proportional to the mass, or a- F / m." This is the second law of motion. "Every action has an equal and opposite re-action." This is Newton's third law and it also has plays a role in exactly how both airbags and crumple zones work. Newton's third law would probably be the most important of all of them. When you have an impact with your car, not only does your car hit another object, but the object pushes back on your car. Since the crumple zone on your car is made soft to absorb impacts, it will take the brunt of the collision so that the time it takes the car to come to a complete stop is spread out over a longer period of time. Not only may this save other people's lives it may also save money in low speed collisions.

24 The Real World of Newton s Laws To study an auto accident, you would have to study the changes in momentum that a car faces during the collision. According to Newton's laws, "A body in motion will stay in motion until it is acted up by an outside force." This is the same idea when it comes to an accident. An accident is the same as if you were to stop immediately and go from 50 miles per hour to 0 in less than a second. The amount of momentum your body has is still 50 miles per hour where as the car has now stopped. This is where airbags help in the event of an accident. Before your head gets the chance to smash into the steering wheel, the air bag deploys and cushions the impact that is about to happen. The fact that your body when in motion, wants to resist changes in their state of motion, like when coming in contact with an air bag, is called it's inertia. Air bags do not just cushion your body in the even of an impact, but they also spread the impact over a larger area. By doing this, the force is not all concentrated in one small area of your body. This in turn will cause the seriousness your injuries to be reduced because the force you feel is spread out. As seen in the illustration below. Airbag deploys in about 15msec

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26 A little more on Newton s 3 rd Law A bug hitting the windshield of a car exerts the same amount of contact force on the car that the car exerts on the bug, of course the bug having a much smaller mass will experience a much greater acceleration than the car. In fact, the bug won't have the guts to do it again. 1