Chapter 1: The Prime Movers

Transcription

1 What is force?

2 Chapter 1: The Prime Movers

3 Force is a push or pull. It is a vector, meaning that it has a magnitude and direction. A vector is a physical quantity that has both magnitude and direction Magnitude; numbers (10 lbs of force versus 50 lbs) Direction- you can push/pull an object in different directions, we measure those directions by measuring the angle between the force (drawn as an arrow) and a horizontal line You can also just say a direction North Right/left Up or down A scalar quantity only has magnitude Example: speed, distance, temperature, pressure, and mass.

4 We will talk about force in four energy systems Mechanical systems A force exerted on an object can cause a change in the objects motion, so force is called the Prime Mover in a mechanical system. When solids interact, they exert forces on one another which causes change in motion. Fluid Systems Fluids such as air, steam, water, oil or gasoline are moved Usually through pipes, lines or tubes. Forces are distributed on a surface. Pressure is the force on a surface divided by the area of a surface. When a can of hairspray says CAUTION CONTENTS UNDER PRESSURE it is because the molecules of the hairspray are rapidly colliding with the walls of the container each exerting a force on the wall. Fluids move through these tubes because of pressure, therefore pressure is the prime mover in fluid systems.

5 Electrical system Electric charge (electrons) move through wires or complex circuits. Force causes the motion of the electrons (current), but there has to be an electric potential difference, or voltage. Thermal System A system where energy in the form of heat flows from one place to another (always from hot to cold). Heat flows because of a temperature difference (aka: a temperature gradient). The amount of energy that is transferred can be measured.

6 Multiple forces can act on an object at the same time. If the forces are opposite and equal (opposite directions, but same magnitude) then the object is at rest, and is considered to be in equilibrium. For the object to be moving, there has to be a net force on the object This means that when you add the numbers together, you don t get zero i.e the forces don t cancel each other out

7 If the crane LIFTS an object off the ground, does it exert a force on the container? YES! Is there a net force acting on the object? YES! If the crane holds an object MOTIONLESS above the ground, does it exert a force on the container? YES! Is there a net force acting on the object? NO!

8 As the high jumper pushes off of the ground is there a net force acting on her? Yes Which direction is it? Up

9 Forces are used in all mechanical systems Transmitted by mechanical parts Gears, belts, valves, chains, pulleys, screws, cams, linkages, rods, sprockets and pistons. These parts are interconnected to form mechanical systems. A Bike is an example of a complex mechanical system Chains and sprockets, Hand brakes and cables Pedals To move or stop the bike requires a long chain of exerting forces What forces are involved in forward movement of the bike?

10 Remember, we said force is a vector, which means it has Magnitude and Direction, both of which we can measure. You measure force whenever you step onto the scale Your weight is a measurement of the force of the earth s gravity on your body. We can measure force of an object with a spring scale. The length that the spring stretches is proportional to the force applied to it.

11 In the world of science, the metric scale is used. The units are called the SI (System International) units. Length: meter (m) Mass: kilogram (kg) Time: second (s) Current: ampere (A) Temperature: Kelvin (K) Amount of substance: mole (mol) Luminous intensity: candela (cd) Force: Newton (N) Some companies in the US still use the English system of units. The unit of force is the pound Length: feet/inches/yards Mass: slug In equations units can be multiplied and divided just like numbers As long as your working within either unit system they don t mix. You can t add apples and oranges 20 pounds cannot be added to 10 Newtons, for example. You would have to convert it first!

12 List and describe the four types of systems of energy that we will be learning about in this class? What are the prime movers in each? Describe force. What are the two components of a vector? Is force a vector? Name two units of measure for force. What is the net force? Two forces act on an object. The forces have a magnitude of 34 Newtons and act along the same line but in opposite directions. What is the magnitude of the net force acting on the object? Three forces act on an object. If the magnitude of the net force is zero, the object is said to be in.

13 We learned previously that if there is no net force there can be no movement. This is not completely true It is true for objects initially at rest Newton s first law states that objects in motion will stay in motion and objects at rest will stay at rest unless acted upon by an unbalanced force (a net force). When there is a net force (sum of forces is nonzero), there is an unbalanced force (don t cancel out).

14 There are two forces acting on these skydivers Gravity (downwards Drag (air resistance, upwards) When they first jump out, the forces are unbalanced. There is a net force acting on them F drag F gravity

15 Speed will increase (acceleration due to gravity) Drag increases with speed Eventually, force of drag is equal to their weight Called terminal velocity When the parachute is engaged Increases drag Net force upward, speed decreases Drag decreases as speed decreases Eventually the force of drag and the force of gravity will be balanced No net force Will continue to fall at constant speed, no acceleration Newtons 1 st Law: inertia, an item will resist change in motion Until acted upon by an unbalanced force (the ground) If an object is moving at constant speed there is NO net force

16 Are mass and weight the same? NO WAY!!!!!!! Mass is a measurement of how much space an object takes up Weight is a measurement of the force of gravity on an object (how hard the earth is pulling something). Mass will never change no matter where you are Same on earth as on the moon or any other planet Take up the same amount of space Your weight on the moon is different because the force of gravity there is only 1/6 of the earths gravity So it doesn t pull you as hard and you would weigh less.

17 Torque Quantity that causes rotation in mechanical systems An effect of a force applied on a body at some distance from the axis of rotation of that body. Clockwise or counterclockwise Torque=FxL F= force L=length of lever arm The shortest distance from rotation axis (center of the wheel or gear) to the line of action of he applied force. In the case of a wheel, it s the radius. Units: measured in Newton*meters or lb*ft

18 Vectors are drawn as arrows Length of the arrow is proportional to magnitude of the vector (how much force is being applied) Arrow points in direction of the applied force. 1) Choose a scale Ex: 1 inch= 10 pounds of force 2) Draw proportional length in correct direction Using a protractor to measure angle

19 In a game of tug and war three people on the left EACH pull with a force of 230 Newtons while two people on the right EACH pull with 300 Newtons. Which side wins? What is the net force? How do you calculate the net force if there are multiple forces acting at different angles to one another? Have to find the resultant!

20 Draw the vectors acting on the object. Draw them in order head to tail F1 Resultant (Net Force) Use the Pythagorean theorem to find out the net force. F1 2 +F2 2 =Resultant 2 Use a protractor to determine the direction F2

21 Mike s car has a flat tire. He is trying to remove a lug nut that the tire shop overtightened with an impact wrench. Mike applies a force of 80 pounds to the end of a lug wrench that is 1.2 feet long Find the torque applied to the lug nut by the wrench The torque Mike applies to the lug nut does not make it turn. What is the net torque on the lug nut.