TAKS Objective 5: Day 2 Energy Isthe ability to do Work Two Types: Kinetic (Energy of Motion) and Potential (Stored Energy) Law of Conservation of Energy Energy can change forms, but is never created nor destroyed Loss in one form = gain in an another form A falling object speeds up as it falls to the ground; PE decreases as KE increases. The KE it has at impact = the PE it had before it fell. Forces and Motion Forces can create changes in motion (acceleration) Deceleration is negative acceleration Motion can be described simply Motion is a change in an object’s position Average velocity (speed) is the change of position of an object over time Velocity Graphs V = distance time Velocity(v) is Velocity the slope (rise over run) of a 60 Distance (m) position (d) 40 Series1 vs. time (t) 20 Series2 graph 0 1 3 5 7 9 11 13 15 Time (sec) 40 The diagram represents the total travel of a teacher on a Saturday. Which part of the trip is made at the greatest average speed? FQ GR How do we work this one? HS Calculate v = d/t for each segment. J T Acceleration is a change in an objects velocity (speed or direction) When an object’s speed changes over time it is accelerating (or decelerating) A = vfinal – vinitial time Units for acceleration m/s/s or m/s2 Acceleration Graphs Acceleration (a) Acceleration is the slope of a Velocity ((m/s)(m) velocity (v) vs. 60 time (t) graph 40 Plotted on a 20 distance vs. time 0 graph, 1 3 5 7 9 11 13 15 acceleration is Time (sec) an exponential curve Forces A pull or push (or lift) that can cause an object to start moving, stop moving or change direction. Definition of a Force A Force is a push or a pull Balanced Force A force that produces no change in an object’s motion because it is balanced by an equal, opposite force. Unbalanced Forces Are forces that results in an object’s motion being + changed. Friction A force that acts in a direction opposite to the motion of two surfaces in contact with each other. Friction Friction causes an object to slow down and stop. Since the amount of energy stays constant, the energy becomes heat. Newton’s 1st Law of Motion Object in motion stays in motion Newton’s 1st Law of Motion And Objects at rest stay at rest Newton’s 1st Law of Motion they are acted upon by Until unbalanced forces. Inertia Tendency for an object to stay at rest or moving in a straight line at a constant speed. The mass (m measured in kg) of an object determines its inertia Unit of Force – Use the chart Unit of mass is kg Unit of Acceleration is m/s/s 1 kg x 1 m/s/s = 1 kg x m/s/s = 1 Newton (N) Resulting in Force unit called a Newton (N) Newton’s 2nd Law of Motion Force = Mass X Acceleration F=ma Weight (pull of gravity) is a commonly measured force, calculated by F=mg, g is the acceleration due to gravity 9.8 m/s2 Same floor = same friction Same room = same air Same car = same mass Newton’s 2nd Law of Motion The greater the mass of an object, the greater the force required to change its motion. Newton’s 2nd Law of Motion Thegreater the acceleration of an object, the greater the force required to change its motion. Your Turn! 8 How much force is needed to accelerate a 1,300 kg car at a rate of 1.5 m/s2? F 867 N F = m a or G 1,950 N = 1300Kg x 1.5m/s2 H 8,493 N F = 1950 N J 16,562 N Newton’s 3rd Law of Motion Forevery action force there is an equal and opposite reaction force. Newton’s 3rd Law of Motion All forces come in action- reaction pairs Ex: feet push backward on floor, the floor pushes forward on feet Newton’s 3rd Law of Motion Rocket and Jets- walls of the fuel compartment push backward on igniting gases, gases push forward on the fuel compartment (and the rocket) so it rises. Newton’s 3rd Law of Motion Rowing a boat Name the Action Reaction pair of forces? Action: Oar pushes water back. Reaction: water pushes boat forward. Newton’s 3rd Law of Motion Bird flying Name the Equal but Opposite Action--Reaction Forces? Action: Wing pushes air down. Reaction: Air pushes up on wing, lifting bird. Machines do Work Work: the product of force times distance W=Fxd The work done by forces on an object = changes in energy for that object. work and energy are measured in Joules 1 Joule=1 Newton • meter Why use a machine? In an ideal (perfect) machine the work put into the machine (Win) = the work put out by that machine (Wout) Machines make work easier The ideal mechanical advantage of a machine (IMA) of a machine is the number of times the output force is larger than the input force IMA=Fout/Fin A machine can only make this happen by moving the input force through a farther distance than the output force Fin • din=Fout • dout Real Machines use Energy No real machine is 100 % efficient. i.e. none put out more work than is put in Efficiency of a machine is work output/work input X 100 % Eff = Wout X 100% W in Machines use power Power: the rate at which energy is used (work is done) P=Work/time Power is measured in H.P. or watts 1 watt = 1 Joule 1 sec 6 Types of simple machines Some Simple Machines: Inclined planes Screws Pulleys Wheel and axle Levers Wedge Universal Law of Gravitation All objects in the universe attract each other by the force of gravity Universal Law of Gravitation 1) the mass of the object doing the pulling, and Gravity varies depending on two factors: 2) the distance from the center of that object On Earth gravity = 9.8 m/s/s Forevery second that an object falls its speed increases by 9.8 m/s Universal Law of Gravitation The acceleration due to gravity may be affected by the air resistance of the falling object. Weight= Mass (m) X acceleration due to gravity (g) Weight Unit of mass = kg Unit of acceleration = m/s/s Unit of weight = Newton 1 Newton= about ¼ pound_ 52 Objects of the same mass but of different sizes and shapes were dropped from a given height. Their rates of free fall were measured and recorded. Which of the following is most likely the question this experiment was designed to answer? F How does height affect the force of gravity? G How does gravity affect objects of different densities? H How do mass and weight affect falling objects? J How do size and shape affect an object’s rate of free fall? The End… of TAKs Physics as we know it. Now its your turn. Take the quiz on the overhead. Be sure to write down your answers. You must turn them in to get your coupon.