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Transcript
Forces and Motion Chapter 12 Chap 12 Pretest • What is relative motion? • Relative motion is movement in relation to a frame of reference. Chap 12 Pretest • What is the difference between distance and displacement? • Distance is the length of a path between two points. • Displacement is the direction from the starting point and the length of the straight line from the starting point to the ending point. Chap 12 Pretest • How is average speed calculated? • Total distance is divided by total time. • v=d/t Chap 12 Pretest • On a distance-time graph, what does the slope represent? • The slope represents the speed. Chap 12 Pretest • What is velocity? • Velocity is speed with direction. Chap 12 Pretest • A backpack falls out of an open window. The backpack starts from rest and hits the ground 1.0 second later with a velocity of 9.8 m/s. What is the average acceleration of the backpack? • A. 9.8 m/s • B. 9.8 m • C. 9.8 m/s2 • D. All of the above Chap 12 Pretest • How is acceleration related to velocity? • Acceleration is the change in velocity, that is, any change in speed, direction, or both. Chap 12 Pretest • How are mass and weight different? • Mass is the measure of inertia; weight is the measure of the force of gravity acting on an object. Objects in Free Fall • What factors affect a falling object? Perform the following simple activity to begin learning about the forces that act on falling objects. • Stand beside your desk. Hold a sheet of notebook paper level at eye level. Release the sheet of paper and watch it fall. Describe the motion of the paper. • Hold a sheet of notebook paper that has been crumpled into a tight ball at eye level. Release the crumpled paper and watch it fall. Describe the motion of the paper. • How do the motions of the flat sheet of paper and crumpled ball of paper compare? What forces do you think are acting on each sheet of paper? 12.1- Forces • A force is a push or a pull that acts on an object. • A force can cause a resting object to move, or it can accelerate a moving object by changing the object’s speed or direction. 12.1 - Forces • Force is measured in newtons. – Newtons are abbreviated as N. • One newton is the force that causes a 1kg mass to accelerate at a rate of 1 meter per second each second. – 1 N = 1 kg•m/s2 • The newton is named after Sir Isaac Newton. 12.1 – Representing & Combining Forces • Forces can be represented by arrows showing the direction and strength of the force. • The net force is the overall force acting on an object after all of the forces are combined. – Force arrows are combined. Forces in the same direction add together, forces in opposite directions subtract from one another. 12.1 – Balanced Forces • Balanced forces are forces that combine to produce a net force of zero. • When the forces on an object are balanced, the net force is zero and there is no change in the object’s motion. • Examples: tug of war, arm wrestling 12.1 – Unbalanced Forces • An unbalanced force is a force that results when the net force acting on an object is not equal to zero. • When an unbalanced force acts on an object, the object accelerates. • Examples: pushing a resting book on table, winners/losers of arm wrestling or tug of war matches 12.1 - Friction • All moving objects are subject to friction, a force that opposes the motion of objects that touch as they move past each other. • Friction acts at the surface where objects are in contact. • There are four main types of friction: static friction, sliding friction, rolling friction, and fluid friction. 12.1 – Friction Types • Static Friction – the friction force that acts on objects that are not moving. – Always acts in the direction opposite of the applied force. • Sliding Friction – a force that opposes the direction of motion of an object as it slides over a surface. – Sliding friction is less than static friction, so less force is needed to keep on object moving that to start it moving. 12.1 – Friction Types • Rolling Friction – the friction force that acts on rolling objects. – Rolling friction is about 100 to 1000 times less than the force of static or sliding friction. – Examples – moving dollies, ball bearings • Fluid Friction – the force that opposes the motion of an object through a fluid. – Fluid friction acting on an object moving through air is known as air resistance. – Examples – stirring cake batter 12.1 - Gravity • Gravity is a force that acts between any two masses. Gravity is an attractive force, pulling objects together. • Earth’s gravity acts downward toward the center of Earth. 12.1 – Falling Objects • Both gravity and air resistance affect the motion of a falling object. • Gravity causes objects to accelerate downward, whereas air resistance acts in the direction opposite to the motion and reduces acceleration. • Terminal velocity is the constant velocity of a falling object when the force of air resistance equals the force of gravity. 12.2 – Newton’s First Law of Motion • According to Newton’s first law of motion, the state of motion of an object does not change as long as the net force acting on the object is zero. – Unless an unbalanced force acts, an object at rest remains at rest, and an object in motion remains in motion with the same speed and direction 12.2 – Newton’s First Law of Motion • Newton’s first law of motion is sometimes called the law of inertia. • Inertia is a tendency of an object to resist change in its motion. 12.2 – Newton’s Second Law of Motion • According to Newton’s second law of motion, the acceleration of an object is equal to the net force acting on it divided by the object’s mass. • Mass is the measure of inertia of an object and depends on the amount of matter the object contains. • Acceleration = net force / mass, or a = F/m 12.2 – Practice Problems • A boy pushes forward a cart of groceries with a total mass of 40.0 kg. What is the acceleration of the cart if the net force on the cart is 60.0 N? • a = F/m = 60 N / 40kg=1.5 m/s2 • What is the upward acceleration of a helicopter with a mass of 5,000 kg if a force of 10,000 N acts on it in an upward direction? • a= F/m = 10,000 N / 5,000 kg = 2 m/s2 12.2 – Practice Problems • An automobile with a mass of 1,200 kg accelerates at a rate of 3.0 m/s2 in the forward direction. What is the net force acting on the automobile? • a = F/m, or F = ma = 1,200kg*3.0 m/s2 =3,600 N • A 25 N force accelerates a boy in a wheelchair at 0.5 m/s2. What is the mass of the boy and the wheelchair? • a= F/m, m = F/a = 25 N / 0.5 m/s2 = 50 kg 12.2 – Weight and Mass • Weight and mass are not the same thing. – Weight is the force of gravity acting on an object. – Mass is the measure of inertia of an object and depends on the amount of matter the object contains. • Weight = Mass x Acceleration due to gravity – W = mg 12.2 – Weight and Mass • Do you weigh more on Earth or the moon? • Hints: – Your mass is the same in both locations. – The acceleration due to gravity on Earth is 9.8 m/s2, and the moon’s acceleration due to gravity is 1/6th that on Earth. • Answer: – You weigh only 1/6th as much on the moon as on Earth! 12.3 – Newton’s Third Law of Motion • According to Newton’s third law of motion, whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object. • These two forces are called action and reaction forces. 12.3 – Action and Reaction Forces • Action-Reaction forces are equal in size and opposite in direction. • Action-Reaction forces may or may not result in motion. • Action-Reaction forces do not act on the same object so they do not cancel or have a net force of zero. – Only when equal and opposite forces act on the same object do they result in a net force of zero. • Examples: – Bumper cars (motion occurs) – Pushing against a wall (no motion occurs) 12.3 - Momentum • Momentum is the product of an object’s mass and its velocity. – Momentum = Mass x Velocity • Momentum is measured in units of kilogram-meters per second. • An object has a large momentum if the product of its mass and velocity is large. – An object with large momentum is hard to stop. • The momentum for any object at rest is zero. 12.3 - Momentum • Which has more momentum, a 0.046kilogram golf ball with a speed of 60.0 meters per second, or a 7.0-kilogram bowling ball with a speed of 6.0 meters per second? • Momentum golf ball = 0.046 kg x 60.0 m/s =2.8kg*m/s • Momentum bowling ball = 7.0 kg x 6.0 m/s = 42 kg*m/s 12.3 – Conservation of Momentum • According to the law of conservation of momentum, if no net force acts on a system, then the total momentum of the system does not change. • In a closed system, the loss of momentum of one object equals the gain in momentum of another object – momentum is conserved. Momentum vs Time graph • At what time did the ball have zero momentum? – At t = 0 s; the ball has zero momentum before it is released. • At what time did the ball have the greatest momentum? – At t = 2.5s; about 6.5kg*m/s • What is the ball’s speed after 1.25 seconds? – – – – – – Momentum = mass x velocity Momentum = 3.25 kg*m/s Mass = 0.25 kg Velocity = momentum/mass Velocity = 3.25 kg*m/s / 0.25 kg Speed = 13 m/s