Physics 1000 - Newton's Laws of Motion PDF

Physics 1000 – 001/002 Newton’s Laws of Motion  Inertia The property of objects to resist changes in motion  Newton’s 1st Law of Motion Every object continues in a state of rest or a state of uniform motion in a straight line unless...

Physics 1000 – 001/002 Newton’s Laws of Motion  Inertia The property of objects to resist changes in motion  Newton’s 1st Law of Motion Every object continues in a state of rest or a state of uniform motion in a straight line unless acted on by net external force  Force Force In the simplest sense, a force is a push or a pull Net Force The vector sum of forces that act on an object  Equilibrium Mechanical Equilibrium The state of an object when the net force acting on it is zero When an object is at rest, it is in static equilibrium When an object is moving at a constant velocity, the net force acting on it is still zero (there is no change in velocity) The object is still in mechanical equilibrium, but we call this dynamic equilibrium Equilibrium Rule The vector sum of the forces equals zero  Checkpoint Question A jet cruises at a constant velocity of 1000 km/h when the thrusting force of its engines is a constant 100,000 N. What is the acceleration of the jet? What is the force of air resistance on the jet?  Friction The resistive force that opposes the motion or attempted motion of an object either past another object with which it is in contact or through a fluid Static Friction Friction between objects that are not moving relative to each other Kinetic friction (aka sliding friction) Friction between objects moving relative to each other Static friction is typically higher than kinetic (sliding) friction This means it takes more force to get an object moving that to simply keep it moving  Checkpoint Questions Marie exerts a 100 N horizontal force on a table on the floor, and it doesn’t slide. This indicated that 100 N isn’t great enough to make the table slide. How does the friction force between the table and the floor compare with Marie’s push? She then pushes with an extra 10 N, and the table still doesn’t slide. How much friction acts on the table? She pushes even harder, and the table moves. Once in motion she finds that a push of 115 N is enough to keep the table sliding at a constant velocity. How much friction then acts on the table? What net force acts on the sliding table when pushed with 125 N which the friction with the floor is 115 N? In the above cases, when does static friction occur, when does sliding friction occur, and when does no friction occur?  Vector Terminology Vector Quantity A quantity that has both magnitude and direction Vector The arrow used to represent a vector quantity Scalar Quantity A quantity that has magnitude but not direction Resultant The net result of a combination of two or more vectors  Newton’s 2nd Law of Motion The acceleration of an object is directly proportional to the net force acting on the object, is in the direction of the net force, and is inversely proportional to the mass of the object  Newton’s 2nd Law of Motion Written as an equation, Newton’s second law looks like: Acceleration = Net Force / Mass As an equation, If we multiply both sides by mass, we get the more commonly seen:  Mass and Weight These are commonly used interchangeably, but are distinct in physics Mass is the quantity of matter in an object Mass is also the measure of an object’s inertia Weight is a force, usually due to gravity For constant gravity, weight and mass are proportional This is why they are commonly used interchangeably in everyday life As an example, gravity on the Moon is about 1/6 as strong as on Earth, so objects only weigh 1/6 as much, while still having the same mass SI Units kilogram (kg) for mass Newton (N) for weight, as it is a force  Checkpoint Questions Does a 2 kg iron block possess twice as much inertia as a 1 kg iron block? Twice as much mass? Twice as much volume? Twice as much weight? Does a 2 kg iron block possess twice as much inertia as a 1 kg bunch of bananas? Twice as much mass? Twice as much volume? Twice as much weight? How does the mass of a bar of gold vary with location? If you were a passenger on the International Space Station and were confronted with two cans, one fills with beans and the other empty, how would you determine which is full and which is empty? In which case would it be easier to lift a cement truck: on Earth’s surface or on the Moon’s surface?  Units of Force The SI unit of force is the Newton (N), but how is a Newton related to the other units? We know that: Using dimensional analysis: So  Checkpoint Questions In Chapter 2, acceleration was defined as the change in velocity over time. Are we saying in this chapter that acceleration is instead the ratio of force to mass, F/m? Which is it? What three controls in an automobile allow the driver to produce acceleration? How does the automobile’s mass affect the acceleration? In a vacuum, a coin and a feather fall at the same rate, side by side. Would it be correct to say that equal forces of gravity act on both the coin and the feather when in a vacuum?  Force Types Contact Force Any force that occurs as a result of two objects making contact with each other Normal Force A component of a contact force that is perpendicular to the surface in contact In resting objects, is also knows as support force Support Force An upward force opposite to the force of gravity This force comes from the supporting object wanting to keep its shape Friction as a Component of a Contact Force  Non-Free Fall When objects fall on Earth in real life, they aren’t falling in a vacuum, so they don’t act as in free fall This is because air resistance (drag) resists the gravitational force, a form of friction Written as an equation, Unless R=0, acceleration will be less than gravitational acceleration Air resistance depends on two things, both of which cause the object to have to move through more air molecules, which is what actually creates the drag Frontal surface area of the falling object The speed of the falling object  Terminal Velocity Air resistance will continue to increase until it completely counteracts gravity, and equilibrium is reached When this occurs, an object has reached its terminal speed When combined with direction, we say that the object has reached terminal velocity Increasing frontal surface area is the primary way of decreasing terminal velocity This is what makes parachutes effective at creating a safe terminal velocity  Problem #1 What is the acceleration produced by a net force of 2000 N exerted on a 1000 kg car?  Problem #1 What is the acceleration produced by a net force of 2000 N exerted on a 1000 kg car?  Problem #1 What is the acceleration produced by a net force of 2000 N exerted on a 1000 kg car?  Problem #1 What is the acceleration produced by a net force of 2000 N exerted on a 1000 kg car?  Problem #2 Find the amount of force, or thrust, on a 20,000 kg jet plane to achieve an acceleration of  Problem #2 Find the amount of force, or thrust, on a 20,000 kg jet plane to achieve an acceleration of  Problem #2 Find the amount of force, or thrust, on a 20,000 kg jet plane to achieve an acceleration of  Problem #2 Find the amount of force, or thrust, on a 20,000 kg jet plane to achieve an acceleration of  Problem #2 Find the amount of force, or thrust, on a 20,000 kg jet plane to achieve an acceleration of  Problem #3 Consider a problem expressed in symbols only: A force F acts in a forward direction on a carton of doughnuts of mass m, which a friction force f opposes the motion. Find the acceleration of the carton. Then, express this in numbers is the mass of the carton is 4.0 kg and the applied force is 12.0 N against a friction force of 6.0 N.  Problem #3 Consider a problem expressed in symbols only: A force F acts in a forward direction on a carton of doughnuts of mass m, which a friction force f opposes the motion. Find the acceleration of the carton. Then, express this in numbers is the mass of the carton is 4.0 kg and the applied force is 12.0 N against a friction force of 6.0 N.  Problem #3 Consider a problem expressed in symbols only: A force F acts in a forward direction on a carton of doughnuts of mass m, which a friction force f opposes the motion. Find the acceleration of the carton. Then, express this in numbers is the mass of the carton is 4.0 kg and the applied force is 12.0 N against a friction force of 6.0 N.  Problem #3 Consider a problem expressed in symbols only: A force F acts in a forward direction on a carton of doughnuts of mass m, which a friction force f opposes the motion. Find the acceleration of the carton. Then, express this in numbers is the mass of the carton is 4.0 kg and the applied force is 12.0 N against a friction force of 6.0 N.  Problem #3 Consider a problem expressed in symbols only: A force F acts in a forward direction on a carton of doughnuts of mass m, which a friction force f opposes the motion. Find the acceleration of the carton. Then, express this in numbers is the mass of the carton is 4.0 kg and the applied force is 12.0 N against a friction force of 6.0 N.  Problem #3 Consider a problem expressed in symbols only: A force F acts in a forward direction on a carton of doughnuts of mass m, which a friction force f opposes the motion. Find the acceleration of the carton. Then, express this in numbers is the mass of the carton is 4.0 kg and the applied force is 12.0 N against a friction force of 6.0 N.  Problem #3 Consider a problem expressed in symbols only: A force F acts in a forward direction on a carton of doughnuts of mass m, which a friction force f opposes the motion. Find the acceleration of the carton. Then, express this in numbers is the mass of the carton is 4.0 kg and the applied force is 12.0 N against a friction force of 6.0 N.  Problem #3 Consider a problem expressed in symbols only: A force F acts in a forward direction on a carton of doughnuts of mass m, which a friction force f opposes the motion. Find the acceleration of the carton. Then, express this in numbers is the mass of the carton is 4.0 kg and the applied force is 12.0 N against a friction force of 6.0 N.  Force as an Interaction Previously, we defined a force simply as a push or a pull More accurately, a force is part of an interaction between one thing and another These interactions actually happen in pairs, with both objects exerting a force on the other Pair of forces Even when it may seem like only one object is exerting a force on another object, it is actually a pair of forces These pairs of forces are treated as one interaction  Examples of Pairs of Forces Pushing on a wall The wall pushes back, keeping you from falling over A boxer hitting a punching bag The bag exerts a force on the glove, which is what stops its motion A hammer hitting a nail The nail stops the motion of the hammer Other examples previously mentioned Normal force Static Friction force  Checkpoint Questions Is it possible to push on something without that same something pushing back on you?  Checkpoint Questions Does a high-speed baseball possess force?  Newton’s 3rd Law of Motion Whenever one object exerts a force on a second object, the second object simultaneously exerts an equal and opposite force on the first.  Newton’s 3rd Law of Motion If we consider one force the action force and the other the reaction force, we can rephrase Newton’s third law to: To every action there is always an opposed equal reaction.  Identifying Action and Reaction When trying to determine the force pair of an interaction, identify the action, then the reaction. Action: Object A exerts a force on object B. Reaction: Object B exerts a force on object A.  Internal vs External Forces If force pairs are equal and opposite, why don’t they just cancel out? It depends on your reference frame For a system to accelerate, the external net force needs to be nonzero Interactions that happen completely within a system are called internal  Checkpoint Questions On a cold, rainy day, you find yourself in a car with a dead battery. You must push the car to move it and get it started. Why can’t you move the car by remaining comfortably inside and pushing against the dashboard?  Checkpoint Questions A car accelerates along a road. Identify the force that moves the car.  Action and Reaction on Different Masses While the forces in an interaction are the same, differences in mass can create different magnitudes of acceleration If one object is more massive that the second object, it will undergo less acceleration than the less massive object for the same force  Homework Read Chapter 3 Conceptual Questions: 3, 10, 14, 20, 26, 32, 38, 44, 49, 56 Exercises: 63, 68, 70, 72, 76