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Transcript
Chapter 4 : Forces and the laws of motion Definition Force –describes the interaction between and object and its environment and it’s a vector quantity Forces can act on an object in three ways They can cause objects to… Start moving-golf ball Stop moving -soccer Change directionvolleyball Units The SI unit for force is the Newton This was named after Sir Isaac Newton because of the work he contributed to the understanding of force. The Newton is the amount of force that when it is acting on an object of 1 kg it produces and acceleration of 1 meter/second squared This is why one Newton equals 1 kg times 1 m/s ² The way forces act Forces are able to act through contact or at a distance When forces touch each other the motion the object previously had is changed When a soccer ball is kicked the object changes direction because of the physical contact between the forces Field Forces The second way that forces act is called force fields This could be a gravitational force- happens as objects are falling The object will accelerate due to the earth’s gravity even thought the earth is on in contact with the object. Another example is the attraction or repulsion between electric charges Force Diagrams The acceleration of an object depends on the magnitude of the force acting on it and the direction Force is a vector quantity Force diagrams show force vectors and the direction they act in The force vector will point in the direction of the force and its length is proportional to the magnitude of the force Free body diagrams A free-body diagram is a way to show just the forces acting on the object The free body diagram is used to find only the forces that are affecting the motion of the object Inertia- the objects nature to maintain its state of rest Mass is the measure of inertia The greater the mass of a body the less the object will accelerate under an applied fore Example: What this means for a racecar. The greater the mass of the car, the more energy required to get the stationary car moving. If two cars use the same amount of propulsive energy, the car with the lower mass will accelerate faster. Section 2 Net Force The sum of all forces acting on an object is the net force: add up normal, friction, gravity and applied forces F-friction: the resistance force, opposite force applied F-applied: the direction the force is acting F-normal upward force exerted F-gravity the force gravity has on objects Newton’s first Law Newton’s first Law: an object will continue the motion it is in unless acted on by a force An object will stay in rest unless acted on by a force the same is true for an object in motion it will continue in the same speed and direction unless acted on by a force-two cars crashing Other definition: when the net external force on an object is zero the net acceleration of the object is zero External forces-single forces acting on the object and they result from the interacting between an object and its environment Equilibrium Objects that are either at rest or moving at a constant velocity are said to be at equilibrium This happens when the vector sum of the forces acting on the object equals zero When you find the net force acting on an object and it equals zero then the object is at equilibrium Section 3 Newton’s 2nd Law: the acceleration (change in velocity) of an object is directly proportional to the net force acting on the object It is also inversely proportional to the objects mass According to this law if equal forces are applied to objects of different masses the one with the greater mass will experience a larger acceleration Net Force A net force acting on an object causes the object to accelerate and the speed to increase ΣF =ma Net Force = mass x acceleration Σ is the Greek symbol sigma, which means the sum Practice Working out how much force is used to push a F1 car when broken down or run out of fuel If the car has a mass of 700kg And a driver pushes the car with an acceleration of 0.05m/s/s What’s the force needed? Answers Race Car F = ma Force = 700kg x 0.05m/s/s Force = 35kN (kilo Newtons) Bus F=ma F= 2000kg x .05 m/s² F= 100 Newtons Newton’s third Law Forces always exist in pairs For every action there is an equal and opposite reaction When two objects interact, the forces that act on each other are called an actionreaction pair Action-Reaction The action force is equal in magnitude and opposite in direction to the reaction force These two forces occur at the exact same time Because they coexist either force can be called the action or reaction depending on which way you are looking at the situation More action and reaction Action and reaction forces act on different objects Because of this an action-reaction pair does not mean the net force is zero This can best be described by the act of driving a nail into wood with a hammer The hammer exerts a for on the nail and the nail exerts a force back on the hammer However the nail is only affected but the forces acting on it It is not affected by the force on the hammer Therefore they don’t balance out and acceleration is able to occur on the nail Field Forces Newton's third law applies to field forces too Newton’s third law says that as an object falls to the Earth and accelerates the Earth also accelerates towards this object However if you think back to Newton’s second law you will remember the greater the mass the less acceleration occurs Therefore in the case of a falling object such as an apple or a book, the object will have a large acceleration due to gravity The Earth will have an acceleration towards the object, but it will be negligibly small because of the Earth’s large mass Section 4: Everyday forces Fg is a vector quantity of the force exerted toward the Earth Weight is the magnitude of this force Fg is a scalar quality Weight is not always the same for the object It depends on location of the object Weight •You can calculate the weight of an object using Fg=mag •ag= the magnitude of acceleration due to gravity or free-fall acceleration •In this book ag= 9.81m/s² •On the moon ag= 1.6m/s² •Because of this the object weighs less on the moon The Normal Force The normal force is used to explain how if gravity is action on all objects, they don’t continue to fall into the earth When objects are at Equilibrium they don’t move anymore, much like a desk or dresser in your room or the tires on your car Because we know gravity acts on everything thing there must be a force acting opposite to it so all these objects remain at equilibrium More normal force The definition of the word normal is “perpendicular” The normal force is always perpendicular to the contact surface, but not necessarily opposite the force due to gravity In the absence of other forces the normal force is opposite the Fg How to find Normal Force The magnitude of the normal force can be calculated by mgcosθ the angle θ is the angle between the normal force and a vertical line And is also the angle between the contact force surface and a horizontal line The force of friction Friction opposes the applied force The static friction is the resistive force that keeps an object from moving This happens when you push on an object but you don’t apply enough force to move the object and it remains at equilibrium Fs= -Fapplied Kinetic friction Kinetic friction is less than static friction The magnitude of the net force acting on an object is equal to the difference between the applied force and the forces of kinetic friction (Fapplied= –Fk) In free-body diagrams the force of friction is always parallel to the surface of contact. The force of kinetic friction is always opposite the direction of motion. The Force of Friction The force of friction is proportional to the normal force Heavier objects also experience greater normal force and more friction than lighter objects The relationship between normal force and the force of friction is one factor that affects friction More Friction The force of friction depends on the composition and qualities of the surfaces in contact One example of this would be skiing or snowboarding When the snow is more compact there would be less friction present and more acceleration will take place and you will be able to go much faster Coefficient of friction μk = Fk / Fn μk Stands for the coefficient of kinetic friction The coefficient of friction is a ratio of the force due to friction, and the normal force between two surfaces Coefficient of static and the force of friction μ = Fs,max / FN this equation represents the ratio s of maximum value of the forces of static friction Ff = μFN If you know both the value of μ and the normal force you can find the magnitude of the force of friction Example problems A desk has a mass of 52.5 kilograms. If the coefficient of static friction between the desk and the floor is 0.84, what force must be used to move the desk from rest? A dresser has a mass of 61.25 kilograms Once is in motion, what force must be used to keep it moving at a constant velocity if the coefficient of kinetic friction is 0.6533333333333333? Answers 432.621 N You must do 52.5 times 9.81m/s² to get the FN which equals 515.025n then you multiply that by the .84 and you get the Fs,max Second answer =392.5635 N You need to first change Kg’s into Newtons by multiplying by 9.81m/s² you will get around 600.9N then you multiply this by the coefficient of kinetic friction which gives you the answer Air resistance •Whenever an object moves through a fluid medium (like air or water) the fluid provides a resistance to the objects motion this is called FR When an object is in free fall its velocity increases As the velocity increase so does the the resistance on the object when the upward force of air resistance balances out the downward gravitational force the net force on the object will be zero When it reaches zero the object will continue downward at a constant maximum velocity This is called terminal speed The 4 fundamental forces Electromagnetic forces-interaction between protons and electrons Gravitational force-due to gravity Strong Nuclear forces Weak Nuclear forces All four of these forces are field forces The nuclear forces have small ranges so you can not directly see their effects Any force you can see at a macroscopic level is either gravitational or electromagnetic Out of these four forces Gravity is the weakest and strong Nuclear is the strongest