# chapter 3 – force, mass & acceleration some review… a force is a push or pull. the si unit for...

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• Slide 1
• Slide 2
• chapter 3 force, mass & acceleration
• Slide 3
• Some review A force is a push or pull. The SI unit for force is the Newton. SIewton What IS a Force ? Newtons 1 st law of motion (Law of Inertia) states that the motion of an object changes only if an unbalanced force acts on the object.
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• Mass, Force, & Acceleration Newton's Second Law of Motion: F = ma This means that the net force (F) acting on an object equals the product of the mass (m) times the acceleration (a) of the object. The direction of the force is the same as that of the acceleration. kgm ss kg m. ss
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• In other words, The more force that hits an object, the faster it moves. force acceleration directly related
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• Calculating Force mass: m=0.70 kg acceleration: a=3500m/s/s The maximum acceleration of a fist in a karate blow has been measured at 3500 m/s/s. The mass of the fist is 0.70 kg. If the fist hits a wooden block, what force does the wood place on the fist? Unknown: force (F) What info is given in the question? = ( 0.70 kg )( 3500m/s/s ) = 2450 kgm/s/s = F = m a 2450 N
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• Whats different about throwing a baseball and throwing a bowling ball? You can throw them with the same force, but will get much different results! Force and Mass 3.1 Newtons Second Law
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• acceleration mass inversely related big mass = small acceleration small mass = big acceleration F = ma
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• Suppose you give a skateboard a push with your hand. An Important Force 3.1 Newtons Second Law According to Newtons first law of motion, it will keep moving forever. Does the skateboard keep moving with constant speed after it leaves your hand? No!! It acceleratesnegatively!!! According to Newtons second law (F=ma), if the skateboard is accelerating, there must be a net force acting on it.
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• The force that slows and brings things to a stop is friction. Friction 3.1 Newtons Second Law The amount of friction between two surfaces depends on two factors the kinds of surfaces and the force pressing the surfaces together. Were going to learn about 3 kinds of friction static, sliding, and rolling.
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• Suppose you have filled a cardboard box with books and want to move it. Static Friction 3.1 Newtons Second Law Its too heavy to lift, so you start pushing on it, but it doesnt budge.
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• Static Friction 3.1 Newtons Second Law Static friction is the frictional force that prevents two surfaces from sliding past each other.
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• You ask a friend to help you move the box. Sliding Friction 3.1 Newtons Second Law Pushing together, the box moves. You have exerted enough force to overcome the static friction.
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• If you stop pushing, the box quickly comes to a stop. Sliding Friction 3.1 Newtons Second Law Sliding friction is the force that opposes the motion of two surfaces sliding past each other.
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• Rolling Friction 3.1 Newtons Second Law Rolling friction is the frictional force between a rolling object and the surface it rolls on. As a wheel rolls over a surface, the wheel digs into the surface, causing both the wheel and the surface to be deformed.
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• Air resistance acts in the opposite direction to the motion of an object through air. Air Resistance 3.1 Newtons Second Law
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• The amount of air resistance on an object depends on the speed, size, and shape of the object. Air Resistance 3.1 Newtons Second Law If you drop two identical plastic bags from the exact same height, but crumple one and open one, the crumpled one will fall faster. It falls faster because it is smaller, therefore it has less air resistance. Mass has nothing to do with it!!
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• As an object falls, the downward force of gravity causes the object to accelerate. Terminal Velocity 3.1 Newtons Second Law However, as an object falls faster, the upward force of air resistance increases. This causes the net force to decrease as the object falls.
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• The terminal velocity is the highest speed a falling object will reach. Terminal Velocity 3.1 Newtons Second Law Force of air resistance Force of gravity
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• 3.1 Section Check Question 1 A.) acceleration B.) momentum C.) speed D.) velocity Newtons second law of motion states that _________ of an object is in the same direction as the net force on the object. A!A! Everything accelerates in the direction of the force that hits it.
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• 3.1 Section Check Question 2 A.) joule B.) muscle C.) newton D.) watt The unit of force is __________. C!C! 1 newton = 1 kg m/s 2
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• 3.1 Section Check Answer Friction results from the sticking together of two surfaces that are in contact. Question 3 What causes friction?
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• What is gravity? Gravity is an attractive force between any two objects that depends on the masses of the objects and the distance between them. 3.2 Gravity No matter how far apart two objects are, the gravitational force between them never completely goes to zero.
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• 3.2 Gravity Earths Gravitational Acceleration When all forces except gravity acting on a falling object can be ignored, the object is said to be in freefall. Close to Earths surface, the acceleration of a falling object in freefall is 9.81 m/s 2. This acceleration is given the symbol g and is sometimes called the acceleration of gravity.
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• A space shuttle in orbit is in free fall, but it is falling around Earth, rather than straight downward. 3.2 Gravity Floating in Space Objects in the shuttle seem to be floating because they are all falling with the same acceleration.
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• The gravitational force exerted on an object is called the objects weight. Weight Weight and mass are not the same. Weight is a force and mass is a measure of the amount of matter an object contains. Weight increases as mass increases. F = ma weight = mg
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• How much does a 100-kg object weigh? 3.2 Gravity Calculating Weight weight = mg weight = (100 kg)(9.81 m/s 2 ) weight = 981 kgm/s 2 = 981 N
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• Projectile motion If you throw an object, it doesnt travel in a straight line. If you throw a ball, it will curve down. Definition: A projectile is an object acting under only one force GRAVITY
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• The acceleration of an object moving in a circular path is toward the center of a circle. The force that causes acceleration is also towards the center of the circle. This is called centripetal force. Centripetal force this allows a car or a bike to round a sharp turn.
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• 3.2 Question 1 Gravity depends on the masses of the objects and the distance between them. Gravity is an attractive force between any two objects and depends on __________. Answer Hint: there are 2 things that the strength of gravity depends on!!!
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• Newtons Third Law Newtons third law of motion describes action-reaction pairs this way. When one object exerts a force on a second object, the second one exerts a force on the first that is equal in strength and opposite in direction. 3.3 The Third Law of Motion For every action, there is an equal and opposite reaction.
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• Action and Reaction When you jump on a trampoline, for example, you exert a downward force on the trampoline. 3.3 The Third Law of Motion Simultaneously, the trampoline exerts an equal force upward, sending you high into the air.
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• The swimmer acts on the water. The reaction of the water pushes the swimmer forward. 3.3 The Third Law of Motion Name the action and reaction in this picture:
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• Action/Reaction Pairs in Rockets In a rocket engine, burning fuel produces hot gases. The rocket engine creates momentum by exerting a force on these gases, causing them to escape out the back of the rocket. By Newtons third law, the gases exert a force on the rocket and push it forward. 3.3 The Third Law of Motion
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• task Come up with at least 10 examples of action reaction pairs.
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• Momentum A moving object has a property called momentum that is related to how much force is needed to change its motion. The momentum of an object is the product of its mass and velocity. 3.3 The Third Law of Motion momentum = mass x velocity
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• Momentum Momentum is given the symbol p and can be calculated with the following equation: The unit for momentum is kg m/s. Momentum has a direction because velocity has a direction. 3.3 The Third Law of Motion
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• Conservation of Momentum All matter in motion has momentum. 3.3 The Third Law of Motion Momentum, however, can be transferred from one object to another. In a collision, the total momentum of the bodies does not change. (This is just another way of stating the third law of motion)
• Slide 40
• Momentum & Colliding Objects When two objects collid