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**chapter 3 – force, mass & acceleration**

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**What IS a Force? A force is a push or pull. Some review…**

The SI unit for force is the Newton. Newton’s 1st 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. m . s·s kg·m s·s kg

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**In other words, The more force that hits an object, the faster it moves.**

acceleration directly related

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**2450 N Calculating Force F = m a mass: m=0.70 kg acceleration:**

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? What info is given in the question? Unknown: force (F) F = m a mass: m=0.70 kg acceleration: a=3500m/s/s = (0.70 kg)(3500m/s/s) = 2450 kg·m/s/s = 2450 N

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Newton’s Second Law 3.1 Force and Mass What’s different about throwing a baseball and throwing a bowling ball? You can throw them with the same force, but will get much different results!

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**F = ma inversely related big mass = small acceleration mass**

small mass = big acceleration mass F = ma acceleration

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**An Important Force No!! It accelerates—negatively!!! 3.1**

Newton’s Second Law 3.1 An Important Force Suppose you give a skateboard a push with your hand. According to Newton’s first law of motion, it will keep moving forever. Does the skateboard keep moving with constant speed after it leaves your hand? No!! It accelerates—negatively!!! According to Newton’s second law (F=ma), if the skateboard is accelerating, there must be a net force acting on it.

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Newton’s Second Law 3.1 Friction The force that slows and brings things to a stop is friction. The amount of friction between two surfaces depends on two factorsthe kinds of surfaces and the force pressing the surfaces together. We’re going to learn about 3 kinds of friction—static, sliding, and rolling.

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Newton’s Second Law 3.1 Static Friction Suppose you have filled a cardboard box with books and want to move it. It’s too heavy to lift, so you start pushing on it, but it doesn’t budge.

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Newton’s Second Law 3.1 Static Friction Static friction is the frictional force that prevents two surfaces from sliding past each other.

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**Sliding Friction 3.1 You ask a friend to help you move the box.**

Newton’s Second Law 3.1 Sliding Friction You ask a friend to help you move the box. Pushing together, the box moves. You have exerted enough force to overcome the static friction.

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Newton’s Second Law 3.1 Sliding Friction If you stop pushing, the box quickly comes to a stop. Sliding friction is the force that opposes the motion of two surfaces sliding past each other.

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Newton’s Second Law 3.1 Rolling Friction As a wheel rolls over a surface, the wheel digs into the surface, causing both the wheel and the surface to be deformed. Rolling friction is the frictional force between a rolling object and the surface it rolls on.

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Newton’s Second Law Air Resistance Air resistance acts in the opposite direction to the motion of an object through air. 3.1

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Newton’s Second Law 3.1 Air Resistance The amount of air resistance on an object depends on the speed, size, and shape of the object. 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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Newton’s Second Law 3.1 Terminal Velocity As an object falls, the downward force of gravity causes the object to accelerate. 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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Newton’s Second Law 3.1 Terminal Velocity Force of air resistance The terminal velocity is the highest speed a falling object will reach. Force of gravity

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Section Check 3.1 Question 1 Newton’s second law of motion states that _________ of an object is in the same direction as the net force on the object. A! A.) acceleration B.) momentum C.) speed D.) velocity Everything accelerates in the direction of the force that hits it.

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**C! 1 newton = 1 kg · m/s2 Question 2 3.1**

Section Check 3.1 Question 2 The unit of force is __________. C! A.) joule B.) muscle C.) newton D.) watt 1 newton = 1 kg · m/s2

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Section Check 3.1 Question 3 What causes friction? Answer Friction results from the sticking together of two surfaces that are in contact.

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Gravity 3.2 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. No matter how far apart two objects are, the gravitational force between them never completely goes to zero.

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**Earth’s Gravitational Acceleration**

Gravity 3.2 Earth’s 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 Earth’s surface, the acceleration of a falling object in freefall is 9.81 m/s2. This acceleration is given the symbol g and is sometimes called the acceleration of gravity.

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Gravity Floating in Space A space shuttle in orbit is in free fall, but it is falling around Earth, rather than straight downward. Objects in the shuttle seem to be floating because they are all falling with the same acceleration. 3.2

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Weight weight = mg F = ma The gravitational force exerted on an object is called the object’s 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.

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**= 981 N 981 kg•m/s2 Calculating Weight weight = mg**

Gravity 3.2 Calculating Weight How much does a 100-kg object weigh? weight = mg weight = (100 kg)(9.81 m/s2) weight = 981 kg•m/s2 = 981 N

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Projectile motion If you throw an object, it doesn’t 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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Motion in Circles! 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 Hint: there are 2 things that the strength of gravity depends on!!! Question 1 Gravity is an attractive force between any two objects and depends on __________. Answer Gravity depends on the masses of the objects and the distance between them.

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**For every action, there is an equal and opposite reaction.**

The Third Law of Motion 3.3 Newton’s Third Law Newton’s 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. For every action, there is an equal and opposite reaction.

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The Third Law of Motion 3.3 Action and Reaction When you jump on a trampoline, for example, you exert a downward force on the trampoline. Simultaneously, the trampoline exerts an equal force upward, sending you high into the air.

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**Name the action and reaction in this picture:**

The Third Law of Motion 3.3 Name the action and reaction in this picture: The swimmer “acts” on the water. The “reaction” of the water pushes the swimmer forward.

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**Action/Reaction Pairs in Rockets**

The Third Law of Motion 3.3 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 Newton’s third law, the gases exert a force on the rocket and push it forward.

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task Come up with at least 10 examples of action reaction pairs.

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**momentum = mass x velocity**

The Third Law of Motion 3.3 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. momentum = mass x velocity

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The Third Law of Motion 3.3 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.

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**Conservation of Momentum**

The Third Law of Motion 3.3 Conservation of Momentum All matter in motion has momentum. 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)

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**Momentum & Colliding Objects**

When two objects collide, momentum can be used to help understand what happens. When two pool balls collide (one at rest and one in motion) the one in motion loses momentum and the resting ball gains exactly the amount the other one lost.

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The Third Law of Motion 3.3 When Objects Collide If two pucks are speeding toward each other with the same speed, and if they collide, the total momentum is zero.

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**Question 1 3.3 According to Newton’s third law of motion,**

Section Check 3.3 Question 1 According to Newton’s third law of motion, For every action, there is an equal and opposite reaction. Sooooo, if you push it…

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If you push on it... It pushes back!

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Section Check 3.3 Question 2 The momentum of an object is the product of its __________ and __________. A.) mass, acceleration B.) mass, velocity C.) mass, weight D.) net force, velocity B! momentum = mass x velocity p = mv

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Section Check 3.3 Question 3 If a ping pong ball and a cue ball have the same momentum, which is travelling faster? A.) They’re going the same speed. B.) the ping pong ball C.) the cue ball D.) impossible to know B! momentum = mass x velocity p = mv

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**(the law of conservation of momentum)**

Section Check 3.3 Question 4 When two objects collide, what happens to their momentum? Answer Their total momentum doesn’t change, but momentum can be transferred from one object to another. (the law of conservation of momentum)

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Section Check 3.3

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