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Chapter 6 Forces and motion

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Think about it…. If Wile E. Coyote and a boulder fall off a cliff at the same time, which do you think will hit the ground first? Would it matter if the cliff were very high or particularly low? How could Mr. Coyote slow down his fall?

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Gravity and Motion Gravity and Falling Objects Gravity and Acceleration Demo 1: Which will hit the ground first, second, third? Basketball, baseball, ping pong ball? Were you correct? Demo 2: Prediction and Observation 1: Prediction and Observation 2:

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**Acceleration Due to Gravity**

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**Velocity of Falling Objects**

Equation: V = g x t Example Problems: EX 1: A penny at rest is dropped from the top of a tall stairwell What is the penny’s velocity after it has fallen for 2 s? EX 2: The same penny hits the ground in 4.5s. What is the penny’s velocity as it hits the ground?

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**Velocity of falling objects**

EX 3: A marble at rest is dropped from a tall building. The marble hits the ground with a velocity of 98 m/s. How long was the marble in the air? EX 4: An acorn at rest falls from an oak tree. The acorn hits the ground with a velocity of 14.6 m/s. How long did it take the acorn to land?

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**Air Resistance and Falling Objects**

Demo1: Which falls faster? Crumpled paper or flat sheet of paper? Why did this happen?

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**Air Resistance and Falling Objects**

Air Resistance - force that opposes the motion of objects through air

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**Acceleration Stops at Terminal Velocity**

As the speed of a falling object increases, air resistance increases. The upward force of air resistance continues to increase until it is equal to the downward force of gravity. The object then falls at a constant velocity called the terminal velocity. Is terminal velocity a good thing or bad thing?

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**Free Fall Occurs When There Is No Air Resistance**

An object is in free fall only if gravity is pulling it down and no other forces are acting on it. What are the 2 places that have no air and free is possible? Vacuum and space

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**Orbiting Objects are in Free Fall**

Two Motions Combine to Cause Orbiting A space shuttle moves forward at a constant speed and it is in constant free fall towards the earth causing orbiting The unbalanced force that causes objects to move in a circular path is centripetal force (ex. Moon orbiting the earth, earth around sun.)

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**Why does the shuttle not fall to earth if gravity is pulling down on it?**

Because it is traveling forward What would happen if the shuttle started moving much faster or slower? Faster – out of orbit Slower – fall towards earth

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**Projectile Motion and Gravity**

Projectile Motion- the curved path that an object follows when thrown, launched, or otherwise projected near the surface of earth. Horizontal Motion - When you throw a ball, the forward force your hand exerts on the ball Vertical Motion- Pulled down towards the earth by gravity

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**Newton’s Laws of Motion**

Newton’s First Law of Motion An object at rest remains at rest, and an object in motion remains in motion at constant speed and in a straight line unless acted on by an unbalanced force. What does this mean? This means that there is a natural tendency of objects to keep on doing what they're doing. All objects resist changes in their state of motion. In the absence of an unbalanced force, an object in motion will maintain this state of motion.

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**Newton’s first law of motion**

What is the motion in this picture? Forward motion – rolling What is the unbalanced force in this picture? Rock What happened to the skater in this picture? Fall off the skateboard What other force do you know that can stop an object from moving? Friction, gravity

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**Newton’s first law of motion**

Newton’s 1st Law is also the reason you wear a seatbelt! Newton’s 1st Law is also known as inertia Inertia – the tendency of an object to resist being moved, or if the object is moving to resist a change in speed or direction until an outside force acts on it Does mass have an affect on inertia? Would you rather catch a baseball or bowling ball? Yes mass has an affect on inertia

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**Newton’s Second Law of Motion**

Acceleration is produced when a force acts on a mass. The greater the mass of the object being accelerated the greater the amount of force needed to accelerate the object. What does this mean? Everyone knows the Second Law. The heavier objects require more force to move the same distance as lighter objects

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**Newton’s 2nd Law of Motion**

The 2nd Law gives us an exact relationship between force, mass, and acceleration. It can be expressed as a mathematical equation: or FORCE = MASS times ACCELERATION SI Unit for force = Newtons (N)

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**Newton’s 2nd law of motion**

Mike's car, which weighs 1,000 kg, is out of gas. Mike is trying to push the car to a gas station, and he makes the car go m/s2. Using Newton's Second Law, you can compute how much force Mike is applying to the car.

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**Newton’s 2nd law of motion**

What is the acceleration of a 3 Kg mass if a force of 14.4 N is used to move the mass? What force is necessary to accelerate a 1,250 Kg car at a rate of 40 m/s2?

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Newton’s third law For every action there is an equal and opposite re-action. What does this mean? This means that for every force there is a reaction force that is equal in size, but opposite in direction. That is to say that whenever an object pushes another object it gets pushed back in the opposite direction equally hard

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Newton’s third law Let's study how a rocket works to understand Newton's 3rd Law. The rocket's action is to push down on the ground with the force of its powerful engines, and the reaction is that the ground pushes the rocket upwards with an equal force. Newton’s Cradle

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Momentum Momentum - Depends on the object’s mass and velocity. The more momentum an object has, the harder it is to stop the object or change directions. What would have more momentum? A bowling ball or a basketball rolled at the same velocity? Bowling ball b/c it has more mass A tractor trailer or a Honda civic at the same velocity? Tractor trailer

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**Momentum p = m x v Equation: Momentum = mass x velocity**

(Kg m/s) (Kg) (m/s) Example: A 100 Kg car falls off a cliff from rest and hits the ground with a velocity of 35 m/s. What is the car’s momentum when it hits the ground? A 35 Kg bowling ball was thrown at Tommy at a velocity of 15 m/s. What was the bowling ball’s momentum?

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Momentum Problems What is the momentum of an ostrich with a mass of 120 Kg that runs with a velocity of 16 m/s north? What is the momentum of a 6 Kg cat that is moving at 10 m/s down the alley toward the mouse? An 85 Kg man is jogging with a velocity of 2.6 m/s to the north. Nearby, a 65 Kg person is skateboarding and is traveling with a velocity of 3 m/s north. Which person has a greater momentum? Show your work.

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**law of conservation of momentum**

Any time objects collide, the total amount of momentum stays the same. This is true for all collisions if no other forces act on the colliding objects Conservation of Momentum can be explained by Newton’s third law of motion. 5 Reaction Force Action Force

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