Download - Newton’s 2 nd Law of Motion C11, S2
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Flashback: Newton’s Laws of Motion
• Sir Isaac Newton was a central figure in the Scientific Revolution during the 17th century. His ideas went against the more accepted ideas of motion. The popular idea before Newton was that a moving object, if left on its own, would eventually come to rest, and a force was necessary to keep the object moving.
• We now know this to be false, thanks to Isaac Newton.
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Flashback: Newton’s Laws of Motion
• Today, Newton’s three laws of motion are considered to be the backbone of physics. These ideas were major breakthroughs that set history on a new course. Because of these ideas we now have safety restraints (seat belts) in multiple types of travel.
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Flashback: Newton’s 1st Law of Motion
• An object at rest remains at rest, and an object in motion remains in motion at a constant speed and in a straight line unless acted on by an unbalanced force.
• Newton’s 1st law is sometimes called the law of inertia.
• Inertia is the tendency of all objects to resist any change in motion.
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Law of Intertia
Tennis ball wants to keep
sitting still
Baseball wants to keep moving!
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Law of Intertia, continued
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Don’t let this be you. Wear seat belts!!
Because of inertia, objects (including you) resist changes in their motion. When the car
going 80 km/hour is stopped by the brick wall, your body keeps moving at 80
km/hour.
Law of Intertia
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Law of Intertia
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Flashback: Newton’s First LawBalanced Forces = No Change in
Velocity• A stationary object
will never move without unbalanced forces acting on it
Balanced forces = no change in speed
12N
12N
0N 0N
• A moving object will never stop moving without unbalanced forces acting on it
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• If the forces are in the same direction, then the net force is the sum between the two forces.
12 N 8 N
NET FORCE = 12N + 8N = 20N, left
Combining forces to form NET FORCE
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Combining forces to form NET FORCE
• If two forces are in opposite directions, then the net force is the difference between the two forces, and it is in the direction of the larger force.
12 N
8 N
NET FORCE = 12N – 8N = 4N, right
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Flashback: Types of Friction – Static Friction
• Static friction is the type of friction that prevents an object from moving when a force is applied.
• For example, when you push or pull on the refrigerator and at first it doesn’t want to move but then it suddenly starts to move.
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Flashback: Types of Friction – Static Friction
• Generally, the rougher the surface or the heavier the object, the more static friction.
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• Sliding friction slows down an object that slides.
• Sliding friction is due to microscopic roughness of two surfaces.
• Brakes on a car are an example of sliding friction.
Flashback: Types of Friction - Sliding Friction
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• Rolling friction is needed to provide grip to make a wheel or a tire turn.
• Rolling friction occurs between the ground and the part of the tire touching the ground.
Flashback: Types of Friction - Rolling
Friction
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• Fluid Friction occurs between an object and air or water.
• For example, fluid friction occurs when skydivers jump out of a plane, or when boats travel through the water
Flashback: Types of Friction - Fluid Friction
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Flashback: Mass vs. Weight
• Mass is the amount of matter in an object and DOES NOT change with gravity.
• Mass is measured in Kilograms (Kg).
• Weight is a force measured by the pull of the Earth’s gravity.
• Weight is measure in Newtons (N).
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• Your weight would change if you were standing on a planet other than Earth, as shown.
Flashback: Mass vs. Weight
• If your mass was 60 kg, what would your weight in Newton’s be on Earth? On Jupiter?
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• When the motion of an object changes, the object is accelerating.
• Acceleration occurs any time an object speeds up, slows down, or changes its direction of motion.
Flashback: Force and Acceleration
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Newton’s 2nd Law of Motion
• The acceleration of an object depends on the mass of the object and the amount of force applied (f=ma)
• Newton’s 2nd law of motion connects force, acceleration, and mass.
• In the presence of a NET FORCE, an object experiences acceleration in the direction of the force.
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Newton’s 2nd Law of Motion
• With any given force:• an object with less
mass accelerates quicker (i.e. volleyball)
• An object with more mass accelerates slower (i.e. bowling ball)
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Your _______ on Earth is the gravitational force between you and Earth.
A. accelerationB. inertiaC. massD. weight
Question 1
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Answer
The answer is D, “weight”. Since weight is dependant on gravity, your weight would be different if you were standing on a planet other than Earth.
Question 1
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Question 2
Explain Newton’s second law of motion.
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Explain Newton’s second law of motion.
Newton’s Second Law states that an object acted upon by a net force will accelerate in the direction of the force. And, that the net force is equal to the acceleration times the mass of the object (f=ma).
ANSWER
Question 2
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Newton’s 2nd Law Formulas
• ALL THESE ARE DIFFERENT WAYS TO EXPRESS THE SAME EQUATION!!
• In these equations, a is the acceleration (m/s2), m is the mass (kg), and f is the net force (N).
• Force is measured in Newtons, abbreviated N.
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Calculations, Force
• Newton’s second law of motion can be used to calculate NET FORCE.
• A baseball has a mass of 0.2 kg. If its acceleration is 40 m/s2, what is the net force on the baseball?
f = ma f = ? m = 0.2kg a = 40m/s2
f = 0.2kg * 40m/s2 f =8 N
f = ma
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Calculations, Force
•A 6 kg cat accelerates at 1.5 m/s2. Find the net force.
f = ma f = ? m = 6kg a = 1.5m/s2
f = 6kg * 1.5m/s2 f = 9 N
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(Advanced Classes)Calculations, Force
• A 2 kg book is pushed along a table with a thrusting force of 4 N. Find the frictional force on the book if the book’s acceleration is 1.5 m/s2
f = ma f = ? m = 2kg a = 1.5m/s2
f = 2kg * 1.5m/s2 f = 3 N (net force) 4 N (thrusting force) – 3 N (net
force) =
1 N (frictional force)
f = ma
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Calculations, Acceleration
• Newton’s second law of motion can be used to calculate ACCELERATION.
• Suppose you pull a 10-kg sled so that the net force on the sled is 5 N. What is the acceleration?
f = ma f = 5N m = 10kg a = ?
5N = 10kg * a
0.5 m/s2
f = ma
a = 5N/10kg =
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Calculations, Acceleration
A net force of 4,500 N acts on a car with a mass of 1,500 kg. What is the acceleration of the car?
f = ma f = 4,500N m = 1,500kg a = ?
4500N = 1500kg * a
3.0 m/s2
f = ma
a = 4500N/1500kg =
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Calculations, Acceleration
•A book with a mass of 2.0 kg is pushed along a table. If the net force on the book is 1.0 N, what is the acceleration of the book?
f = ma f = 1.0N m = 2.0kg a = ?
1.0N = 2.0kg * a0.5 m/s2
f = ma
a = 1.0N/2.0kg =
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Calculations, Mass
• Newton’s second law of motion can also be used to calculate MASS.
• A basketball is thrown with a net force of 9 N and accelerates at 1.5 m/s2, what is the mass of the basketball?
f = ma f = 9N m = ? a = 1.5 m/s2
9N = m * 1.5m/s2 6 kg
f = ma
m = 1.5m/s2 / 9N =
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Calculations, Mass
• An object acted on by a force of 3.6 N has an acceleration of 1.2 m/s2. What is the mass of the object?
f = ma f = 3.6N m = ? a = 1.2 m/s2
3.6N = m * 1.2m/s2
3 kg
f = ma
m = 3.6N / 1.2m/s2 =
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