Dr. Jie Zou PHY 1151 Department of Physics

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Chapter 5

Newton’s Laws of Motion

Dr. Jie Zou PHY 1151 Department of Physics

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Outline Force and Mass Newton’s First, Second, and Third Laws of

Motion Free-Body Diagrams Examples of applying Newton’s 2nd Law of

Motion One-dimension Inclined Plane

Dr. Jie Zou PHY 1151 Department of Physics

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Force and Mass Force: A force, simply put, is a push or a pull.

Force is a vector quantity, specified by the magnitude (or strength) of the force, and the direction in which the force acts

Total force (or the resultant force, or the net force): The total force exerted on an object is the vector sum of the individual forces acting on it.

Units for forces: newtons (N). 1N (1/4) of a pound. Mass: Mass is a measure of the quantity of

matter in an object. It is a scalar. It is an inherent property of an object. Units = kg

Dr. Jie Zou PHY 1151 Department of Physics

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Fg

Book

N

Typical Examples of Forces Force of gravity Fg: The attractive force

exerted by the earth on an object. Direction: Directed toward the center of the earth. Magnitude: The magnitude of the force of gravity

on an object is called the weight of the object W. Weight: W = mg

m: the mass of the object. g: the acceleration due to gravity (or free fall

acceleration)

Normal force N: It is a contact force that is acting perpendicular to the surface.

Fg

N

Dr. Jie Zou PHY 1151 Department of Physics

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Newton’s First Law of Motion (The Law of Inertia) Newton’s First Law of Motion: If the net

force on an object is zero, its velocity is constant. “Net force is zero” means: 1) no force acts on the

object or 2) forces act on the object, but they sum to zero.

Inertia: The property of objects to resist changes in motion is called inertia. (The literal meaning of the word inertia is “laziness”.)

Mass is that property of an object that specifies how much inertia the object has.

Dr. Jie Zou PHY 1151 Department of Physics

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Standing in a moving bus

STOP

The bus suddenly slows down. Do you lurch forward or backward? And why? How about when the bus suddenly speed up from rest?

An Example of Inertia

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Newton’s Second Law of Motion Newton’s Second Law of Motion: The

acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. a = ( F)/m, or F = ma The above equation is equivalent to three

component equations: Fx = max, Fy = may, Fz = maz.

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The Force Exerted by Foamcrete: Foamcrete is a solid paving material soft enough to collapse under the weight of an airliner. It is designed to stop an airplane that has run off the end of a runway. Suppose a 747 jetliner with a mass of 1.75 105 kg and an initial speed of 26.8 m/s is slowed to a stop in 122 m. What is the magnitude of the retarding force exerted by the Foamcrete on the plane?

Active Example 5-1

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(a) The physical situation

(b) The free-body diagram for the book

Free-Body Diagrams Free-body diagram: A sketch that

indicates each and every external force acting on a given object.

Dr. Jie Zou PHY 1151 Department of Physics

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Active Example 5-2 Find the Speed of the Sled:

A 4.60-kg sled is pulled across a smooth ice surface. The force acting on the sled is of magnitude 6.20 N and points in a direction 35.0 above the horizontal. If the sled starts at rest, how fast is it going after being pulled for 1.15 s?

Dr. Jie Zou PHY 1151 Department of Physics

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Another Example To give a 19 kg-child a ride,

two teenagers pull on a 3.7 kg-sled with ropes. Both teenagers pull with a force of 55 N at an angle of 35° relative to the forward direction, which is the direction of motion. In addition, the snow exerts a retarding force on the sled that points opposite to the direction of motion, and has a magnitude of 57 N. Find the acceleration of the sled and child.

Dr. Jie Zou PHY 1151 Department of Physics

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Example 5-9: Object on an Inclined Plane

A child of mass m rides on a toboggan down a slick, ice-covered hill inclined at an angle with respect to the horizontal. (a) What is the acceleration

of the child? (b) What is the normal force

exerted on the child by the toboggan?

Dr. Jie Zou PHY 1151 Department of Physics

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Newton’s Third Law of Motion Newton’s Third Law of Motion: If two

objects interact, the force F12 exerted by object 1 on object 2 is equal in magnitude to and opposite in direction to the force F21 exerted by object 2 on object 1. For every action there is an equal and opposite

reaction. The action-reaction forces always act on different

objects. As a result, the two forces do not cancel. Forces always come in pairs.

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Examples of Action-Reaction Pairs

The action-reaction forces always act on different objects. As a result, the two forces do not cancel.

Since the action-reaction forces act on different objects (different masses), they generally produce different accelerations.

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Homework See online homework assignment at

www.masteringphysics.com