© 2010 Pearson Education, Inc. Conceptual Physics 11 th Edition Chapter 2: NEWTON’S FIRST LAW OF MOTION — INERTIA

© 2010 Pearson Education, Inc. Aristotle’s Ideas of Motion Aristotle’s classification of motion Heavier objects fall faster than lighter objects Any object not in its proper place will strive to get there. Once in its proper place, it will come to rest. Examples: Stones fall. Puffs of smoke rise.

© 2010 Pearson Education, Inc. Galileo’s Concept of Inertia Galileo demolished Aristotle’s assertions in the early 1500s. Galileo’s discovery: Objects of different weight fall to the ground at the same time in the absence of air resistance. A moving object needs no force to keep it moving in the absence of friction.

© 2010 Pearson Education, Inc. Galileo’s Concept of Inertia Force is a push or a pull. Inertia is a property of matter which resists changes in motion. depends on the amount of matter in an object (its mass).

© 2010 Pearson Education, Inc. Galileo’s Concept of Inertia Balls rolling on downward-sloping planes picked up speed. Balls rolling on upward-sloping planes lost speed. So a ball on a horizontal plane must maintain speed forever. If the ball comes to rest, it is not due to its “nature,” but due to friction.

© 2010 Pearson Education, Inc. Newton’s First Law of Motion An object at rest tends to stay at rest, an object in motion tends to stay in motion in a straight line at constant speed unless acted upon by a net outside force.

© 2010 Pearson Education, Inc. Scalar quantity a quantity which can be completely described with a magnitude (how much) only. Examples: –15 kg(mass) –23.7 cm (length) –19.5 m/s (speed)

© 2010 Pearson Education, Inc. Vector quantity a quantity whose description requires both magnitude (how much) and direction. A vector can be represented by arrows drawn to scale. –length of arrow represents magnitude and the arrowhead shows direction. –Examples: 279 N downward(force) 88.0 km/h East(velocity) 3.24 m/s 2 33° N of E(acceleration)

© 2010 Pearson Education, Inc. Net Force Net force is the combination of all forces that change an object’s state of motion. Example: If you pull on a box with 10 N and a friend pulls oppositely with 5 N, the net force is 5 N in the direction you are pulling.

© 2010 Pearson Education, Inc. A cart is pushed to the right with a force of 15 N while being pulled to the left with a force of 20 N. The net force on the cart is…? A.5 N to the left. B.5 N to the right. C.25 N to the left. D.25 N to the right. Net Force CHECK YOUR NEIGHBOR

© 2010 Pearson Education, Inc. A cart is pushed to the right with a force of 15 N while being pulled to the left with a force of 20 N. The net force on the cart is A.5 N to the left. B.5 N to the right. C.25 N to the left. D.25 N to the right. Net Force CHECK YOUR NEIGHBOR Two forces are in opposite directions, so they subtract. The direction is determined by the direction of the larger force.

© 2010 Pearson Education, Inc. What is the net force acting on the box? A.15 N to the left B.15 N to the right C.5 N to the left D.5 N to the right Net Force CHECK YOUR NEIGHBOR ?

© 2010 Pearson Education, Inc. What is the net force acting on the box? A.15 N to the left B.15 N to the right C.5 N to the left D.5 N to the right Net Force CHECK YOUR NEIGHBOR

© 2010 Pearson Education, Inc. The Equilibrium Rule The vector sum of forces acting on a non- accelerating object equals zero. In equation form:  F = 0.

© 2010 Pearson Education, Inc. The Equilibrium Rule : Example A string holding up a bag of flour Two forces act on the bag of flour: –The force of tension on the string acts upward. –The force of weight acts downward. Both are equal in magnitude and opposite in direction. –When added, they cancel to zero. –So, the bag of flour remains at rest.

© 2010 Pearson Education, Inc. The equilibrium rule,  F = 0, applies to…? A.vector quantities. B.scalar quantities. C.Both of the above. D.None of the above. The Equilibrium Rule CHECK YOUR NEIGHBOR

© 2010 Pearson Education, Inc. The equilibrium rule,  F = 0, applies to A.vector quantities. B.scalar quantities. C.Both of the above. D.None of the above. The Equilibrium Rule CHECK YOUR NEIGHBOR Explanation: Vector addition accounts for + and – quantities. So, two vectors in opposite direction can add to zero.

© 2010 Pearson Education, Inc. Normal Force Normal force (support force) is an upward force on an object that is perpendicular to the surface which supports the object. Example: A book on a table compresses Atoms in the table, and the compressed atoms produce the normal force.

© 2010 Pearson Education, Inc. Understanding Normal Force When you push down on a spring, the spring pushes back up on you. Similarly, when a book pushes down on a table, the table pushes back up on the book.

© 2010 Pearson Education, Inc. When you stand on two bathroom scales with one foot on each scale and with your weight evenly distributed, each scale will read A.your weight. B.half your weight. C.zero. D.more than your weight. Support Force CHECK YOUR NEIGHBOR

© 2010 Pearson Education, Inc. When you stand on two bathroom scales with one foot on each scale and with your weight evenly distributed, each scale will read A.your weight. B.half your weight. C.zero. D.more than your weight. Support Force CHECK YOUR NEIGHBOR Explanation: You are at rest, so  F=0. Forces from both scales add together to equal your weight. The Force exerted by each scale is one-half your weight

© 2010 Pearson Education, Inc. Equilibrium of Moving Things Equilibrium: Equilibrium: a state of no change in a bodies motion with no net force acting –Static –Static equilibrium Example: hockey puck at rest on slippery ice –Dynamic –Dynamic equilibrium Example: hockey puck sliding at constant speed on slippery ice

© 2010 Pearson Education, Inc. Equilibrium of Moving Things Equilibrium test: whether something undergoes changes in motion Example: A crate at rest is in static equilibrium. dynamic Example: When pushed at a constant speed, it is in dynamic equilibrium.

© 2010 Pearson Education, Inc. A bowling ball is in equilibrium when it A.is at rest. B.moves at constant speed in a straight-line path. C.Both of the above. D.None of the above. Equilibrium of Moving Things CHECK YOUR NEIGHBOR

© 2010 Pearson Education, Inc. A bowling ball is in equilibrium when it A.is at rest. B.moves steadily in a straight-line path. C.Both of the above. D.None of the above. Equilibrium of Moving Things CHECK YOUR NEIGHBOR Explanation: Equilibrium means no change in motion, so there are two options: If at rest, it continues at rest. If in motion, it continues at constant speed in a straight line.

© 2010 Pearson Education, Inc. You are pushing a crate at a constant speed in a straight line. If the friction force is 75 N, how much force must you apply? A.more than 75 N B.less than 75 N C.equal to 75 N D.not enough information Equilibrium of Moving Things CHECK YOUR NEIGHBOR

© 2010 Pearson Education, Inc. Equilibrium of Moving Things CHECK YOUR NEIGHBOR Explanation: The crate is in dynamic equilibrium, so,  F = 0. Your applied force balances the force of friction. You are pushing a crate at a steady speed in a straight line. If the friction force is 75 N, how much force must you apply? A. more than 75 N B. less than 75 N C. equal to 75 N D. not enough information

© 2010 Pearson Education, Inc. The Moving Earth Copernicus proposed that Earth was moving. This idea was disputed by people in his day. Example: If Earth moved, why aren’t all the people and animals thrown off? Solution: As it turns, everything on the Earth moves with it at the same speed due to inertia.

© 2010 Pearson Education, Inc. You are riding in a van at a constant speed and toss a coin straight up. Where will the coin land? A.behind you B.ahead of you C.back in your hand D.There is not enough information. The Moving Earth CHECK YOUR NEIGHBOR

© 2010 Pearson Education, Inc. You are riding in a van at a steady speed and toss a coin up. Where will the coin land? A.behind you B.ahead of you C.back in your hand D.There is not enough information. The Moving Earth CHECK YOUR NEIGHBOR Explanation: The coin has inertia. It continues to move with the van and your hand and lands back in your hand.