Presentation on theme: "Reading Quiz - Newton’s Laws 1. Which of these laws is not one of Newton’s laws? ___ 1. Action equals reaction. ___ 2. F = ma. ___ 3. All objects fall."— Presentation transcript:
Reading Quiz - Newton’s Laws 1. Which of these laws is not one of Newton’s laws? ___ 1. Action equals reaction. ___ 2. F = ma. ___ 3. All objects fall with equal acceleration. ___ 4. Objects at rest stay at rest, etc.
2. The law of inertia ___ 1. is not covered in the reading assignment. ___ 2. expresses the tendency of bodies to maintain their state of motion. ___ 3. is Newton’s 3rd law.
3. Is the normal force on a body always equal to its weight? ___ 1. yes ___ 2. no ___ 3. not covered in the reading assignment
Newton’s First Law Force - a vector quantity: a push, pull or influence that affects the motion of an object. Inertia: property of an object that resists changes in the state of motion. Newton’s First Law: Every object continues in its state of rest, or of uniform motion in a straight line, unless it is compelled to change that state by forces impressed upon it.
Conceptual Questions 1) Beaker and Tablecloth: video demo 2) What happens as a result of the downward motion and sudden stop of a hammer when it’s end (not the head) is slammed against a table? 3) The inertial ball: video demo INERTIA Common theme: INERTIA
Newton’s Second Law Mass: quantitative measure of the inertia of an object (kilograms). Newton’s Second Law: The acceleration of an object is directly proportional to the net force acting on it and is inversely proportional to its mass. The direction of the acceleration is in the direction of the net force acting on the object. or
Note: this is a vector equation meaning Force: an action capable of accelerating an object. Allows one to quantify forces - the Newton is the unit for forces. One Newton is that force that would impart an acceleration of 1 to an object of mass one kilogram.
Quantitative Problems 1) A person stands on a bathroom scale in a motionless elevator. When the elevator begins to move, the scale briefly reads only 0.75 of the person’s regular weight. Calculate the acceleration of the elevator, and find the direction of the acceleration. 2) Starting from rest, a 4.0 kg body reaches a speed of 8.0 m/s in 2.0 s. What is the net force acting on the body? What can you say about the force?
Newton’s Third Law A force exerted on an object is always exerted by another object. Newton’s Third Law: Whenever one object exerts a force on a second object, the second exerts an equal and opposite force on the first - action and reaction. Note: The pair of action and reaction forces always act on different objects!
Weight - the force exerted on an object by the gravitational attraction of the earth: downwards Normal force - when objects are in contact, forces are exerted by each object on the other (nature of forces?). The contact force that is perpendicular to the surface of contact is called the normal force. Note: Weight and normal force are not action-reaction pairs!
Quantitative Problems 1) A 10.0 kg box is placed on top of a 20.0 kg box that is resting on top of a table. Determine the normal force that the table exerts on the 20.0 kg box, and the normal force that the 20.0 kg box exerts on the 10.0 kg box. 2) A 1.0 kg block and a 3.0 kg block are in contact on a frictionless horizontal surface. A 10.0 N force is applied to the first block. Find the force this block exerts on the second block. If the same force is now applied to the second block, what force is exerted by it on the first block?
Applications Draw a sketch of the problem. Free-body diagram - for each body (hence the word free), draw a diagram with all forces acting on that body. Apply Newton’s 3rd law where possible. Do not include forces exerted by this body on other bodies (why?). Apply Newton’s 2nd law to each body - note vector nature. Solve the (simultaneous) equations for the unknowns.
Conceptual Questions 1) A car is towing a trailer at constant speed on a level road. The force the trailer exerts on the car ____ a) is less than the force the car exerts on the trailer ____ b) is equal to the force the car exerts on the trailer ____c) is equal to the force the road exerts on the trailer ____d) is equal to the force the trailer exerts on the road
2) A 250 lb man holding a 30 lb bag of potatoes is standing on a scale in an amusement park. He heaves the bag straight up into the air, and before it leaves his hands, a card pops out of the slot with his weight and fortune. It reads ____a) 250 lb ____b) 280 lb ____c) less than 250 lb ____d) more than 250 lb
3) A skydiver is observed to have a terminal speed of 55 m/s in a prone position and 80 m/s in a vertical position. Which of the following can be concluded from this observation? ____a) The force of gravity on the skydiver is less in the prone position ____b) The force of air resistance on the skydiver is proportional to the speed of the body ____c) The force of air resistance is greater at 55 m/s in the prone position than at 80 m/s in the vertical position ____d) The force of air resistance at 55 m/s in the prone position is the same as at 80 m/s in the vertical position
Quantitative Problem A 100 kg basketball player can leap straight up in the air to a height of 80 cm. How? Let us analyze this in the following way: a) The player bends his legs until the upper part of his body has dropped by 60 cm, then he begins his jump. Explain in words what the player is doing from the time he begins his jump until his feet leave the ground. b) Draw separate free body diagrams for the player and for the floor as he is jumping, but before his feet leave the ground.
c) Is there a net force on the player as he jumps (before his feet leave the ground)? How can that be? Explain. d) What is the speed with which he leaves the ground? e) What was his acceleration, assumed to be a constant, as he jumped? f) Suppose he jumps while standing on metric bathroom scales that read in N. What do they read: i) Before he jumps? ii) As he is jumping? iii) After his feet leave the ground?