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Unit 2 Newton’s Laws of Motion in 1D A. Vectors and Scalars B. Addition of Vectors & RESULTANT In one dimension, simple addition and subtraction is all that is needed. RESULTANT VECTOR Aristotle vs Galileo FORCES NET FORCE D. TYPES OF FORCES 1) Force of Gravity (Weight) 2) Normal Force (Support Force) If the box weighed less, what would happen to the normal force acting on the box? 3) Tension Force 4) Friction Force E. NEWTON’S LAWS Newton’s 1st Law of Motion Also called the law of inertia. Mass - Without gravity, how can we distinguish between a 1kg and a 2kg mass? Why do all objects fall at the same rate? (|a|=g=9.8m/s2) Newton’s 3rd Law of Motion Explain the movement of a rocket using the 3rd Law. Question: A loaded school bus hits a bug and kills it. Which body receives the greater force of impact, bug or bus? Who pulls harder on the rope? Who wins the tug of war? Force Body Diagrams (FBD) using vectors In order to solve problems involving forces, we need to draw an FBD. A box is dragged by a rope towards the right on a smooth floor. Draw the force vectors on the box. Newton’s 2nd Law of Motion Constant velocity equates to what in regards to Fnet? Example 1 A crane lowers a 1306kg car by a cable with an acceleration of 0.73 m/s2. The car starts 20.0m above the ground with an initial speed of zero. a) What is the tension in the cable? Draw FBD b) How much time will it take the car to reach the ground? Example 2: A force of 75N pushes on 2 boxes as shown. The mass of b1 is 20kg and the mass of b2 is 35kg. Surface is smooth. a) Determine the acceleration of the two boxes. b) Determine the net force on b2. c) Determine the net force on b1 . Why is it different? Example 3 A person stands on a bathroom scale in an elevator at rest on the ground floor of a building. The scale reads 836N. As the elevator begins to move upward, the scale reading briefly increases to 935N but then returns to 836N after reaching a constant speed. a) Determine the acceleration of the elevator. b) If the elevator was moving at 3.0m/s upwards and then uniformly decelerated to rest in 4.7s, determine the scale reading. Force of Friction (Ff) On a microscopic scale, most surfaces are rough. Two Types of Friction: 1) Static Friction (Ffs) 2) Kinetic Friction (Ffk ) Force of friction tends to oppose the motion of objects Friction depends on two things: In the case of static friction, there is a maximum value at which the static friction force will resist motion between surfaces. This means that if you push a table with 50N of force where maximum static friction is 75N, the table won’t break free. You need to push with just a smidge over 75N where we say you just have to equal maximum static to break free. The static frictional force increases as the applied force increases, until it reaches its maximum. Example1 A person crosstrains by pushing a 950N man (who sits on a 45N metal box). The box is pushed with a force of 335N at a constant acceleration. If the coefficient of kinetic friction is 0.30, determine the speed of the box after 3.0s if it starts from rest. Example2 What minimum amount of force is needed to start to make a 250N crate move across a floor if the coefficient of static friction is 0.65 and the coefficient of kinetic friction is 0.40? Example2 A physics book is pushed and released across a table. The book is sent sliding with a speed of 4.3m/s. If it takes the book 1.6m to stop, determine the value of the coefficient of kinetic friction. Terminal Velocity Consider a skydiver who steps off a hovering helicopter at high altitude. NOW consider the effect of air resistance (friction) during the fall. a) Initially at t=0, what forces act on the skydiver? b) Initially at t=0, what is the acceleration and velocity of the skydiver? c) As the skydiver begins to fall, what happens to the force of air resistance on skydiver? d) As the skydiver continues to fall, describe what happens to their speed and acceleration? Why? e) Eventually what happens to the speed of the skydiver?