Sara Rwentambo. PHYS 1007 AB

Transcription

1 Topics: Free body diagrams (FBDs) Static friction and kinetic friction Tension and acceleration of a system Tension in dynamic equilibrium (bonus question) Opener: Find Your Free Body Diagram Group Activity! (20 minutes) For each situation, draw a free body diagram labeling all the forces acting on the object. Don t forget to include a coordinate system for each case. In all situations, neglect air friction and other forms of friction (unless mentioned otherwise). Find your free body diagram and find the x and/or y components for all forces acting on each FBD and theoretical equations for the net force in both the x and y direction for your situation ( Fx = and Fy = ). (see last page of this worksheet) Situation Free Body Diagram Fnet in x direction Fnet in y direction 1. An apple falling from a tree (undergoing free fall) 2. A ball attached to a string hanging from the celling at rest 3. A curling rock sliding on ice (assume ice is a frictionless surface). Your physics textbook sliding down an inclined plane (kinetic friction is present)

2 Activity 1: Static and Kinetic Friction (20 minutes) Adrian is moving into residence and applies a pushing force to move a 0kg crate of his personal belongings to the right at a constant velocity across the horizontal floor (Hint: constant velocity means ax = 0m/s 2 ). The coefficient of static friction (µs) between the crate and the floor is a) Draw a free body diagram of all the forces acting on the crate. b) What maximum applied force (magnitude and direction) is required just as the crate is about to move? Use fs = µsn and take g = 9.81m/s 2. c) After a moment, Adrian applies 88N to get the crate accelerating to the right. If the net horizontal force is 10N[right], what is the coefficient of kinetic friction (µk)? Use fk = µkn

3 Activity 2: Chalkboard Relay Race! Acceleration and Tension of Two Objects on a Pulley: (35 minutes) Two crates are connected to a massless rope that runs through a frictionless pulley at the edge of a horizontal tabletop. The mass of the crate on the frictionless table (m1) is 10kg and the mass of the crate hanging off the table (m2) is 15kg. The entire system accelerates off the tabletop as shown in the diagram. a) Draw a free body diagram for m1 and for m2, labeling all the forces acting on each mass. (5 minutes) b) What are the x and/or y components of each force on each crate? Form theoretical equations for net force in x and/or y direction of each crate (ie. ( Fx = and Fy = ). (10 minutes) c) Find the magnitude of acceleration of m1. Take g = 9.81m/s 2 (Hint: acceleration is the same for both crates) (10 minutes) d) Plug your answer from part c) into one of your theoretical equations from part b) to solve for the tension on the rope (Hint: rope attached to m1 has the same tension as rope attached to m2) (3 minutes)

4 Closer: (5 minutes) Analyze Why It Works: With a partner or group, briefly discuss why the opener activity of this workshop is a good learning tool for solving Newtonian mechanics problems (motion of objects influenced by forces of a system) for this course. Bonus question! : A 12kg traffic light is suspended from two different weightless cables attached poles of equal height. Tension on the first cable (T1) makes an angle of 30 below the horizontal, and tension of the second cable (T2) makes an angle of 5 below the horizontal. a) Draw a free body diagram of the situation. b) What are the x and y components for T1 and T2? c) Use what you found from part b) to find a theoretical equation for the net force in x and y direction of each cord ( Fx = and Fy = )? c) Use your equations from part c) to solve for T1 and T2.

5 Here is a chart to help you organize components and net forces acting on an object from a FBD: Force (eg. Ff, N, Tension, force of gravity ) x component y component Fnet Fx = Fy = Example: A ball attached to a string hanging from the celling at rest (up is positive down is negative) FBD Force x component y component tension, T none +T Force of gravity (mg) none -mg Fnet Fx = none Fy = T mg = 0