Using Newton’s Second Law

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Presentation transcript:

Using Newton’s Second Law These problems take many steps. You must have memorized some facts (the laws, terms, etc) but also analyze the situation, nd create a diagram so you can modify the second law.

Problem Solving Steps Identify the variable(s) the problem asks you to calculate. Draw FBD (required any time 2nd law must be expanded) Expand 2nd Law sometimes in two dimensions (required when individual forces used) Substitute given or computed values into the formula Solve

Steps that might help Draw a picture State all the given variables in a list, (or put them on the picture) Try to draw your FBD to scale (longer arrows for stronger forces) Remember, if the object accelerates, there is a net force (forces are NOT balanced) and the net force is in the direction of the acceleration.

Practice Problem: Riding an Elevator How hard does the floor push on an 80kg person that is riding in an elevator that is: a) still? accelerating upwards at 0.5 m/s/s? rising at a constant 2.0m/s? Rising, but slowing by 0.5m/s/s?

“Apparent Weight” The size of the “other” force acting on an object, most often the normal force. The normal force is the apparent weight in the previous problem.

Assignment Read 5-3, study sample problems in book, and posted examples: 26, 29

Practice Problem An 80 kg man with an initial speed is 5.00m/s is slowed to a stop by a 400N friction force. Draw a free body diagram and determine his displacement while sliding to a stop.

Example of 2nd Law with trig In the figure below, the person exerts a 100N force directed 30° below horizontal onto the 50 kg desk. A 61.6N friction force acts on the desk (exerted by the floor). Determine the acceleration of the desk.

Examples to be posted: CH5 # 77, 23, 39, 34

The “Slackline” A 60kg person stands in the center of a slackline in a back yard. The webbing makes an angle of 10° with horizontal on both sides of the person. Determine the tension in the slackline. Now the person moves closer to one end. On that side, the slackline makes an angle of 30° with horizontal, and on the other side, the slackline makes an angle of 5° with horizontal. Determine the tension in the slackline on each side of the person.