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2nd law of motion and sig figs
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2nd law of motion and sig figs

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Transcript

  • 1.
    • Read Investigation 2.3 beginning on page 157.
    • Start a new page in your lab notebook. Title it “Newton’s Second Law of Motion” – don’t forget your table of contents!
  • 2.
    • Predict what will happen when a cart is pushed with a constant force. Write an “if, then” statement in your lab book.
    • Predict what will happen when you use the same amount of force but have a cart with less mass. Write an “if, then” statement in your lab book.
  • 3.
    • Predict what will happen when you push a cart with increased mass with the same force. Write an “if, then” statement in your lab book.
  • 4.
    • Make a list of materials you will use in today’s lab. Put this list in your lab book.
  • 5.
    • Today you will need:
      • calculator
      • spiral
      • pen or pencil
    • Today we will:
      • review what we learned Friday
      • take some notes
      • complete a worksheet
  • 6.  
  • 7.
    • There are two types of observations we can make during an investigation
      • Qualitative – qualities of objects, events , or processes : something smells spicy, tastes sweet, feels slippery
      • Quantitative – observations based on counting or measuring: the temperature, the distance, the speed
  • 8.
    • What type of measurements did we make in this investigation?
  • 9.
    • Newton’s Second Law can be expressed with an equation:
    • Which can be re-arranged to isolate Force like this:
  • 10.
    • Force is measured in Newtons
    • A Newton = the amount of force needed to accelerate a 1 kilogram mass at a rate of 1 m/s 2
    • Since F = m . a
    • 1 N = 1 kg . m/s 2
  • 11.
    • If there is acceleration (speed up, slow down, change direction), there is an unbalanced force.
    • remember: another name for unbalanced force is net force
  • 12.
    • Look at the relationship between force, mass, and acceleration in this equation:
    • To keep the same acceleration, what must force do if mass is increased?
      • force must increase just as much
  • 13.
    • To be more specific:
      • If acceleration stays constant and the mass of the object doubles, what must the force be doing?
      • doubling
  • 14.
    • Turn in your book to page 163
  • 15.
    • IN YOUR SPIRAL:
    • Describe the motion of the object this graph depicts
    • Using F = ma, calculate the mass of the object
    • Collaborate with your table partners – make sure answers agree
  • 16.
    • “Sig Figs”
    • When you perform calculations that use measurements ( quantitative ), you need to express the results of your calculations in a way that makes sense of the precision of the measurements you used.
  • 17.
    • In other words, if you measured your distance as 2.3 centimeters and the time as 1.1 s, you wouldn’t report the speed as 2.0909 cm/s because it implies a precision you didn’t have with your measurements
  • 18.
    • So, how do you figure out how many figures to use?
    • There are five rules
      • 1. All non zero numbers are significant figures
      • So…if the measurement is 125.5 m, how many sig figs are there?
      • four
  • 19.
    • 2. A zero at the end of a decimal number IS significant. So, if the measurement is given as 1.50N, the zero IS significant
      • How many sig figs in 31.40 g?
      • four
  • 20.
    • 3. A zero between nonzero digits is significant. In the measurement 405 km, the zero is significant.
      • How may sig figs in 307.89 g?
      • five
  • 21.
    • 4. A zero at the beginning of a decimal point is not significant. In the measurement 0.027kg, the zeros are not significant. That measurement has two sig figs.
      • How many sig figs in the measurement 0.0306 g?
      • three
  • 22.
    • 5. In a large number without a decimal point, the zeros are not significant. In the measurement 2000 km, there is only one significant figure.
      • how many sig figs in the measurement 500m?
      • one
      • how many sig figs in 501 g?
      • three
  • 23.
    • Adding & Subtracting
      • the final result should have the same number of decimal places as the measurement with the fewest decimal places
  • 24.
    • Multiplying and Dividing
      • the result should have no more significant digits than the factor having the fewest number of significant digits
  • 25.
    • 27.09 km has how many significant figures?
      • four
    • 3600 g has how many significant figures?
      • two
    • 54.009 has how many significant figures?
      • five
  • 26.
    • 10.2 + 201 = ?
      • 211 – you can’t say it’s 211.2 because that would be four significant figures and both numbers you are adding have three significant figures
  • 27.
    • 0.02991 x 10.0 = ?
      • 0.299 – there are three significant figures in 10.0, so that’s how many significant figures you should have in your answer.
  • 28.
    • We already know that the formula for the second law of motion is F = ma
    • We can use this formula to calculate the force of weight
    • Weight = mass x acceleration gravity
  • 29.
    • What happens when an unbalanced force acts on an object?
      • the object accelerates
    • When two forces act on an object at the same time, the direction as well as the magnitude of the force determine the motion of the object
  • 30.
    • If the two forces are in the same direction, the sum of the forces (net force) will cause a larger acceleration than either force on its own
  • 31.
    • If the two forces are in opposite directions then the net force could be zero – in which case there would be no acceleration
    • OR
    • the sum of the two forces will result in a net force in one direction
  • 32.
    • Draw a free body diagram that you think would represent two forces in opposite directions that would result in no net force (and thus, no acceleration)
  • 33.
    • Draw a free body diagram with two forces in opposite directions that would result in a net force (and acceleration)
  • 34.
    • It’s really, really important to remember:
    • a net force results in acceleration
    • BUT
    • no net force does not mean no motion – an object will move at a constant speed in a straight line with no net force!!!
  • 35.
    • What if the forces aren’t in line with each other?
  • 36. You add the vectors using tip-to-tail and then use the Pythagorean theorem to solve a 2 + b 2 = c 2 (40 N) 2 + (60 N) 2 = c 2 1600 N 2 + 3600 N 2 = c 2 5200 N 2 = c 2 c = 72 N