1. 6.2 Newton’s Second Law

2. Newton’s Second Law Concepts
Acceleration is proportional to force
↑ F = ↑ a
Have one student draw the graph of speed vs. number of rubber bands from their lab manual on the board
Ask: If we increase the number of rubber bands, we are increasing the [force on the car]
Ask: From the graph of this data, if we increase the force, what happens to the speed? [speed should increase]
Ask: What type of relationship exists between variables if they both increase or both decrease? [direct relationship]
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3. Newton’s Second Law Concepts
Acceleration is inversely proportional to mass
↑ m = ↓ a
Have one student draw the graph of speed vs. mass from their lab manual on the board
Ask: From the graph of this data, if we increase the mass of the car, what happens to the speed? [speed should decrease]
Ask: What type of relationship exists between variables if one variable increases and the other decreases? [inverse relationship]
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Ask: Using concepts related to Newton’s first law, how can you explain this relationship? [inertia]
Ask: And what is the definition of inertia? [the property of an object that resists changes in motion]

4. Newton’s Second Law Concepts
Acceleration is the result of unbalanced forces
Acceleration is in the same direction as the net force
Ask: According to Newton’s first law, what happens to the motion of an object if the forces on it are unbalanced? [there is a change in motion - it speeds up or slows down]
Ask: And if an object is speeding up or slowing down, what do we call that? [acceleration]
Ask: If I move this car this direction, what direction is the net force? [the same direction of motion]

5. Units of Force: The Newton
Newton: the amount of force needed to accelerate a 1 kg object by 1 m/s every second
1 N = 1 kg•m/s2
Write 1 N = 1 kg•m/s2 on the board
Ask: Looking at the units we have here, how can we write an equation for force?
Replace N with F
Circle kg and Ask: What quantity does kg correspond to? [mass]
Circle m/s2 and Ask: What quantity that we have discussed does m/s2 correspond to? [acceleration]

6. Newton’s Second Law
Ask: Using this equation, how can I write an equation for a= ? [ a = F/m ]
Ask: Using the equation, how can I write an equation for m= ? [ m = F/a ]

7. Newton’s Second Law Exit Slip
John threw a 5.0 kg shot put with an acceleration of 28.5 m/s2. What net force did John exert on the shot put?
Ask: Using this equation, how can I write an equation for a= ? [ a = F/m ]
Ask: Using the equation, how can I write an equation for m= ? [ m = F/a ]