Goal: To understand Newton’s 2nd law and its applications to motions. Objectives: To learn about how accelerations work To understand Newton’s Force Law To learn how accelerations change velocities To apply this to position and motion charts

Breakdown Distance = change in position can be either a scalar or a vector Velocity = rate of change of position Velocity = change in distance / change in time Velocity is always a VECTOR (speed is the scalar) Acceleration = the rate of change of velocity Acceleration = change in velocity / change in time Since velocity is a vector and acceleration depends on velocity therefore acceleration is always a VECTOR NOTE: change = final - initial

Units Distance: meters (m) and sometimes meters direction Velocity: m/s direction Acceleration: m/s2 direction

Law 2: Force Law Force = mass * acceleration Force is how much you are pushing or pulling on something Mass is how much stuff you have (NOTE: mass is NOT weight!!!) Acceleration is the rate at which you change your velocity. Net force = mass * net acceleration

Example Force of gravity I have a mass of 90 kg. What is my force of gravity F = ma Here a = g So, F = mg NOTE: force of gravity is NOT 9.8!

Demo time! I will attempt to walk off the skateboard To do this I will exert a force on the skateboard and a force will push me forward (this is Newton’s 3rd law but we will look at that more later. So, both I and the skateboard will get a force and it will be the same force. The question is, what will happen to the skateboard?

Acceleration Acceleration = net force / mass So, with equal forces, the smaller the mass, the bigger the acceleration. Half the mass means double the acceleration.

But Um but doesn’t gravity pulls everything at the same acceleration? Yes! So, what does that tell you about the force of gravity?

Follow up An astronaut is in orbit around the earth. The mass of the astronaut is 100 kg and the mass of his spaceship is 5000 kg. What are the “weights” of the astronaut and spacecraft (hint what is the net force on them?)? Suppose the astronaut applies a 1000 N force on the spacecraft. What will the acceleration of the spacecraft be? If the same force were applied to the astronaut, what would the acceleration of the astronaut be? Why are the two accelerations so different?

Net force vs acceleration The net acceleration depends on net force. However, an object in motion will remain in motion until the acceleration can stop it. For example, if a car is moving forward and you hit the breaks the breaks will create an acceleration backwards. This will stop the car but not for a few seconds.

Example A car is moving forward at 20 m/s forward. A force with an acceleration of 4 m/s2 backwards is applied. How long does it take for the car to stop?

Thrown ball Acceleration has direction. Imagine I throw a ball horizontally. What will the acceleration in the horizontal direction be?

Two balls I will drop one and throw the other horizontally. Which one will hit the ground first?

Conclusion We have seen from Newton’s 2nd law that force = mass * acceleration Your acceleration depends on your mass – smaller masses have bigger accelerations with the same force. If you are in freefall you have no weight but still have gravity If you are not in freefall then your net acceleration depends on your net force. We have examined how accelerations affect velocities