Goal: To understand Newton

1
Goal To understand Newtons 2nd law and its
applications to motions.

• Objectives
• To learn about how accelerations work
• To understand Newtons Force Law
• To learn how accelerations change velocities
• To apply this to position and motion charts

2
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

3
Units

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

4
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

5
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!

6
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
  Newtons 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?

7
Acceleration

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

8
But

• Um but doesnt gravity pulls everything at the
  same acceleration?
• Yes!
• So, what does that tell you about the force of
  gravity?

9
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?

10
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.

11
Example

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

12
Thrown ball

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

13
Two balls

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

14
Conclusion

• We have seen from Newtons 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