Forces

1
Chapter 3

• Forces

2
Section 1

• Newtons Second Law

3
Force, Mass and Acceleration

• Compare hard thrown ball vs. gently tossed ball
• Force? Velocity? Acceleration?
• Throw hard a baseball vs. softball
• Who goes faster? Why?
• Velocity and Acceleration depend on mass

4
Newtons 2nd Law of Motion

• Def acceleration is in the same direction as net
  force

5
Net Force

• Net Force mass x acceleration Fnet ma
• kg x m/s2 N (Newton)

6
Practice!

• Rocket has 28,913 N of thrust and a mass of 2,350
  kg. What is the acceleration? (frictionless
  surface)
• What is the mass of a rocket that has a thrust of
  105 N and an acceleration of 70 m/s2?

7
Friction

• Roll a ball in the parking lot
• Newtons 1st law says ball will keep constant
  speed
• Stops because of friction (negative acceleration
  force is friction)
• Def force that opposes the sliding motion

8
Microwelds

• Def points where two surfaces come in to contact
• Larger force stronger microwelds
• Break microwelds need force

9
Static Friction

• Push box doesnt move
• Acceleration 0 so net force 0
• Whats canceling your force?
• Static Friction force prevents two surfaces from
  sliding past each other

10
Sliding Friction

• Def force that opposes the sliding motion of two
  surfaces
• Stop pushing box stops

11
Rolling Friction

• Car stuck in mud
• Def frictional force between rolling object and
  surface.

12
Air Resistance

• Falling object pulled by gravity opposed by air
  resistance
• Def friction like force opposed motion as moves
  through air
• No air objects fall with same acceleration
  bowling ball vs. feather

13
Terminal Velocity

• Def highest speed a falling object will reach
• When air resistance balances downward force
• Depends on size, shape, mass
• Sky diver no parachute vs. with parachute

14
Section 2

• Gravity

15
What is gravity?

• How would you throw a long pass with a football?
• Anything that has mass is attracted by gravity
• Gravity attractive force between two objects
  that depends on masses and distance
• Stronger force?
• More mass/closer together

16
Gravity

• We only feel earths gravitational force
• 1 of 4 Basic Forces
• Others
• electromagnetic force electricity and magnetism
• strong and weak nuclear force inside nuclei of
  atoms

17
Law of Universal Gravitation

• Sir Isaac Newton

18
Earths Gravitational Acceleration

• When only force acting on two things is gravity
  they fall at the same rate
• Close to earths surface acceleration due to
  gravity is 9.8 m/s2
• Newtons second law ? F ma
• Fgravity(N) mass (kg) x acceleration of gravity
  (m/s2)
• F mg

19
Weight

• Def gravitational force on a object
• Calculate your weight in N

20
Weightlessness and Free Fall

• When you free fall gravity pulls everything at
  same rate no gravity
• Spaceship in orbit experiences 90 of earths
  gravity
• Ship is in free fall in orbit no gravity

21
Projectile Motion

• Projectile anything thrown or shot through air
• Gravity causes path to be curved
• Horizontal motion throwing
• Vertical motion gravity

22
Horizontal and Vertical Distance

• Same vertical distance same mass same
  acceleration same time
• Different horizontal distance

23
Centripetal Force

• Centripetal acceleration acceleration towards
  the center of a curved path.
• Newtons 2nd law acceleration in direction of
  force ? centripetal force
• Anything moving in circle is because cent. force
  is accelerating towards center

24
Gravity
25
Section 3

• Newtons 3rd Law of Motion

26
Newtons 3rd Law

• Def when one object exerts a force on another,
  the 2nd object exerts an equal but opposite force
• Action Reaction
• Jumping on trampoline

27
Action/Reaction Do NOT Cancel

• Forces are equal, but acting on different objects
• Act on water reaction pushes forward
  unbalanced force

28
Rocket Propulsion

• Rocket exerts force on gas backwards gas exerts
  force on rocket forwards

29
Momentum

• How much force is needed to change motion?
• Def product of mass and velocity

30
Practice!

• What is the momentum of a 1,300 kg car traveling
  28 m/s?
• What is the velocity of a 0.15 kg baseball that
  has a momentum of 6 kg m/s?

31
Force and Changing Momentum

• Catch a foul ball bare handed?

32
Force and Changing Momentum

• F ma a (vf vi)
• t
• Put the two together
• When that baseball hits calculate the force

33
Force and Changing Momentum

• What is the force of a 0.15 kg baseball with a
  velocity of 40 m/s when it comes to a stop in 1
  second?

34
Law of Conservation of Momentum

• Momentum doesnt change unless mass, velocity or
  both change
• Can be transferred pool balls
• Def group of objects exerts forces only on each
  other, their total momentum doesnt change

35
When Objects Collide

• 1st puck will give more momentum to the 2nd puck
  in the same direction
• Moving towards each other same speed 0 momentum
• Collide 0 momentum moving same speed opposite
  direction