Forces. Definition Friction Falling Objects Projectiles Newton s Laws of Motion Momentum Universal Forces Fluid Pressure Hydraulics Buoyancy

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1 Forces Definition Friction Falling Objects Projectiles Newton s Laws of Motion Momentum Universal Forces Fluid Pressure Hydraulics Buoyancy

2 Definition of Force Force = a push or pull that causes a change in the motion of an object. Change may be in speed or direction Act in a certain direction (vector) Can be shown using a Free Body Diagram Forces transfer/transform energy Unbalanced forces change motion, balanced forces do not. Measured in Newtons, N (1 N = 1 kg m/s 2 )

3 Friction A force that opposes the movement of objects that touch each other. 4 main types Static friction resists motion from rest Sliding friction continues to resist motion as an object is sliding over a surface Rolling friction friction of an object on wheels, ball bearings, or rollers Fluid friction resists movement of an object through a fluid (air, water, other liquids)

4 Comparison of Friction Types Static Sliding Rolling Fluid

5 Falling Objects Accelerate due to force of gravity. Acceleration on Earth is at a rate of 9.8 m/s 2 Air resistance (friction) increases with speed. Objects reach a terminal velocity when the force of gravity is opposed by air resistance.

6 Projectile Motion Projectiles experience a horizontal force and the force of gravity. This leads to a curved path of motion.

7 NO MORE VEGGIES!

8 Newton s 1 st Law of Motion The Law of Inertia The motion of an object remains unchanged unless acted upon by an unbalanced force. An object in motion stays in motion and an object at rest stays at rest, unless acted upon by an unbalanced force.

9 Newton s 2 nd Law of Motion The acceleration of an object is equal to the net force acting on it divided by the object s mass. F=ma Or a = F/m or m = F/a

10 Weight vs. Mass Weight = the force of gravity acting on the mass of an object If F = ma then W = mg W is weight in Newtons m is mass in kilograms g is acceleration due to gravity, m/s 2

11 Newton s 3 rd Law of Motion Action-Reaction Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object. List 3 examples in your notes from your textbook (page 373) or from class discussion.

12 Momentum Momentum = mass x velocity The larger the object and the faster it is moving, the more momentum it has Similar to force, but with only velocity instead of acceleration

13 Conservation of Momentum The total momentum of a closed system remains the same before and after collisions For a collision between objects A and B... (Momentum A ) before + (Momentum B ) before = (Momentum A ) after + (Momentum B ) after (m A x v A ) before + (m B x v B ) before = (m A x v A ) after + (m B x v B ) after Elastic collisions (objects hit & bounce apart with no friction or loss of momentum) Inelastic collisions (objects stick together)

14 Electromagnetic Force Can Attract or Repel Opposite charges attract, Like charges repel Moderate strength over medium distance

15 Nuclear Forces Strong and Weak Nuclear Forces Forces inside the nucleus of an atom that hold it together or play a role in its decay Very strong, but only over short distances (like the size of a proton or so)

16 Gravitational Force Newton s law of Universal Gravitation Any two objects in the universe have a force of gravitational attraction between them Affected by the mass of the objects and the distance between them Weakest force but acts over very long distances

17 Centripetal Forces Describes any force that is center directed Acts to change the direction of motion of an object to keep it in orbit NOT Centrifical or Centrifugal

18 Pressure Definition: Force per unit of area Equation: P = F / A Units: if F is in Newtons and A is in m 2, then P is in Pascals Pascals are really small units of pressure, so we often use kilopascals or kpa (1 kpa = 1000 Pa) Other units (like psi, atm, torr, mmhg, bar, in. can all be converted into kpa)

19 Pressure in Fluids Pascal s Principle Pressure on a fluid is transmitted equally and unchanged in all directions throughout the fluid. Application : Hydraulics Piston systems are the simplest

20 Hydraulics F= 100 N F= 1000? N N A= 1 m 2 A= 10 m 2 P= P= 100 F/A? Pa / Pa1 P= 100 F 100 = / F A Pa / 10 x10 x10 Seems like a magical way to multiply force. What s the catch?

21 Bernoulli s Principle As the speed of a fluid increases, the pressure within the fluid decreases 2 important applications Wings and lift Atomizers, aspirators, spray bottles

22 Airplane Wing

23 Lift & Wings

24 Perfume atomizer Atomizer

25 Hose-end sprayer

26 Atomizers

27 Archimedes Principle The buoyant force on an object is equal to the weight of the fluid displaced by the object Pressure increases with depth in a fluid Upward force due to pressure on the bottom of an object is greater than the downward force due to pressure on the object

28 Buoyancy Buoyant force = the weight of the fluid displaced

29 Floating and Sinking Buoyant force > Weight of the of the object the object floats Buoyant force < Weight of the object, the object sinks

30 Floating and Sinking

31 Density Density = mass / volume (D=m/V) Units for density depend on the units for mass and volume. kg/m 3 g/cm 3 g/ml g/l

32 Density It measures how compact the matter is in an object Density can also be used to predict if an object will float or sink in a fluid If D object < D fluid the object will float If D object > D fluid the object will sink Average density is important!

33 Density Benchmarks Density of pure water at 4 C (near freezing) is exactly 1 g/ml Density of pure water at 20 C (room temp.) is g/ml (close enough to 1 for our purposes) Density of air (at sea level, at 20 C) is approximately 1.2 kg/m 3 or g/cm 3