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Chapter 10 - Forces

Question – What causes an object to start moving, stop moving, or change direction Answer - Forces. Chapter 10 - Forces. Force – a push or a pull. 30 lbs 30 lbs. Equal forces in balance. Balanced Forces. Equilibrium – when all forces acting on an object are in balance

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Chapter 10 - Forces

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  1. Question – What causes an object to start moving, stop moving, or change direction Answer - Forces Chapter 10 - Forces Force – a push or a pull 30 lbs 30 lbs Equal forces in balance Balanced Forces

  2. Equilibrium – when all forces acting on an object are in balance If all forces are in balance there is no acceleration. (The object is not moving or is at constant velocity 100 lbs Ground 100 lbs

  3. Balanced Forces 50 mph • The force exerted by the motor in a forward direction is equal the force of friction in the opposite direction. • Frictional force – a slowing force caused by objects or substances rubbing against each other. • If the forces are not in balance there will be an acceleration From wind resistance, gears, etc..

  4. NEWTON’S FIRST LAW OF MOTION • An object at rest will stay at rest and an object that is moving will stay in motion unless acted on by an unbalanced force • An object will tend to do what it is already doing. • Inertia – the tendency of an object to resist changes in motion

  5. NEWTON’S SECOND LAW OF MOTION Fnet = m(a) a = Fnet/m m/sec2 (meter) • Newton – the force required to accelerate a 1 kg mass 1 m/sec2 • Acceleration and force are directly proportional . If we double the force, we double the acceleration. • Acceleration and mass are inversely proportional. If we double the mass, the acceleration is cut in half. kg Newton

  6. 3 types of friction • Sliding (when objects slide over each other) • Rolling friction (less than sliding) • Fluid – when objects move through a fluid such as water or air

  7. Gravity – attraction between all objects that have mass • The greater the mass the greater the gravitational attraction • Free fall – acceleration due to gravity is 9.8 m/sec2 • Air resistance – fluid friction of air • The higher the velocity, the greater the air resistance. • Terminal velocity – when air resistance equals gravitational force

  8. Skydiver terminal velocity 120 mph if body is horizontal Over 200 mph if body is vertical

  9. Projectile Motion 9.8 m/sec2 or 9.8 m/sec2 They will strike the ground at the same time. Gravity accelerates them both at the same rate

  10. Weight – measure of gravitational pull measured in lbs on a spring scale • Is dependent on location • Mass – measure of the amount of matter an object contains • Measured on a balance scale (with counter weights) • Independent of location

  11. NEWTON’S THIRD LAW OF MOTION • If one object exerts a force on another object, then the second object exerts an equal force (but in the opposite direction) on the first object. • These forces can be thought of as action and reaction forces.

  12. Examples: Rocket Action force – rocket pushing out gases Reaction force – gases pushing back against rocket Gun Action force – powder and gun push against bullet Reaction force – bullet pushes back against powder and gun Canoe Action force – paddle pushes against water Reaction force – water pushes back against paddle and canoe

  13. Momentum Momentum = mass (velocity) The greater the momentum, the harder an object is to stop. Momentum can be increased by increasing velocity or mass. Which football player will be hardest to stop? • Light and slow • Heavy and slow • Light and fast • Heavy and fast

  14. Law of conservation of momentum – the total momentum of any group of objects remains the same unless outside forces act on the objects. (friction is a force). This is used to analyze collisions. Examples: Before After 1 kg 0 m/sec 2 kg 2.5 m/sec 1 kg 5m/sec 2 kg 0 m/sec Momentum = 1 kg (5m/sec) Momentum = 5 kg(m) /sec Momentum = 2 kg (2.5 m/sec) Momentum = 5 kg(m) /sec

  15. Before Shot bullet and gun momentum = 0 After Shot Gun Bullet In opposite directions Momentum = 1 kg (10 m/sec) Momentum = 10 kg(m)/sec Momentum = .01 kg (1000 m/sec) Momentum = 10 kg(m)/sec

  16. Collision of bullet and clay Bullet Clay M = .01 kg (1000 m/sec) M = .99 kg (0m/sec) M = 10 kg(m)/sec M = 0 After Collision Bullet in Clay Momentum = (.01 kg + .99 kg) (10 m/sec) Momentum = 10 kg(m)/sec

  17. Satellites What keeps a satellite in orbit? Imagine you are standing on the top of a mountain and can throw a rock as fast as you want. Can you put the rock in orbit by throwing the rock straight out? How? Draw a diagram and explain the forces involved. If the velocity is perfect the drop of the rock due to gravity matches the curvature of the earth.

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