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Forces and Motion

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  1. Forces and Motion • Chapter 23

  2. What is motion? • Motion- an object changing position

  3. Types of Motion • There are 2 types of motion: • Distance • Displacement

  4. Distance • Distance is the total path traveled. Look at the map- Is the dotted line or the straight line distance?

  5. Displacement • This is the distance and direction between the starting and ending point. • It is the straight line distance between start and finish.

  6. Relative Motion • How do you know what direction something is? • Is Florida north or south? • Is Alaska north or south?

  7. Relative Motion • Motion must have a reference point. This means you must compare the motion of an object to something else.

  8. Describing Motion • There are 3 main ways to describe the motion of an object. • Speed • Velocity • Acceleration

  9. Speed • Speed is distance traveled divided by the time taken to travel the distance. • speed (in meters/second)= distance/time • s= d/t

  10. Speed • Constant Speed= the speed at any given time will be the same. • When can we achieve this? • Changing Speed- When your speed is changing, not constant. We call this instantaneous speed. • What measures this for us?

  11. Calculating Speed • Here is a simple way to solve speed equations! • Practice Problems!!!

  12. Velocity • Velocity= speed with a direction • 15 mph NE • It is calculated just like speed except with a direction!

  13. Acceleration • Acceleration is the change in velocity. • Acceleration can occur 3 ways: • Speed Up • Slow Down • Change Directions

  14. Calculating Acceleration • To calculate acceleration you must know: • Final Speed • Starting or Initial Speed • Time

  15. Acceleration Equation • The equation looks like this: • a= sf - si t

  16. Forces • An object motion changes because of forces. • A force is a push or a pull • A force has a size and a direction. • Why must we have both?

  17. Forces • Contact Forces- a force that is exerted when two objects are touching each other. • Long range forces- a force exerted when the objects are NOT in contact. • Can you think of any examples. • Examples- Gravity, Magnetism, Electricity

  18. Unit of Forces • Forces are measured in newton (N). • It is named after Isaac Newton. • One Newton is equal to the force it takes to lift half a cup of water.

  19. Newton’s First Law of Motion • This law states: “An object remain at rest or move in a straight line with constant speed unless it is acted upon by a force”

  20. Inertia and Mass • The first law is often called the law of inertia. • Inertia= an object’s tendency to resist a change in motion. It wants to keep moving or sit still. • What causes inertia? • inertia=mass

  21. Adding Forces • Motion depends upon the size and direction of all forces acting on an object. • Balanced Forces- Forces that are equal but in opposite directions. • No motion will occur

  22. Unbalanced Forces • Unbalanced Forces- Forces of an object are not equal and opposite. • Motion will occur in the direction of the largest force!

  23. Newton’s Second Law • This is used to predict where and how motion will occur!

  24. Newton’s Second Law • This law states: “An object acted on by an unbalanced force will accelerate in the direction of the force.”

  25. Newton’s Second Law • Newton’s Second Law says that the acceleration of an object can be calculated by the following: Acceleration= force (N) mass (kg)

  26. How can you use the Second Law? • You can determine the acceleration of an object if you know the mass and the force. • OR • You can determine the force that is needed to make an object accelerate if you know the acceleration and mass!

  27. Gravity • Every object exerts an attractive force on every other object! • Gravitational Force- the attractive force between two objects, it depends on two things: • Mass (More Mass=More Gravity) • Distance (Closer Together=More Gravity)

  28. Gravity • The acceleration due to gravity is ALWAYS 9.8 m/s2

  29. Mass vs. Weight • What is the difference between mass and weight?

  30. Mass • The amount of matter in an object • This cannot change!

  31. Weight • This is the force of gravity on an object! • This can change... • On Earth, 150 lbs.

  32. On other planets • Mercury- 56.7 Jupiter- 394 • Venus- 136 Saturn- 179 • Moon- 24 Uranus 133 • Mars- 56.5 Neptune- 168.7

  33. Friction • Friction- a force that resists sliding motion between surfaces that are touching. • There are 4 types of friction.

  34. Static Friction • Static friction is keeps an object at rest.

  35. Sliding Friction • This is the friction that occurs while an object is in motion. • It typically slows down objects. • When would you want to increase or decrease sliding friction? • How would you do this?

  36. Rolling Friction • Rolling Friction allows a wheel to roll on a surface. • Walking is an example of rolling friction.

  37. Air Resistance • This is also called fluid friction. • Molecules in the air collide with forward moving objects. This will slow down motion. • This is called air resistance. • It acts as a force in the opposite direction of motion.

  38. Newton’s 3rd Law • This law states- “Forces always act in equal but opposite pairs” • You probably know it better, “For every action there is an equal and opposite reaction!”

  39. Action and Reaction Forces • Any time there is motion there are 2 forces! • When you jump, what are the forces? • You push on the ground, it pushes up on you.

  40. Using the 3rd Law • These are not balanced forces. • These forces act on different objects! • When you kick a ball a force is put on your foot and an equal one on the ball. • That is why the harder you kick the more it hurts.

  41. Think about this... • If Newton’s 3rd Law is true, then why don’t we slide backwards when we push a door closed? • Because of friction. When the door pushes you it is also pushing on the Earth!

  42. What about Gravity? • Do we exert the same force on the Earth as the Earth’s gravity exerts on us? • So why don’t we move when we jump into a swimming pool? • Because we have so little mass!