2. FORCES Tahoma Jr. High 8 th Grade Science Maple Valley, WA

3. Force is a push or a pull. In your everyday life, you experience a variety of forces. You apply a force to a ball when you throw it up in the air. As the ball rises, the force of gravity slows it down.wn. Force

4. Force of Gravity Gravity is a force that pulls objects on Earth toward the center of the Earth. Gravity will create a force that would accelerate objects at a rate of 10 m/s 2

5. As the ball falls, gravity’s attractive force makes it fall more rapidly. When you catch the ball, it applies a downward force to your hand – BUT your hand applies an equal upward force to the ball to stop it.

6. There are force meters added to important packages (like big screen TVs) to indicate if they’ve experienced a sudden force change ( measured in “gravities” or “g-forces” or just “g’s”).

7. Newton Units (Force units) Forces are measured in a unit called a Newton (named after Sir Isaac Newton of course) A Newton is defined as: the amount of force required to accelerate a 1 kg mass 1 m/s 2 (not including friction)

8. This means if you have an object with a mass of 1 kg (with no friction) and you apply a force of 1 N, it will be moving 1 m/s at the end of 1 second, 2 m/s at the end of 2 seconds, 3 m/s at the end of 3 seconds, etc.

9. We can use the amount of force and the mass to figure out the acceleration. Newton’s 2 nd Law of Motion is a mathematical formula. F = ma (F = m x a) Force = mass times the acceleration or using algebra: a = F / m acceleration = Force divided by mass or algebra again: m = F / a mass = Force divided by acceleration

10. If an object weighed 5 kg, it would exert a force of 50 Newtons on the floor (it is “accelerating into” the floor – which is pushing back with equal force). F = m x a F = 5 kg x 10 m/s 2 = 50 N

11. FORCE How much Force an object has depends on its mass (how “big” it is) and acceleration (how fast it’s moving) F = m × a SMALL going FAST = BIG going SLOW

13. An object does not require a force to continue moving (space probe traveling through deep space with motor off) An object does require a force to change its motion!!!

14. The Force of Friction Friction is a force that opposes motion between two surfaces in contact. The amount of friction depends on: a. kinds of surfaces in contact b. amount of force pressing surfaces together (rubbing hands together pressing hard compared to just lightly pressing)

15. Friction occurs when irregular (rough) surfaces move against each other.

16. The net force is what’s found by adding the forces together. The green force to the left can be thought of as a negative force ( in a car the mechanical and air friction ) because it is in the opposite direction of the blue ( the force of the engine pushing forward ). Net force +15 -9 +6 NET FORCE (unbalanced forces) The remaining red net force would cause the object to accelerate (more engine than friction).

17. Forces occur in certain directions – and by adding a direction it is now called a “VECTOR”. Below since both vectors are the same size but simply in opposite directions, the net force must be zero. = 0 these arrows are called vectors (vectors include direction, not just overall force)

18. Lets take a look at three cars and the forces that are applied

19. If all three cars weight the same, it appears that the blue car has a greater force applied than the red car

20. The green car appears to have a smaller force applied in the beginning but the force is increased over time.

21. Work and Power

22. WORK How much Work is done, depends on how much Force is applied across a certain distance W = F × d

23. POWER How much Power there is depends on how much Work is done over a certain time P = W t Dragsters do a lot of work at a fast rate – they are VERY powerful !

24. HOW THEY RELATE TO EACH OTHER You can see the progression from simple to more complex below: F = m × a W = F × d P = W t FORCE (how much needed to do Work) WORK (how much needed to show Power) POWER (how much work done how quickly)

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