Chapter 12

1. Chapter 12 Forces and Motion

2. What is a Force? • Force: A push or a pull that acts on an object • Can cause an object to move, or accelerate a moving object by changing speed or direction • We measure force using newtons (N) • Named after Sir Isaac Newton

3. Combining the Force • Much like speed and acceleration, forces can combine • Forces in the same direction add • Forces in the opposite directions subtract • Net Force: Overall force acting on an object

4. Bringing Balance to the Force • The net force can be zero • When this happens, an objects movement is zero • Examples: Tug o’ war Arm Wrestling

5. Unbalanced Forces • When an unbalanced force acts on an object, the object accelerates • Examples:

6. Friction • Friction: Force that opposes the motion of objects that touch as they move past each other. • All objects are subject to some kind of friction • Four types of Frictions: • Static, Sliding, Rolling, Fluid

7. Static Friction • Static Friction: Force that acts on objects that are not moving. • Always act in the direction opposite to the applied force • Examples: Ground, Heavy Item

8. Sliding Friction • Sliding Friction: Force that opposes the direction of motion of an object as it slides over a surface. • Sliding friction is less than static friction, this means it takes less force to keep the object moving then to stop and start over.

9. Rolling Friction • Rolling Friction: Force that acts on rolling objects • Less force than static or sliding • Examples: Dollies, Ball bearings

10. Fluid Friction • Fluid Friction: The force against an object through fluid • Example: Cake Batter • Air Resistance: Fluid friction acting on an object moving through the air

11. Gravity • Gravity: Force that acts between any two masses • Earth’s Gravity acts down toward center of Earth

12. Falling Objects • Gravity causes objects to accelerate down • Air Resistance acts opposite and reduces accel • If an object falls long enough, these two forces equal themselves • Terminal Velocity: constant velocity of a falling object when gravity=air resistance

13. Projectiles • Projectile motion: the motion of a falling object (projectile) after it is given an initial forward velocity. • The combination of a forward velocity and downward force of gravity causes a projectile to take a curved path.

14. Sir Isaac Newton and his Laws • 1st Law of Motion: The state of motion of an object does not change as long as the net force acting on the object is zero • Simplified: An object in motion stays in motion, and an object at rest stays at rest unless acted upon by a force.

15. 1st Law: Inertia • 1st Law also called Law of Inertia. • Inertia: tendency of an object to resist a change in its motion.

16. 2nd Law of Motion • The acceleration of an object is equal to the net force acting on it divided by the object’s mass. • Mass: measure of the inertia of an object and depends on the amount of matter an object has.

17. 2nd Law put into a formula • Acceleration= Net Force/ Mass • A= F/m

18. Conceptual 2nd Law • The acceleration is always in the same direction as the net force. • The net force can be a result when forces act in different directions • Example: Seat Belt

19. Weight and Mass: What’s the difference? • Weight: Force of gravity acting on an object • Weight= mass x gravity • W= mg • Basically Newton’s 2nd Law • Mass is a measure of the inertia of an object • Weight is a measure of gravity on an object

20. Newton’s 3rd Law • When one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object. • For every action there is an equal and opposite reaction

21. Action/Reaction Force Examples • Action-Reaction forces don’t cancel out because the forces do not act the same

22. Momentum • Momentum: product of an object’s mass and its velocity • An object has a large momentum if the product of its mass and velocity is large • When an object is at rest, momentum is zero

23. Momentum Formula • Momentum= Mass x Velocity

24. Conservation of Momentum • Law of Conservation of Momentum: if no net force acts on a system, total momentum does not change • The loss of momentum of one object equals the gain in momentum of another object

25. Electromagnetic Forces • Electric Forces: force that acts between charged objects or particles • Attraction and Repel (Positive and Negative) • Magnetic Forces: force that acts on certain metals, on the poles of magnets, and on moving charges • Magnets can also attract and repel (North and South)

26. Nuclear Forces • Nuclear Force: The force that is strong enough to hold the nucleus together in an atom • Strong Nuclear: Powerful force that acts only on holding neutrons and protons together • Weak Nuclear: Not as strong, acts only on a short range

27. Gravitational Force • Gravitational force: attractive force that acts between any two masses • Every object in the universe attracts every other object • The reason why we don’t see these is because our masses are so small

28. Gravity over distances • The greater the mass of the objects, the greater gravitational force • It is the weakest force but is the most effective force over long distances

29. Earth, Moon, and Tides • The moon has inertia, so it should go in a straight line • But the Earth holds it in place because of gravity • Centripetal force: center-directed force that continuously changes the direction of an object to make it move in a circle • The gravity the moon exerts on the Earth causes bulges in the oceans, causes high tides and low tides

30. Satellites • Satellites work very similar to the moon, by using the Earth’s gravity and centripetal force to stay in orbit • Today, we use satellites from monitoring the weather to getting and receiving cell phone calls