1. Forces: Gravitational, Magnetic, & Electrical

2. Forces between objects act when the objects are in direct contact or when they are not touching. Magnetic, electrical and gravitational forces can act at a distance.

3. Gravity Sir Isaac Newton is credited with the discovery of gravity. Now, of course we know that he didn ’ t really discover the thing – let ’ s face it, people knew about gravity for as long as there have been people. Gravity didn ’ t have to be discovered for crying out loud! Your basic tiny little toddler figures it out in the first few months of life. So what is the big deal with Newton and gravity? Well, what Newton did was to describe gravity, extend its effects from the surface of the earth (which everyone understood) out into space to explain the behavior of the planets (which nobody suspected), and to formulate a mathematical law that accurately described the force of gravity between objects with mass.

4. Gravity Newton ’ s theory is very simple. Gravity is a force of attraction between any two objects that have mass. Two objects sitting on a desktop attract each other with a force that we call gravity. They don ’ t go flying together because gravity is a very weak force and is only significant when one or the other of the masses is enormous – planet size. This is why we aren ’ t attracted to objects that we pass on our daily wanderings.

5. Gravity The size of the force of gravity is given by this equation: G is the universal gravitational constant. m1 is the mass of one of the bodies and m2 is the mass of the other body. r is the distance between the two bodies. The value of G is the same everywhere throughout the universe. Newton ’ s law of gravity

6. Gravity Newton ’ s law of gravity is an inverse-square Law. This means that the force of gravity gets smaller or larger by the square of the distance. The force is directly proportional to the masses, so if the mass of one of the objects doubles, the force of gravity would double. But if the distance doubled, the force of gravity would decrease by a factor of four. That ’ s because it decreases by the square of the distance. Inverse-square laws are very common in physics. We ’ ll see more of them in our explorations.

7. Gravity Universal Law of Gravitation 1.Gravitational force is a field force between two particles -- in all mediums. 2.Force varies as the inverse square of the distance 3.Force is proportional to mass of objects. 4.The gravity force acts from the center of the two objects. 5.The gravitational force is always attractive. 6.The gravitational force cannot be shielded or canceled.

8. Gravity Solving gravity problems is quite simple. Let ’ s do one. A girl, Brandy (42.5 kg), sits 1.50 m from a boy (63.0 kg), George. What is the force of gravity between them? (This will tell us how attracted they are to each other.) You can see that this is a tiny force, one so small that Brandy will never notice its presence. George must generate some other attractive force if there is to be a relationship between the two of them.

9. A field model can be used to explain how two objects can exert forces on each other without touching. An object is thought to have a region of influence, called a field, surrounding it. When a second object with an appropriate property is placed in this region, the field exerts a force on and can cause changes in the motion of the object.

10. In physics, a gravitational field is a model used to explain the influence that a massive body extends into the space around itself, producing a force on another massive body. Thus, a gravitational field is used to explain gravitational phenomena, and is measured in newtons per kilogram (N/kg).

11. Uniform Gravitational Fields We live in what is essentially a uniform gravitational field. This means that the force of gravity near the surface of the Earth is pretty much constant in magnitude and direction. The green lines are gravitational field lines. They show the direction of the gravitational force on any object in the region (straight down). In a uniform field, the lines are parallel and evenly spaced. Near Earth’s surface the magnitude of the gravitational field is 9.8 N / kg. That is, every kilogram of mass an object has experiences 9.8 N of force. Since a Newton is a kilogram · meter per second squared, 1 N / kg = 1 m/s 2. So, the gravitational field strength is just the acceleration due to gravity, g. Earth’s surface continued on next slide

12. Uniform Gravitational Fields (cont.) Earth’s surface 10 kg 98 N A 10 kg mass is near the surface of the Earth. Since the field strength is 9.8 N / kg, each of the ten kilograms feels a 9.8 N force, for a total of 98 N. So, we can calculate the force of gravity by multiply mass and field strength. This is the same as calculating its weight ( W = mg ).

13. Nonuniform Gravitational Fields Near Earth’s surface the gravitational field is approximately uniform. Far from the surface it looks more like a sea urchin. Earth The field lines are radial, rather than parallel, and point toward center of Earth. get farther apart farther from the surface, meaning the field is weaker there. get closer together closer to the surface, meaning the field is stronger there.

14. How are Weight and Mass related?  Mass is the measure of the amount of matter in an object  Weight is a measure of the gravitational force exerted on an object  Weight varies with the strength of the gravitational force, but mass does not

15. How does gravity affect motion? Free fall –occurs when gravity is the only force acting on an object An object in free fall is accelerating because the force of gravity is an unbalanced force

16. Free Fall Near the surface of the Earth the acceleration due to gravity is 9.8m/s 2 For every second an object is falling its speed increases by 9.8 m/s All objects in free fall accelerate at the same rate

17. How does Air Resistance Affect Gravity? Objects falling through air experience a type of fluid friction called air resistance Friction is a force in the opposite direction of motion so air resistance is an upward force Falling objects with greater surface area experience more air resistance In a vacuum there is no air, all objects fall at the same rate of acceleration

18. How does Air Resistance Affect Gravity? Air resistance increases with velocity Eventually the falling object will fall fast enough that the upward force of air resistance will equal the downward force of gravity At this point, the forces are balanced and the objects stops accelerating The object continues to fall at constant speed This is called terminal velocity – when the force of air resistance = weight of the object

19. Terminal Velocity

20. Every object exerts a gravitational force on every other object with mass. These forces are hard to detect unless at least one of the objects is very massive (e.g., sun, planets). The gravitational force increases with the mass of the objects, decreases rapidly with increasing distance and points toward the center of objects. Weight is gravitational force and is often confused with mass. Weight is proportional to mass, but depends upon the gravitational field at a particular location. An object will have the same mass when it is on the moon as it does on Earth. However, the weight (force of gravity) will be different at these two locations.

21. Show video : (Techbook) Field Forces: Gravity: No Contact Between Objects

22. Electric fields exist around objects with charge. If a second object with charge is placed in the field, the two objects experience electric forces that can attract or repel them, depending on the charges involved. Electric force weakens rapidly with increasing distance.

24. Show video(Techbook) Field Forces: Electromagnets

25. Electric Field Hockey Electric Field Hockey https://phet.colorado.edu/en/simu lation/electric-hockey

26. Magnetic fields exist around magnetic objects. If a second magnetic object is placed in the field, the two objects experience magnetic forces that can attract or repel them, depending on the objects involved. Magnetic force weakens rapidly with increasing distance. Magnetic field lines can be seen when iron filings are sprinkled around a magnet.

27. Electricity is related to magnetism. In some circumstances, magnetic fields can produce electrical currents in conductors. Electric currents produce magnetic fields. Electromagnets are temporary magnets that lose their magnetism when the electric current is turned off.

28. Generators convert mechanical energy into electrical energy and are used to produce electrical energy in power plants. Electric motors convert electrical energy into mechanical energy. Motors are in blenders and washing machines. Both motors and generators have magnets (or electromagnets) and a coil of wire that creates its own magnetic field when an electric current flows through it.