Force and Motion

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**1. **Force and Motion Newton’s Laws

**2. **Development of Ideas of Motion and Inertia Aristotle’s Ideas
Motion was in two forms
Natural-- straight up or down on earth, circular in
heavens
Violent--result of forces making things move
The natural state of things was to be at rest
Motion only happens if a continual force makes it
happen
Basis of the Geocentric theory

**3. **Copernicus Stated that earth was in motion
Based on observations from astronomy
Galileo
Showed that friction causes slowing and stop of motion
Friction is caused by surfaces in contact
Without friction, moving object would not stop
Force is needed only to overcome friction

**4. **Galileo (cont.) Developed idea of inertia
Every material object has a resistance to
change in its state of motion
At rest, stays at rest
In motion, keeps its velocity
Earth demonstrates inertia in revolution, no force pushes it around the sun and none is needed since there is no friction

**5. **The Principia Based on work done by Galileo and Kepler
Principle of Inertia - Galileo
Principle of Inverse Square Force - Kepler
Attraction of sun on planet seemed to be inversely
proportional to the distance squared
Newton expanded on both of these ideas

**6. **Force Generally, a push or pull
Can cause motion, but may not
Vector quantity, with direction
Total sum of forces on a system is Net Force
Two types: Contact forces and
Fundamental forces

**7. **Fundamental Forces Gravity - attraction between bodies due to mass
Electromagnetic - attraction/repulsion in electricity and magnetism
Strong Nuclear Force - hold atomic nucleus together
Weak Nuclear Force - interactions of subnuclear particles

**8. **Inertia - The 1st Law Every object continues in its state of rest or of motion in a straight line at constant speed unless it is compelled to change that state by forces exerted upon it.
Examples
Magician pulls tablecloth from under dishes
Air hockey games at arcades
Voyager and Pioneer spacecraft

**9. ** Mass-a Measure of Inertia What is mass?
Not volume
Volume is space taken up by object--cm3 or L—m3 in Phyz
Mass is the amount of matter contained by object-- kg or g
Compare pillow with car battery

**10. ** Mass-a Measure of Inertia Not weight
Weight depends on gravity--more gravity more weight
Mass is in object no matter the gravity-bowling ball in space
Weight and mass are proportional--more mass makes more weight

**11. **Inertia and the Moving Earth Bird catching worm—should miss by 30 km
Inertia says bird, tree, worm all moving at 30 km/s--no relative motion
Ancients had no fast motion, thus did not see inertia

**12. **Newton’s Second Law of Motion Force and Acceleration Force Causes Acceleration
Single force causes a start or change in motion-- acceleration
Multiple forces may act together which add or subtract to make net force
Amount of acceleration is proportional to net force

**13. **Fnet=ma Acceleration depends both on mass and force jointly
“The acceleration produced by a net force on an object is directly proportional to the magnitude of the net force, is in the same direction as the net force, and is inversely proportional to the mass of the body”
With consistent units Fnet--> Newtons m--> kg
a--> m/s2 the equation becomes exact
acceleration = net force or a = F net
mass m

**14. **Example A prospector pushes a 2030 kg cart with a horizontal net force of 700N for 5.0sec. If the cart starts from rest, how far will it go during the time the force is applied?
F = 700N m = 2030 kg t = 5.0 sec v0 = 0
x = ? Find a a = F/m = 700/2030 m/s2
x = v0t +1/2 at2 = 0 + (0.5) (700/2030) (25)
x = 4.3m

**15. **Homework #1 p. 104 ff 1, 3, 5, 7, 9 (F = ma)

**16. **Weight The pull of gravity on an object is a force
w = mg
Weight is offset by the surface a body rests upon
If the surface is parallel to the ground, the offset force balances the weight force
If the surface is at an angle, only (w cosq) of the weight force is offset, and the object will have acceleration.

**17. **Example A furniture van has a smooth ramp for making deliveries. The ramp makes an angle q with the horizontal. A large crate of mass m is on the ramp. What is its acceleration if the surface is frictionless?
w= mg

**18. **Action and Reaction - The 3rd Law For every action there is an equal and opposite reaction
Every object which touches another has an equal touch upon itself.

**19. **Interactions Interactions Produce Forces
Force applied causes motion - hammer to nail
Opposing force causes slowing - nail to hammer
Law of Action and Reaction
Whenever one object exerts a force on a second object, the second objects exerts an equal and opposite force on the first

**20. **Force Denomination Are called action and reaction arbitrarily
Identifying the forces may be complex
Gravity - What is action and reaction for a falling rock?
If we identify action force by relating objects, A and B, action can be discovered
Earth (A) pulls rock (B) & Rock (B) pulls earth (A)

**21. ** Action and Reaction on Different Masses Earth does accelerate to meet falling rock,
but since its mass is so large, its
acceleration is very tiny

**22. **Action and Reaction on Different Masses Firing rifle, bullet causes reaction at rifle
Acceleration is different because of mass difference
Bullet acceleration large, mass small
a= F
m
Rifle acceleration small, mass large
a = F
m

**23. **Action and Reaction on Different Masses Rocket propulsion is similar - gases forced out
cause motion of rocket in opposite direction

**24. **Example of Use A 68 kg passenger rides in an elevator which is accelerating upward at 1.0m/s2. What force is exerted by the passenger on the floor?
a = -1.0 m/s2 g = -9.8 m/s2 m = 68 kg
Passenger is accelerating downward when elevator goes up.
Fnet = F - mg - ma
F = ma + mg
= 68 (-1) + 68 (-9.8) = -730 N
Passenger’s actual weight force = 68 (-9.8) = -670 N

**25. **Weightlessness If the elevator was accelerating downward, the passenger’s weight force would be reduced, the opposite of going upward.
If acceleration = g, then weight is cancelled.
In a spacecraft, the value of g toward earth for both the vehicle and occupants are the same, so weightlessness also occurs.

**26. **Applications of Action and Reaction Action and Reaction Don’t Cancel
Action is on one object, reaction on another
Forces only cancel if on same object

**27. ** Horse - Cart Problem Horse pulls on cart, cart pulls on horse - Why no cancel?
The horse is not just acting on the cart, but the ground as well
Action of horse on ground produces reaction of ground on horse, pushing horse forward with cart

**28. **External vs. Internal Forces between horse and cart are internal which doesn’t affect motion
Forces between horse + cart and ground are external and do produce motion
Pushing on dashboard of your car from inside is useless - internal
Pushing on car from outside interacts with ground – external

**29. **Applications of Newton’s Laws

**30. **Example 4.8

**31. **Example 4.9

**32. **Example 4.10

**33. **Homework #2 p 105 ff 11, 13, 20, 21, 22

**34. **Opposing Forces and Application of Forces Friction
Friction is a force which acts opposite to motion
Caused by irregularities in surfaces in contact
Amount depends on the materials involved
Occurs in fluids as well as in solids
Viscosity in liquids
Air resistance in air (similar in other gases)
Friction causes the need for applied force to keep constant velocity

**35. **Friction (cont) Value of friction can be calculated using the coefficient of friction, m , which depends on materials of object and surface
Ff = m FN where FN is the “normal force.”
Discussed earlier, and is equal to (w cos q)
Values of m are tabulated in books.

**36. **Example 4.11

**37. **Homework #3 p107ff 25, 26, 28, 30, 33

**38. **Equilibrium Static - no velocity of the object
Dynamic - constant velocity of the object
Both have Fnet = 0 and a = 0

**39. **Force in Statics Forces can be balanced and thus produce no motion
Book on a table has two forces acting upon it
Support force (of the table) upward (FN)
Gravity downward
Hanging from ropes --gravity downward, support upward divided by number of ropes

**40. **Example 4.12

**41. **Example 4.13

**42. **Example 4.14

**43. **Homework #4 p. 108 ff 41, 44, 45, 48, 52
Homework #5
p. 110ff 54, 56, 60, 69, 70