1. Forces when you throw a football or kick a soccer ball you use a force
force – a push or a pull that one body exerts on another
not all forces are noticeable like a ball being kicked
the floor exerts a force on your feet
the atmosphere exerts a force on your body
gravity pulls your body downwards
forces cause motion of objects to change
think of a baseball getting hit by a bat – the ball's motion & its velocity changes

2. Forces forces do not always change velocity
when two or more forces act on an object at the same time, the forces combine to form the net force
net force – the sum of all of the forces acting on an object
if net force equals 0, then forces are balanced
if net force does not equal 0, then forces are unbalanced

3. Forces
forces have direction, so they follow the same rules for adding as displacement
SI Unit for force is the Newton (N) (kgm/s2)
Ex. it takes about 3 N to lift a full can of soda
insert Table 1 – pg. 73

4. Forces
if equal forces are exerted on an object at opposite sides, the net force on the object is zero
If the forces are equal in size & opposite in direction & the motion of the object does not change the forces are called balanced forces
balanced forces do not cause a change in motion
insert Fig. 2c – pg. 73

5. Forces
when two things are pushing with unequal forces in opposite directions – a net force occurs in the direction of the larger force
this causes the object to move in the direction of the net force
the net force will be the difference of the forces & since they are not equal the net force is unbalanced
insert Fig. 2b – pg. 73

6. Forces
when an object as acted upon by two forces in the same direction, the forces are combined or added together
the net force on the object is found by adding the two forces ether
unbalanced forces cause a change in motion
insert Fig. 2a – pg. 73

7. Friction if you push a ball across a floor, it will not go forever
if it is slowing down it has a negative acceleration
if it is accelerating it must have a force acting on it
the force that slows down & stops the ball is friction
friction – the force that opposes motion between two surfaces that are touching each other

8. Friction amount of friction between two surfaces depends on 2 factors
the kind of surfaces
the force pressing the surfaces together

9. Friction
when two surfaces come into contact, the highest bumps of the surfaces come into contact (see Fig. 4)
these areas where the bumps stick together are called microwelds – these are the source of friction
insert Fig. 4 – pg. 74

10. Friction
the stronger the force pushing the surfaces together, the stronger the microwelds will be
to break the microwelds and move one surface over the other, a force has to be applied
insert Fig. 4 – pg. 74

11. Friction imagine you are pushing a box that is so heavy it won’t slide
then the acceleration = 0 & the net force on the box = 0
there must be an opposite force canceling out your push
static friction – friction between 2 surfaces that are not moving past each other
push is not strong enough to break microwelds
insert Fig. 5a – pg. 75

12. Friction you get a friend to help push the box
the box begins to slide, but there’s still some resistance
there’s now enough force to overcome the microwelds
sliding friction – friction that opposes the motion of 2 surfaces sliding past one another – microwelds are constantly broken & reformed
force of sliding friction is usually less than static friction
insert Fig. 5b – pg. 75

13. Friction not all friction is inconvenient
wheels use a special type of static friction to keep moving
rolling friction – the friction between a rolling object & the surface it rolls on
due partly to the microwelds between a wheel & the surface breaking & reforming
rolling friction is much less than static or sliding friction
insert Fig. 6 – pg. 75

14. Gravity
you are always exerting attractive forces on everything all around you (your desk, the building, Mars)
it is the attractive force of gravity
gravity – an attractive force between any two objects that depends on the masses of the objects & the distance between them

15. Gravity gravity is 1 of 4 basic forces – fundamental forces
others are electromagnetic, strong nuclear, & weak nuclear
in 1660s British scientist Isaac Newton looked at the motions of planets to figure out gravity
relationship called the Law of Universal Gravitation – states that the gravitational force increases as the mass of either object increases & as the objects move closer together
insert Fig. 7 – pg. 76

16. Gravity Law of Universal Gravitation explains many things
why we feel Earth’s gravity & not the Sun’s
why we don’t feel our pencil’s gravity
it gives the universe its structure (all stars exert a gravitational force on each other)

17. Weight anytime you are standing still, your acceleration is zero
if your acceleration is zero, then the net force on you must be zero – has Earth’s gravitational attraction for you disappeared? No
Earth is still pulling you downward, but the floor exerts an upward force on you that keeps you from falling down

18. Weight Earth is always going to exert a gravitational force on you
weight – the gravitational force on an object (Fg)
use Fg since weight is the force due to gravity
SI Unit for weight is the Newton (N) (kgm/s2)
m = mass & g = gravity (9.8 m/s2)

19. Weight weight and mass ARE NOT the same
weight is a force & mass is a measure of the amount of matter an object has
the greater an object’s mass, the stronger the force between it & Earth
so the more mass, the more it will weigh
insert Table 2 – pg. 78

20. If you were to go to another planet, the gravity would be different
If you were to go to another planet, the gravity would be different. Your weight would change, but your mass would be the same because the matter in your body would be the same.

21. Weight a mass of 1 kg weighs 1 kg×9.8 m/s2 or 9.8 N
you can calculate the weight of anything in Newtons is you know the mass
1 N = lbs. (1 lb. = N)
on the moon g = 1.6 m/s2
so you would weigh about 1/6 as much on the moon
insert Fig. 9 – pg. 79

22. Weight Example: Calculate your weight in Newtons.
to get your mass in kg multiply your pounds by

23. Newton’s 1st Law of Motion
remember that forces can change an object’s motion
Newton’s 1st Law of Motion – an object moves at a constant velocity unless an unbalanced force acts on it
an object at rest will stay at rest unless a net force acts on it
also called the Law of Inertia

24. Newton’s 1st Law of Motion
inertia – the tendency of an object to resist any change in its motion
inertia is related to the object's mass
the block keeps moving forward since it is not attached to the car
insert Fig. 10 – pg. 80

25. Newton’s 1st Law of Motion
the greater the mass of an object, the greater its inertia
Which would be easier to change the direction of, a moving bowling ball or tennis ball?
harder to change the direction of a moving a bowling ball compared to a tennis ball
bowling ball has more inertia

26. Newton’s 1st Law of Motion
think about a car crash – cars hitting head on
if someone is not wearing a seatbelt they will keep moving forward at the same speed that the car was going
insert Fig. 15a – pg. 86

27. Newton’s 2nd Law of Motion
Newton’s 1st Law described how motion & force are connected
if a force is acted upon an object, it will have a motion with an acceleration

28. Newton’s 2nd Law of Motion
if a bigger force is exerted on an object, the object will have more acceleration
think of throwing versus tossing a ball
insert Fig. 11 – pg. 81

29. Newton’s 2nd Law of Motion
mass will also effect how much speed an object has when a force is applied to it
think of throwing a baseball compared to a softball
softball has more mass & it goes slower with the same amount of force applied to it
softball will also have less acceleration due to its mass
insert Fig. 12 – pg. 82

30. Newton’s 2nd Law of Motion
Newton’s 2nd Law of Motion describes how force, mass, & acceleration are connected
Newton’s 2nd Law of Motion – the net force acting on an object causes the object to accelerate in the direction of the net force

31. Newton’s 2nd Law of Motion
acceleration is determined by the size of the force & the mass of the object

32. Newton’s 2nd Law of Motion
can also use the 2nd Law to calculate the net force on an object
to solve for the net force multiply both sides by the mass

33. Newton’s 2nd Law of Motion
Example: When a tennis player hits a ball with an acceleration of 5,000 m/s2. If the ball has a mass of 0.06 kg, what is the net force exerted on the ball?
known: mass (m) = 0.06 kg, acceleration (a) = 5,000 m/s2
set up problem:

34. Newton’s 2nd Law of Motion
Example: You push a wagon that has a mass of 12 kg. If the net force on the wagon is 6 N south, what is the wagon’s acceleration?
0.5 m/s2 south

35. Newton’s 2nd Law of Motion
when objects fall they are pulled down by the force of gravity
there is another force that acts on the objects
air resistance – a friction-like force that opposes the motion of objects that move through the air
like friction it acts in the opposite direction of the motion of the object

36. Newton’s 2nd Law of Motion
amount of air resistance depends on the speed, size, and shape of the object that is falling
this is why feathers fall more slowly than pennies
if there were no air resistance all objects would fall at the same rate (see Fig. 16 – objects falling in a vacuum)
insert Fig. 16 – pg. 87

37. Newton’s 2nd Law of Motion
to decrease air resistance is to make the object less spread out
see Figure 17
insert Fig. 17 – pg. 88

38. Newton’s 2nd Law of Motion
as speed increases, the force of air resistance increases
as objects fall, they accelerate & its speed increases
the force of air resistance increases until it becomes large enough to cancel the force of gravity
at that point the forces on the object are balanced & the object falls with a constant speed

39. Newton’s 2nd Law of Motion
known as terminal velocity – the maximum speed an object will reach when falling through air
terminal velocity depends on object’s size, shape, & mass
insert Fig. 18 – pg. 88

40. Newton’s 2nd Law of Motion
force of gravity also has a direction associated with it
always downward
when an object is only affected by the force of gravity, it is said to be in free fall

41. Newton’s 2nd Law of Motion
when you measure weight with a scale, you are at rest & the net force on you is zero
so the scale supports you & balances your weight by exerting an upward force (seen by the dial moving)
insert Fig. 19a – pg. 89

42. Newton’s 2nd Law of Motion
now if you were in an elevator that was falling
if you and the scale are in free fall, then you are no longer pushing down on the scale (no weight read on the scale)
insert Fig. 19b – pg. 89

43. Newton’s 2nd Law of Motion
same thing is happening in space
everything in an orbiting space shuttle is falling downward toward Earth (freefall)
they are moving very fast in orbit around the Earth so they do not fall to the Earth
if they (or even the Moon) stopped orbiting the Earth they would fall towards the Earth in freefall
since there is no force in the shuttle to support the objects, they seem to be floating

44. Newton’s 3rd Law of Motion
When you push on a wall – what happens?
nothing
What about if you had roller skates on?
you’ll go backwards
Newton’s 3rd Law explains action–reaction pairs

45. Newton’s 3rd Law of Motion
Newton’s 3rd Law Motion – when 1 object exerts a force on a 2nd object, the 2nd one exerts a force on the 1st that is equal in size & opposite in direction
for every action force there is an equal & opposite reaction force

46. Newton’s 3rd Law of Motion
think of a trampoline
when you jump on a trampoline you exert a downward force on the trampoline
the trampoline then exerts an equal force upward on you

47. Newton’s 3rd Law of Motion
if forces are equal in size & opposite in direction, objects do not move
insert Fig. 13a – pg. 84
if one force is bigger than other, object will move in direction of larger force
insert Fig. 13b – pg. 84

48. Newton’s 3rd Law of Motion
if you threw a baseball while you were on skates, what would happen?
rocket’s work the same way
as the rocket’s engine exerts a force on the gases in the engine (burning fuel) they escape out the back of the rocket
the gases then exert a force on the rocket & push it forward
insert Fig. 24 – pg. 92

49. Centripetal Force
when an object goes around a curve, it changes its velocity (it is changing its direction)
the direction is towards the center of the curve
when the ball accelerates towards the center, an unbalanced force is acting on the ball and pushing it towards the center – known as the centripetal force

50. Centripetal Force
when a car goes around a sharp curve, the centripetal force is the friction between the tires & the road
if the road is slippery the centripetal force may not be strong enough to overcome a car’s inertia – so the car would keep going straight
insert Fig. 21 – pg. 90

51. Force & Momentum
if you caught a fast moving baseball it stings because it exerted a force on your hand
remember from before that acceleration is the difference between the initial & final velocity, divided by time
and from Newton’s 2nd Law that the net force on an object equals its mass × its acceleration

52. Force & Momentum we can now rewrite the 2nd Law to say:
mvf is the final momentum mvi is the initial momentum
equation states that the net force exerted on an object can be calculated by dividing its change in momentum by time

53. Force & Momentum
can use 2nd & 3rd laws to describe what happens when objects collide
momentum of an object will not change unless its mass, velocity, or both change
momentum can be transferred from one object to another
Law of Conservation of Momentum – if no external forces act on a group of objects, their total momentum does not change

54. Force & Momentum
see pool balls – when balls get hit, the momentum of the cue ball has all the momentum & then transfers some of its momentum to the other balls
momentum is conserved