1. Chapter 3 Forces & Newton’s Laws

2. Forces Force—a push or pull that one body exerts on another
Forces can be shown as vectors
Vector –an arrow showing the direction and amount of force.
The longer the arrow, the larger the force.

3. Forces cont’d
Net Force—when 2 or more forces act on an object at the same time
Balanced Forces—multiple forces that are equal in size and opposite in direction.
An object will remain still

4. Forces cont’d
Unbalanced Forces—when the size and direction of multiple forces acting on an object are unequal.
cause objects to move.

5. Balanced & Unbalanced Forces

6. Inertia
Inertia—the tendency of an object to resist a change in its motion
The velocity of an object only changes if a force changes it.
The inertia of an object is related to mass
The greater the mass, the greater its inertia.

7. Newton’s 1st Law of Motion
Newton’s 1st Law: The Law of Inertia
An object at rest stays at rest and an object in motion stays in motion with the same speed and direction unless acted on by an unbalanced force.

8. Newton’s 1st Law of Motion

9. Shooting Objects at Rest The object’s inertia causes it to remain still even when a bullet passes through it.

10. Punkin Chunkin: Inertia II

12. Friction
Friction: A force that opposes motion between two surfaces that are touching each other
The amount of friction is dependent upon
Type of Surfaces
Amount of force pressing the surfaces together

13. Types of Friction Static Friction Sliding Friction Rolling Friction
Friction between two objects that are
NOT moving.
Sliding Friction
Friction between two surfaces moving past each other.
Rolling Friction
Friction between a rolling object & the surface it is rolling on.
Fluid Friction
Friction between an object and a gas (like air) or liquid (like water)

14. Air Resistance
Air Resistance: Collisions of the object's leading surface with air molecules.
Dependent upon a variety of factors, most importantly
the speed of the object
Increased speeds = increased air resistance.
the cross-sectional area of the object
The more spread out, the more air resistance

15. Newton’s Second Law Newton’s Second Law as an equation:
Newton’s 2nd Law: The acceleration of an object is dependent upon the force acting upon the object and the mass of the object.
Newton’s Second Law as an equation:
F = m × a   
F = force (N) 
m = mass (kg)   
a = acceleration (m/s/s or m/s2)

16. Force Unit The SI unit for force is the Newton (N)
The Newton is a combined unit
Since F=ma, the unit for force combines the mass unit (kg) with the acceleration unit (m/s2)
N = kg•m/s2

18. Gravity http://www.youtube.com/watch?v=p_o4aY7xkXg
Universal law of Gravity:
Any 2 objects will exert an attractive force on each other
The size of the force is dependant on 2 things
Mass of both objects
Distance between the objects
The range of gravity never disappears
Therefore it is not possible for an object with mass to be weightless.

19. Weight vs. Mass
Weight: Force of gravity pulling you toward the earth (or moon or other object)
F = ma becomes W = mg
Weight = mass × gravitational acceleration
g on Earth is 9.8m/s2
SI Unit for Weight = Newton (N)
Weight can change with a change in location.

20. Mass: A measure of how much matter an object has.
All objects have mass because they have inertia
SI Unit for Mass = Kilograms (kg)
Pounds is a unit of mass NOT weight.

22. Centripetal Force
Centripetal Force: The force exerted toward the center of a curved path.
The smaller the radius, the larger the centripetal force
The radius could be the length of rope or distance between 2 objects
A tighter turn has more force
Video: Roller coaster Loops

23. Gravity & Centripetal Force
Earth’s gravity exerts a centripetal force on the Moon that keeps it in a nearly circular orbit.
Real World/space station & centripetal force

25. Falling Objects on Earth
Near Earth’s surface acceleration due to gravity is 9.8 m/s2
Video: If 2 objects with different masses are dropped from the same height, which would hit the ground first?
Without air resistance, all objects hit the ground at the same time, mass has no effect.
dropping objects on the moon

26. Projectile Motion
Projectile—Anything dropped, thrown, or shot through the air
Because of gravity and inertia, projectiles moving horizontally have a curved path.
An object shot horizontally & one dropped from the same height at the same time, will hit the ground at the same time.
Juggling & Projectile Motion
Mythbusters Bullet Fired and Dropped

27. Monkey and the Zookeeper
The zookeeper must shoot the banana from the banana cannon to the monkey who hangs from the limb of a tree.
This particular monkey has a habit of dropping from the tree the moment that the banana leaves the muzzle of the cannon.
The zookeeper is faced with the dilemma of where to aim the banana cannon in order to hit the monkey.
If the monkey lets go of the tree the moment that the banana is fired, then where should she aim the banana cannon?

28. Monkey and the Zookeeper
Where should the zookeeper aim?
Taking Gravity into effect The zookeeper aims above the monkey
Does the speed of the banana change the effect?
The zookeeper aims at the monkey and shoots the banana very fast
The zookeeper aims at the monkey, yet shoots the banana very slow.
ZOO in SPACE
In a low gravity environment, where should the zookeeper shoot the banana?

30. NEWTON’S 3rd LAW: For every action, there is an equal and opposite reaction
“Action” and “Reaction” are names of forces
Forces ALWAYS occur in pairs
Single forces NEVER happen

31. “Equal & Opposite” In Newton’s 3rd Law, “equal” means: Equal in size
Equal in time.
occur at EXACTLY the same time.
In Newton’s 3rd Law, “opposite” means:
Opposite in direction
forces are EXACTLY 180o apart in direction.
Only forces that act on the same object can cancel.
Newton’s 3rd Law action and reaction forces act on different objects, so they DON’T cancel.

32. Writing Action Reaction Forces
Bowling ball hits the pin to the left.
REACTION
Pin hits the bowling ball to the right

34. Momentum:
A property of how much force is required to change the object’s motion
The object’s mass times its velocity
p = mv
p = momentum
m = mass
v = velocity
SI UNIT = Kg m / s

35. Law of Conservation of Momentum
Momentum may be transferred to another object
The total momentum before a collision is equal to the total momentum after the collision
m1v1 = m2v2

36. Types of Collisions Elastic collisions
the total kinetic energy before the collision is equal to the total kinetic energy after the collision.
Usually the objects bounce off of each other.

37. interactive Inelastic collisions
the total kinetic energy before the collision is NOT equal to the total kinetic energy after the collision.
Usually the objects stick together
interactive

38. The Truck and The Ball A pickup truck is moving with a constant speed.
In the course of its motion, a ball is projected straight upwards by a launcher located in the bed of the truck.
Assume the ball does not encounter a significant amount of air resistance.
What will be the path of the ball and where will it be located with respect to the pickup truck?

39. Many would insist that there is a horizontal force acting upon the ball since it has a horizontal motion.
Yet this is simply not the case. The horizontal motion of the ball is the result of its own inertia.
When projected from the truck, the ball already possessed a horizontal motion, and thus will maintain this state of horizontal motion unless acted upon by a horizontal force.
An object in motion will continue in motion with the same speed and in the same direction ... (Newton's first law).

40. Newton's Law Review
physclips newton's laws