111 Forces change motion 112 Force and mass determine acceleration 113 Forces act in pairs 114 Forces transfer momentum Warmup Questions T or F Speed includes direction while velocity does not ID: 429385
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Slide1
Chapter 11: Forces
11.1: Forces change motion
11.2: Force and mass determine acceleration
11.3 Forces act in pairs
11.4 Forces transfer momentumSlide2
Warm-up Questions (T or F)
Speed includes direction, while velocity does not
A moving object covers the same distance in less time if its velocity is greater
Acceleration measures only change in speedSlide3
11.1 Forces change motion
A force is a push or a pull
Ex: pitcher uses force to set the ball in motion, the batter uses force to change the direction of the pitched ball’s motion, and the fan stops the ball’s motion (using force)
Forces change the motion of objectsSlide4
Types of Forces
Contact Forces: when one object pushes or pulls another object by
touching
itSlide5
Types of Forces
Gravity: force of attraction between two masses
F
is the force of attraction between two objects
G
is the universal gravitational constant;
G
= 6.67*10-11 N-m²/kg². The units of
G
can be stated as
Newton meter-squared per kilogram-squared or Newton square meter per square kilogram. M and m are the masses of the two objects r is the distance between the objects, as measured from their centers GMm/r² is G times M times m divided by r-squared Slide6
Types of Forces
Friction: force that resists motion between two surfaces that are pressed together
Is in the opposite direction of motion
(more on gravity and friction in chapter 12)Slide7
Forces – Push and Pull
Size and Direction of Forces
Velocity is a vector…and so is force
Has size and direction
Balanced and Unbalanced Forces
Net force: the overall force acting on
an
object when all the forces are combined
Knowing size and direction of all forces allows you to predict changes in the object’s motionSlide8
Forces: Balanced and Unbalanced
If the net force on an object = 0, the forces acting on it are
balanced
Same effect as no forces at all
Only
unbalanced
forces can change the motion of an object
It doesn’t matter whether the objected started at rest or was already movingSlide9
Forces – Balanced and Unbalanced
Examples: parachutist moving at a constant speed
The forces acting on him/her
are
balancedSlide10
Balanced Forces
Raindrop Terminal Velocity
When the object is falling, the force of gravity is directed towards the ground and the drag force directed upwards
As the body moves faster, the drag force becomes larger and larger until it equals the force of gravity and the object then falls at a constant velocity
F
drag
F
gravitySlide11
Forces on Moving Objects
If the forces are balanced, an object can be moving at a constant velocity
Ride a bike: force you put in = force of friction
An unbalanced for is needed to change the object’s motion
To increase bike’s speed, may exert more forward force (pedal harder)
To turn, lean to one side – unbalancing the force
To stop: force of friction from brakesSlide12
Newton’s first law relates force and motion
Mid-1600’s: three laws of motion to help describe and predict motions of objects
Built on Galileo’s work, revising the ancient Greeks (push a book and it stops moving because you stopped pushing)
Galileo’s thought experiment
In the absence of friction, a moving object with continue moving even if there is no force acting on it
Meaning:
it does not take a force to keep an object moving…it takes a force to stop an object that is already moving (friction)
Objects at rest and in motion both resist changes in motion
An object at rest is no different than a moving object, except an object at rest has zero velocitySlide13
Newton’s First Law of Motion
Restating Galileo’s conclusions:
Objects at rest remain at rest, and
objects in motion remain in motion with the same velocity
,
unless acted upon by an unbalanced force
Examples?
Anything that changes the motion of an object
Passenger still moving forward at the speed the car was moving
The windshield provides the unbalanced force decreasing the driver’s forward motion
Wear your seat beltSlide14
Newton’s First Law of Motion
Inertia: resistance of an object to a change in the speed or the direction of its motion
(law of inertia)
Closely related to mass:
Easier to push/pull and empty box vs full box
Easier to stop/turn an empty wagon than a full one
**harder to change the motion of the object that has more massSlide15