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Chapter 11: Forces Chapter 11: Forces

Chapter 11: Forces - PowerPoint Presentation

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Chapter 11: Forces - PPT Presentation

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

forces force object motion force forces motion object moving change objects velocity unbalanced direction balanced friction rest gravity speed constant push law

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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