Chapter 6 Forces Force - A push or a pull on an object.

The symbol F is used to represent the force. It is a vector quantity which means it has both direction and magnitude.

Contact Force - A force that acts on an object only by touching it.

Long Range Force - A force exerted without contact.

Four Fundamental Forces of Nature

1. Gravity2. Electromagnetic 3. Strong Nuclear4. Weak interaction

Chapter 6 Forces

Chapter 6 Forces

The agent is the specific, identifiable, immediate cause of the force.

Chapter 6 Forces

Free Body Diagram

Chapter 6 Forces

Chapter 6 Forces

Force Symbol

Definition

Friction Ff The contact force that opposes motion

Normal FN The contact force exerted by the surface of an object

Spring Fsp A restoring force

Tension

FT The pull of a string or rope

Thrust Fthrust A force that moves objects

Weight Fg A force due to gravitational attraction

Chapter 6 Forces

1. An object at rest or in motion will stay that way unless acted upon by an unbalanced force.

2. The acceleration on an object is directly proportional to the fore and inversely proportional to the mass. a=F/m

3. For every action there is an equal and opposite reaction.

Newton’s 3 laws

Chapter 6 Forces Inertia - The tendency of an

object to resist motion.

Equilibrium - When the net force on an object is zero.

Equilibrant - The force equal to and opposite to the resultant.

Chapter 6 Forces

Mass vs Weight

Mass is the amount of matter in an object and weight is the force of gravity acting on the object. Mass never changes.

Chapter 6 Forces

Newton A unit of force. F = ma = kg m/s2

To change mass into weight, multiply the mass by 9.8 m/s2

Chapter 6 ForcesWhat force is required to accelerate an automobile weighing 20,000 N from 40 m/s to 70 m/s in 10 seconds?

m

Wg

20000 N9.8 m/ s2 2041kg

NaF 6123)/3)(2041( 2 smkgm

t

vva if

s

smsma

10

/40/70

2/3 sma

Chapter 6 ForcesNet Force: Thesum of all the forces

Chapter 6 Forces

Chapter 6 Forces

a. the mass of the football.b. the net force.c. the acceleration of the

football.

A force of 100 N is exerted by a punter on a football that weighs 45 N. Find:

a. m

Wg

45N

9.8m/ s2

b. Net Force = Fup- Fdown= 100 N - 45 N = 55 N up

c. a

Fnet

m

55N4.6kg

12m/ s2

= 4.6 kg

Joe notblocking

•Friction: A force that opposes the direction of motion. •Static Friction: The force on one surface by the other when there is no relative motion between the two surfaces.•Kinetic Friction: The force on one surface when the surfaces are in relative motion.

Chapter 6 Forces

•Kinetic Friction Force Ff, kinetic= µkFN

•Static Friction Force 0< Ff, static < µkFN

Chapter 6 Forces

Surface µs µk

Rubber on concrete .80 .65

Rubber on wet concrete .60 .40

Wood on wood .50 .20

Steel on steel (dry) .78 .58

Steel on steel (with oil) .15 .06

Teflon on steel .04 .04

Coefficients of Friction

Chapter 6 Forces

Chapter 6 Forces

Chapter 6 Forces

A 50-kg box is pushed across a wooden floor at 2 m/s. How much force is exerted on the box?

Kinetic Friction Force Ff, kinetic= µkFN

m = 50 kgFN = 490 Nµk = .2

Ff = (.2)(490 N) = 98 N

Chapter 6 Forces

A force of 60 N accelerates a 15 kg block at 3 m/s2 along a horizontal surface.

a. How large is the frictional force?b. What is the coefficient of friction?

a. Fnet = Fapplied- Ff

(15 kg)(3 m/s2) = 60 N - Ff Ff = 15 N

Ff Fapplied

b. µs = Ff/FN

µs = 15 N/(15 kg*9.8m/s2) = .102

Chapter 6 Forces

•Air Resistance The force on a moving object when moving through air.

•Terminal Velocity The fastest something can fall because of air resistance.

•Simple Harmonic Motion (SHM) Motion that returns an object to its equilibrium position as a result of a restoringforce that is directly proportional to its displacement.

Chapter 6 Forces

Chapter 6 Forces

Chapter 6 Forces

T 2

lgSimple

Pendulum

T period (s)l length (m)g acceleration due to gravity 9.8 m/s2

Find the period of a pendulum witha length of 2 meters.

T 2

lg

T 22 m

9.8 m/ s2 T = 2.84 m

Chapter 6 Forces