Force

Chapter 6

Force•Any push or pull exerted on

an object

System•The object with

the force applied

Environment•The world

surrounding the object

Contact Force•A force that acts

on an object by touching it

Contact Force•A baseball bat striking a ball

Long-range Force

•A force that acts on an object w/o

touching it

Long-range Force

•The force of gravity

Agent•Whatever is

causing the force

Inertia•The resistance to

change (in motion)

Equilibrium•When the net forces acting on

an object = zero

Force Vector Diagram

•A Diagram showing the vectors of all forces

acting on an object.

Force Vector Diagram

WeightWeighton tableon table

Force ofForce oftable ontable onthe ballthe ball

Draw Force Vector Diagrams of:

1)A book on a desk

2)A book being pushed across the desk

3)A book falling

Newton’s Laws of Motion

Newton’s 1st LawAn object will remain

at rest or in constant straight-line motion if

the net force acting on it is zero

Newton’s 1st LawThe velocity is

constant and acceleration is zero

when the net force on an object is zero

Newton’s 2nd LawThe acceleration of an

object is directly proportioned to the

net force applied to it

Newton’s 2nd Law

Fnet

maa = =

Newton’s 2nd Law

Fnet = ma

Newton’s 3rd LawFor every action, there is an equal & opposite reaction

Newton’s 3rd Law

FA on B =

-FB on A

Two horizontal forces of 23.5 N & 16.5 N are acting in the same direction on a 2.0 kg object. Calculate: 1) net Force on the object2) its acceleration

Two horizontal forces of 23.5 N & 16.5 N are

acting in opposite directions on a

2.0 kg object. Calculate:1) net force on the object2) its acceleration

Forces of 4.0 N west & 3.0 N north are acting on a 2.0 kg object. Calculate:1) net Force on the object2) its acceleration

Calculate the acceleration of a 1500

g object falling towards Earth when

the Fair friction is 11.7 N.

List Newton’s Laws of Motion

Types of ForcesFriction Tension

Normal Thrust

Spring Weight

Friction (Ff)• The contact force that acts to

oppose sliding motion between surfaces

• Its direction is parallel & opposite the direction of sliding

Normal (FN)•The contact force exerted by a surface on an object

•Its direction is perpendicular & away from the surface

Spring (Fsp)• A restoring force, or the push

or pull a spring exerts on an object

• Its direction is opposite the displacement of an object at the end of a spring

Tension (FT)• The pull exerted by a string,

rope, or cable when attached to a body & pulled taut

• Its direction away from the object & parallel to the string at the point of attachment

Thrust (Fthrust)• A general term for the force

that moves rockets, planes, etc

• Its direction is the same direction as the acceleration of the object barring any resistive forces

Weight (Fg)• Force due the gravitational

attraction between two objects like an object & the Earth

• Its direction is straight down towards the center of the Earth

Name & describe the 6 types of forces

Weight (Fg)

Weight = Fg = mag = mg

Fg = W = mg

When an object is launched, the only

forces action upon it are the forces gravity

& air friction.

No net force is required to keep an object in motion. Frictional forces oppose motion.

Inertia is not a force, but the resistance to the change in motion

or momentum.

Air exerts huge & balanced frictional forces on an object. When in motion, the net Ff of air is large.

Terminal Velocity•The constant velocity that is reached when the force of air friction of a falling object equals its weight

Friction (Ff)

Kinetic frictional force

Ff, kinetic

Static frictional force

Ff, static

Draw Vector Force Diagrams of:

1) a skydiver gaining downward velocity

2) a skydiver at terminal velocity

Draw Vector Force Diagrams of:

3) a rope pulling a ball up at constant velocity

4) a rope acceleration a ball upwards

An object’s weight on Earth is 490 N. Calculate:1) its mass2) its weight in the moon where gmoon = 1.60 m/s2

An 500.0 g object on an unknown planet has a

weight of 250 N. Calculate the acceleration

caused by the planet’s gravity.

Static Ff

•The force exerted on one surface by

another when there is no relative motion

Kinetic Ff

•The force exerted on one surface by

another when in relative motion

Forces acting on an object:

FFNN = -W = -W

FFAA > F > Fff

FFappliedapplied

FFgg or Weight or Weight

FFff

FFNN

Static Ff

Ff, static = sFN

s is proportionality constant called the

frictional coefficient

Kinetic Ff

Ff, kinetic = kFN

A 25 N force is required to pull a 50.0 N sled down the

road at a constant speed. Calculate the sliding frictional coefficient

between the sled & the road.

A person & a sled have a total weight of 490 N. The

sliding frictional coefficient between the sled & the snow is 0.10. Calculate the force required to pull the sled at

constant speed.

Calculate the acceleration of the sled if the applied force pulling on the sled

is 299 N.W = 490 N = 0.10

Calculate the force required to pull a 500.0

g block with an acceleration of 3.0 m/s2.

= 0.50

Periodic Motion•Repetitive or

vibrational motion like that of a spring, swing or pendulum

Simple Harmonic Motion

•Periodic motion in which the restoring force is

directly proportional to the displacement

Period (T)•The time required

to complete one full cycle of motion

Amplitude•Maximum

displacement from the zero point or

equilibrium

Pendulum Motion Formula

T = 2 ----llaagg

Calculate the period of a

pendulum with a length of 49 cm:

Calculate the length of the pendulum of a

grandfather clock whose period is equal

1.0 second:

Fundamental Forces•Gravitational•Electromagnetic•Strong Nuclear•Weak Nuclear

Calculate the force required to pull a 150 g

block at a constant velocity of 180 km/hr. = 0.20

A 9.8 kN car went from 0 to 25 m/s in 5.0 s. between car & road =

0.20. Calculate the force applied by the engine of

the car.

Calculate the force required to start a 2.0 kg block & its acceleration

when moving.s = 0.20, k = 0.10

Calculate the force required to start a 2.0 kg

block & calculate its acceleration when

moving.s = 0.20, k = 0.10

A 6.0 kg ball is attached by a rope over a pulley

to a 4.0 kg ball.1) Draw the problem.2) Calculate each ball’s

acceleration

A 6.0 kg ball is attached by a longrope over a

pulley to a 4.0 kg ball.1) Calculate air friction

at max velocity

A 65 kg boy & a 35 kg girl are in a tug-of-war. The girl’s acceleration is 13 cm/s2. Calculate the boy’s acceleration.

A 150 g baseball, was hit & came to rest in 4.0 s after going 100.0 m.

Calculate: vi, a, & Ff on the ball.

A 50.0 kg box falls off a 0.49 km cliff.

1) Calculate vi, vf, a, & t.2) Calculate Ff if air

friction is included

A 10.0 kg box falls off a 0.49 km cliff & hits the

ground in 20.0 s. 1) Calculate vf & a.2) Calculate Ff if air

friction is included

Calculate the force required to pull a 250 g block at a constant

velocity of 360 km/hr. = 0.30

Calculate the force required to accelerate a 1500 g block along the floor at 3.0 m/s2.

= 0.25

Calculate the apparent weight of a 50.0 kg

person on a scale on an elevator descending at

2.0 m/s2.

Calculate the apparent weight of a 50.0 kg person on a scale on an elevator

ascending at 2.0 m/s2.

Calculate the period of the pendulum on

Big Ben which is 4.9 m long.

Calculate the force required to accelerate

a 10.0 kg block straight up at

25 cm/s2.

Calculate the force required to accelerate

a 50.0 kg block straight up over a pulley at 5.0 m/s2.

Calculate the acceleration of a system of a 55.0 kg block tied

to a 45.0 kg block hanging over a pulley.

Calculate the frictional coefficient of a 100.0 kg block if a 150 N

force causes it to accelerate at 50.0 cm/s2.

Calculate the frictional coefficient

of a 10.0 kg block if a 98 N force causes it to slide at 30.0 cm/s.

A 5.0 N force accelerates a 1000.0 g block at 45.0 cm/s2.

Calculate K.

Calculate the acceleration of a system of a 200.0 kg cart on a plane tied to a 50.0 kg block hanging over a

pulley.