Forces are usually divided into two types. 1.Contact forces occur because of physical contact between objects. Examples: pushing open a door pulling.

Forces are usually divided into two types.

1.Contact forces occur because of physical contact between objects.

Examples: pushing open a door pulling on a rope

2. Field forces – Act at a distance through space. The

presence of an object effects the space around it, creating a region of space around the object called a field.

Example: gravitational field magnetic field around a magnet

CONTACT FORCES

Normal force Force that prevents an object from falling

through the surface of another body Always acts perpendicular to the surface Always equals the forces applied to the

surface (or surface will break!)

mg

Fn Called the ‘normal’ force because it is always normal (perpendicular) to the surface.

CONTACT FORCES

Normal force Force that prevents an object from falling

through the surface of another body Always acts perpendicular to the surface Always equals the forces applied to the

surface (or surface will break!)

mg

Fn

mg

F

Fn

Ffr

Fn

Friction force Ffr

motion

friction

Friction is a force that is created whenever two surfaces move or try to move across each other. 

Friction always opposes the motion or attempted motion of one surface across another surface.

Friction is dependent on the texture/roughness of both surfaces.

Friction is also dependent on the force which presses the surfaces together. 

CONTACT FORCES

Air resistance / Drag When an object moves through air or any other

fluid, the fluid exerts a friction-like force on the moving object. The force is called drag.

Drag depends upon the speed of the object, becoming larger as the speed increases. (UNLIKE FRICTION!)

Drag also depends upon the size and the shape of the object and the density and kind of fluid. (UNLIKE FRICTION!)

CONTACT FORCES

B/c drag increases with speed, object moving through the air reach a terminal velocity – a maximum speed at which Fg = Fdrag so there is no more acceleration.

Air resistance / Drag When an object moves through air or any other

fluid, the fluid exerts a frictionlike force on the moving object. The force is called drag.

Drag depends upon the speed of the object, becoming larger as the speed increases. (UNLIKE FRICTION!)

Drag also depends upon the size and the shape of the object and the density and kind of fluid. (UNLIKE FRICTION!)

CONTACT FORCES

Without drag, raindrops would fall 340 m/h.

With drag, they only fall 17 m/h.

physics

Tension the force that the end of the rope exerts on

whatever is attached to it. Direction of the force is along the rope.

T1

T2T2

CONTACT FORCES

What is the relative force along the two yellow arrows?

Why?

Spring Force Force due to the elasticity of a material Depends on the elasticity of the spring Direction is opposite displacement

CONTACT FORCES

CONTACT FORCES: THINK PAIR SHAREType of Force

Direction

Normal

Friction

Drag

Tension

Spring

CONTACT FORCES: THINK PAIR SHAREType of Force

Direction

Normal Perpendicular to surface, opposite applied / gravitational forces

Friction Opposite motion

Drag Opposite motion

Tension Along the rope & opposite motion

Spring Opposite displacement

Field Forces Relative Strength

Action Distance

Gravitational Forceattraction between objects due

to their masses

10-45 Infinite – but decreases with square of distance

Electromagnetic Forcebetween charges

10-2 Infinite – but decreases with square of distance

Strong Nuclear Forcekeeps nucleus together

1 Very short!

Weak Nuclear Forcearise in certain radioactive

processes

10-8 Very very short!

FIELD FORCES

Field Forces Relative Strength

Action Distance

Gravitational Forceattraction between objects due

to their masses

10-45 Infinite – but decreases with square of distance

Electromagnetic Forcebetween charges

10-2 Infinite – but decreases with square of distance

Strong Nuclear Forcekeeps nucleus together

1 Very short!

Weak Nuclear Forcearise in certain radioactive

processes

10-8 Very very short!

At the atomic level – all contact forces are result of repulsive electromagnetic forces – the

repulsion of atoms’ electric fields

FIELD FORCES

HOW TO SOLVE FORCE PROBLEMS

1. Draw a free body diagram – label all the forces acting on one object.

2. Add up the forces3. Apply Newton’s second law: F = ma.

How to draw a force diagram

2. Make a simple sketch of the system – point system

1. Choose ONE body to be isolated

dog or the cart?

FHHHHHHHHHHHHHH

fr

mgHHHHHHHHHHHHHH

FHHHHHHHHHHHHHH

dog

NHHHHHHHHHHHHHH

FHHHHHHHHHHHHHH

net

3. Identify forces that act on the system Label them on diagram

4. Find out the net force by adding the force vectors

decision: cart

5. Apply Newton’s second law

netF = ma

WE DO PROBLEMS

Superman lifts a 35,000 kg truck off Jimmy Olsen at a constant velocity of 10 m/s. How much force did Superman apply to the truck?

Now Lois Lane is the one trapped under a semi. Superman applies a 450,000 N force to fling the truck off of Lois. How fast does the truck accelerate?

Fnet = 0 because there is no acceleration.

Fapplied = Fgravity = mg = 35000*9.8 =340000 N

Fapp

Fgravity

Fapp

Fgravity

Fapplied - Fgravity = ma

450000 - (35000*9.8) = 3.1 m/s2

35000

YOU DO PROBLEMS

A bookbag is hanging from your shoulder by one strap. Draw a free body diagram, making sure that your forces are proportional.

A fish accelerates as it swims horizontally through the water. It experiences an applied force (forward from its fins), drag, buoyancy (upward force from the water), and gravity. Draw the free body diagram, making sure that your forces are proportional.

Ftension

Fgravity

Fbouyancy

Fapplied

Fgravity

Fdrag

YOU DO PROBLEMS

Does a net force exit? Yes or no.

Yes … it is accelerating

No. It has constant velocity

No. It has constant velocity.

No, it has constant velocity.

Yes, it is accelerating.

Yes, it is accelerating.

YOU DO PROBLEMS

For each problem, determine the value of the unknown forces.

YOU DO PROBLEMS

For each problem, determine the acceleration of each block. Each block has a mass of 10 kg.

A: No acceleration; forces are balanced.

B: Fnet= 5N left a = F/m = 0.5 m/s2

left

C: No acceleration; forces are balanced.

D: Fnet = 15 N up a = F/m = 1.5 m/s2

up