an introduction to forces: student learning goal the student will analyse, in quantitative terms,...

An Introduction to Forces: An Introduction to Forces: Student Learning GoalStudent Learning Goal

The student will The student will analyse, in analyse, in quantitative terms, the forces acting quantitative terms, the forces acting on an object, and use free-body on an object, and use free-body diagrams to determine net force on diagrams to determine net force on the object in one dimensionthe object in one dimension. (B2.9). (B2.9)

An Introduction An Introduction to Forcesto Forces

SPH4CSPH4C

A DefinitionA Definition

A A forceforce is a push or a pull. is a push or a pull.



It is a vector quantity and is symbolized It is a vector quantity and is symbolized by:by:



It is a vector quantity and is symbolized It is a vector quantity and is symbolized by:by:

F



It is a vector quantity and is symbolized by:It is a vector quantity and is symbolized by:

In the SI system, force is measured in In the SI system, force is measured in Newtons (N).Newtons (N).

1 N = 1 kg m/s1 N = 1 kg m/s22

F

Applied ForceApplied Force

Applied ForceApplied Force is a is a general term for any general term for any contact force, e.g.contact force, e.g.

F A



TensionTension

F or TT

F A



TensionTension

FrictionFriction

F or TT

F A

F or Ff fr



TensionTension

FrictionFriction

Normal ForceNormal Force

F or TT

FN

F A

F or Ff fr

TensionTension

Tension is the force Tension is the force exerted by strings, exerted by strings, ropes, cables, etc. ropes, cables, etc. attached to an object.attached to an object.

TensionTension

Tension is the force Tension is the force exerted by strings, exerted by strings, ropes, cables, etc. ropes, cables, etc. attached to an object.attached to an object.

The tension along the The tension along the string is constant.string is constant.

FrictionFriction

Friction acts to oppose any Friction acts to oppose any (attempted) motion.(attempted) motion.

FrictionFriction


Static friction: the force that Static friction: the force that prevents a stationary object from prevents a stationary object from starting to movestarting to move

Fs

FrictionFriction



Kinetic friction: the force that acts Kinetic friction: the force that acts against an object’s motionagainst an object’s motion

FsFk

FrictionFriction




Air resistance (drag): friction on an Air resistance (drag): friction on an object moving through airobject moving through air

FsFk

Fa ir

FrictionFriction

Friction acts to oppose any (attempted) Friction acts to oppose any (attempted) motion.motion.

Static friction: the force that prevents a Static friction: the force that prevents a stationary object from starting to movestationary object from starting to move


Air resistance (drag): friction on an Air resistance (drag): friction on an object moving through air (many physics object moving through air (many physics problems will neglect this)problems will neglect this)

FsFk

Fa ir


The normal force acts to keep objects The normal force acts to keep objects apart.apart.


The normal force is a force exerted by The normal force is a force exerted by a surface (or another object) on any a surface (or another object) on any other object at 90° (perpendicular other object at 90° (perpendicular to the surface)to the surface)

e.g. if you push on a wall, the wall will e.g. if you push on a wall, the wall will exert a normal force on your hand exert a normal force on your hand

e.g. a table exerts a normal force on a e.g. a table exerts a normal force on a textbooktextbook

Action-at-a-Distance Action-at-a-Distance ForcesForces

There exist forces for which There exist forces for which contact between objects is contact between objects is not necessary. These forces not necessary. These forces are called are called action-at-a-action-at-a-distance forcesdistance forces..


There exist forces for which There exist forces for which contact between objects is contact between objects is not necessary. These forces not necessary. These forces are called are called action-at-a-action-at-a-distance forcesdistance forces. .

One example is gravitational One example is gravitational force, the force of attraction force, the force of attraction between between allall objects with objects with mass.mass.

Fg


There exist forces for which contact There exist forces for which contact between objects is not necessary. between objects is not necessary. These forces are called These forces are called action-at-action-at-a-distance forcesa-distance forces. .

Example 1:Example 1:Gravitational force - the force of Gravitational force - the force of

attraction between attraction between allall objects with objects with mass.mass.

(The gravitational force the Earth (The gravitational force the Earth exerts on an object is called its exerts on an object is called its weightweight.).)

Fg


Example 2:Example 2:

Electrostatic force Electrostatic force – the attractive – the attractive or repulsive or repulsive force between force between charged objects charged objects (+ vs. +; + vs. -)(+ vs. +; + vs. -)


Example 3:Example 3:

Magnetic forces – the Magnetic forces – the force between force between objects with objects with magnetic polesmagnetic poles

(i.e. two magnets (i.e. two magnets repel with N-poles repel with N-poles facing each other; facing each other; attract when N-pole attract when N-pole facing the S-pole)facing the S-pole)

Free-Body DiagramsFree-Body Diagrams

Usually an object will have more than Usually an object will have more than one force acting upon it.one force acting upon it.


Usually an object will have more than Usually an object will have more than one force acting upon it. A one force acting upon it. A free-free-body diagrambody diagram (FBD) shows all the (FBD) shows all the forces acting on an objectforces acting on an object


Usually an object will have more than Usually an object will have more than one force acting upon it. A one force acting upon it. A free-free-body diagrambody diagram (FBD) shows all the (FBD) shows all the forces acting on an object – and forces acting on an object – and onlyonly the forces acting on the object.the forces acting on the object.



A representation of the object is A representation of the object is drawn in the centre of the diagramdrawn in the centre of the diagram



A representation of the object is A representation of the object is drawn in the centre of the diagram drawn in the centre of the diagram and the forces acting on it are drawn and the forces acting on it are drawn as arrows pointing outwards.as arrows pointing outwards.


Usually an object will have more than one Usually an object will have more than one force acting upon it. A force acting upon it. A free-body free-body diagramdiagram (FBD) shows all the forces (FBD) shows all the forces acting on an object – and acting on an object – and onlyonly the forces the forces acting on the object.acting on the object.

A representation of the object is drawn in A representation of the object is drawn in the centre of the diagram and the forces the centre of the diagram and the forces acting on it are drawn as arrows acting on it are drawn as arrows pointing outwards. The arrows must be pointing outwards. The arrows must be labelled!labelled!

FBD: Example 1FBD: Example 1

A ball is falling downward through the A ball is falling downward through the air. Draw a FBD for the ball.air. Draw a FBD for the ball.



Fg



Fg

Fair


A book is being pushed rightward A book is being pushed rightward across a table. Draw a FBD for the across a table. Draw a FBD for the book.book.



FA



FAFf



Fg

FAFf



Fg

FN

FAFf

An Introduction to An Introduction to Forces: Forces:

More PracticeMore PracticeDraw a free-body Draw a free-body

diagram for each of diagram for each of the following the following objects:objects:

(a) A car engine (a) A car engine being lifted from a being lifted from a car by a rope car by a rope attached to a pulleyattached to a pulley




(b) an car moving (b) an car moving with constant with constant velocity on a level velocity on a level roadroad




(c) an apple hanging (c) an apple hanging from a tree branchfrom a tree branch




(d) a skydiver being (d) a skydiver being slowed by a slowed by a parachuteparachute


More PracticeMore PracticeMr. Wong is pulling across level snow Mr. Wong is pulling across level snow

a sled on which is sitting his a sled on which is sitting his daughter. Tied to the back of his daughter. Tied to the back of his daughter’s sled is another tiny sled daughter’s sled is another tiny sled on which his daughter’s baby doll is on which his daughter’s baby doll is sitting. sitting.

Draw the free-body diagrams for (a) Draw the free-body diagrams for (a) Mr. WongMr. Wong


More PracticeMore PracticeDraw the free-body diagrams for (b) Draw the free-body diagrams for (b)

his daughterhis daughter


More PracticeMore PracticeDraw the free-body diagrams for (d) Draw the free-body diagrams for (d)

his daughter’s sledhis daughter’s sled


Answers to More PracticeAnswers to More PracticeDraw the free-body diagrams for Draw the free-body diagrams for

(d) the doll’s sled(d) the doll’s sled

Net ForceNet Force

FBDs are drawn to help determine the FBDs are drawn to help determine the net force net force (the sum of all forces) (the sum of all forces) acting on an object.acting on an object.

Resolve all vertical and horizontal Resolve all vertical and horizontal vectors and determine the resultant.vectors and determine the resultant.

FBD: Example 2 with FBD: Example 2 with NumbersNumbers

A book is being pushed across a table with a A book is being pushed across a table with a force of 5 N [right]. The force of friction force of 5 N [right]. The force of friction is 2 N [left], the gravitational force is 10 is 2 N [left], the gravitational force is 10 N [down], and the normal force is 10 N N [down], and the normal force is 10 N [up]. Find the net force on the book.[up]. Find the net force on the book.


A book is being pushed across a table with a A book is being pushed across a table with a force of 5 N [right]. The force of friction force of 5 N [right]. The force of friction is 2 N [left], the gravitational force is 10 is 2 N [left], the gravitational force is 10 N [down], and the normal force is 10 N N [down], and the normal force is 10 N [up]. Find the net force on the book.[up]. Find the net force on the book.

Fg = 10 N

FA = 5 NFf = 2 N

FN = 10 N


A book is being pushed across a table with a A book is being pushed across a table with a force of 5 N [right]. The force of friction is 2 force of 5 N [right]. The force of friction is 2 N [left], the gravitational force is 10 N [down], N [left], the gravitational force is 10 N [down], and the normal force is 10 N [up]. Find the and the normal force is 10 N [up]. Find the net force on the book.net force on the book.

Horizontal forces: FHorizontal forces: FAA + F + Fff = 5 N + (- 2 N) = 3 N = 5 N + (- 2 N) = 3 N

Vertical forces: FVertical forces: Fgg + F + FNN = (- 10 N) + (10 N) = 0 = (- 10 N) + (10 N) = 0

The net force FThe net force Fnetnet = 3 N [right] = 3 N [right]

INERTIA AND NEWTON’S FIRST LAW OF MOTION

QUESTION: When you are standing on the train and it

suddenly starts to move, what happens? You fall backwards.

What happens if you are standing on the train as it is moving and it starts to slow down?

You fall forwards.

What causes this movement?

INERTIA

A property of matter that causes an object to resist changes in its state of motion; it is directly proportional to the mass of the object. The greater the mass, the greater the

inertia an object possesses.

In physics we pretend that there is no friction for some of our problems.

If the carpet of the train did not have any friction and there was no air resistance, when the train starts, you would stay in the same spot as the train moves forward until the back wall hits you.

When the train slows down, you would continue to move forward until you hit a wall.

NET FORCES

As we have already learned, several forces can act on one object at the same time. The vector sum of all the forces acting on the object is known as the net force.

The net force is not an actual force or a separate force of nature; it is the sum of actual forces.

EXAMPLE:

A weight lifter holds a weight above the head by exerting a force of 1.6 kN [up]. The force of the gravity acting on the weight is 1.6 kN [down].

1. Draw a Free body diagram of the system. 2. Find the net force.

Find the net force.

= 1.6 kN [up] - 1.6 kN [down]

= 0

NEWTON’S FIRST LAW OF MOTION (LAW OF INERTIA)

If the Net force acting on an object is zero, the object will maintain its state of rest or constant velocity.

THIS CAN BE STATED IN LAYMAN’S TERMS AS:

Objects at rest or moving with a constant velocity maintain their state of rest or constant velocity unless acted upon by a net force. The net force must be external in order to

change an objects velocity. Internal forces have no effect on an

objects motion.

This law of inertia helps us to understand the principles behind using seatbelts and air bags. Once an object is moving, it tends to keep moving at a constant velocity because of its inertia.

Air bags and seatbelts help to slow us down safely.

EXAMPLE: A 12 passenger jet aircraft of mass 1.6 x 104

kg is travelling at a constant velocity of 850 km/h [E] while maintain a constant altitude. Besides gravity and air resistance, the aircraft also experiences an upward force called “lift” and a forward force of the engines called “thrust.”

Draw an FBD of the aircraft, and state the net force acting on the aircraft.

What is the net force?

LAW OF INERTIA SUMMARY

1. Objects at rest remain at rest unless acted upon by a net force.

2. Objects in motion remain in motion unless action upon by a net force.

3. If the velocity of an object is constant (or zero), the net external force acing it must be zero.

4. If the velocity of an object is changing either in magnitude, direction, or both, the change must be caused by a net external force on the object.

A THOUGHT EXPERIMENT

You are on a level ice field without boundaryImagine slapping a hockey puck forward and it travels 50 m.

1) What stops the hockey puck?2) If you smooth out the ice field, what will

happen to the travelling distance of the puck?

3) Repeat the above until (theoretically) friction is totally eliminated, what will happen to the travelling distance of the puck?

4) What does that mean?

an introduction to forces: student learning goal the student will analyse, in quantitative terms,...

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