Why things move?

Describing forces

What changes the motion of bodies? Forces.

What keeps bodies moving? No need of forces.

Aristotle (384-322 B.C.) Galileo Galilei (1564-1642) Isaac Newton (1642-1727)

More an observer than an experimenter.

Performed several careful experiments with falling objects, and

published a lot. He challenged Aristotle’s views.

was first demonstrated by Galileo.

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Proposed a theory to explain the motion of all objects (celestial or not).

Most important in PHY101: Newton’s 3

laws of motion and his law of gravitation.

Contact Forces Field Forces

Frictional Force Gravitational Force

Tension Force Electrical Force

Normal Force Magnetic Force

Air Resistance Force

Applied Force

Spring Force

Inertia is an object tendency to continue in its motion, in the absence of a force.

It is related to mass.

The bigger the mass, more resistance to changes in the motion.

An object remains at rest, or in uniform motion in a straight line, unless it is compelled to change by an externally imposed force.

Also known as law of inertia or law of equilibrium.

If Fnet = 0 (no net force acting on the body)

• and the body is a rest

• and the body is moving with constant velocity

then the body remains with v = 0.

then the body remains moving with constant velocity.v v

Even today people still believe that a force is required to keep an object

moving.

Balanced forces is the case described by the 1st Law of Newton.

What does “balanced forces” mean??

0or 0 Forcesonaccelerati

Gravity pulls downwards on

the person

The floor pushes upward on the

person

The forces on the person are balanced.

The forces on the book are balanced.

Gravity pulls downward on the book

The table pushes upward on the book

The acceleration of an object is directly proportional to the magnitude of the imposed force and inversely proportional to the mass of the object. The acceleration is in the same direction as that of the imposed force.

amFnet

F

F

a

a

m1

m2

The smaller-mass object experiences a larger acceleration

than the larger-mass object when identical forces are applied to

the two objects.

UNIT of force:1 newton= 1N = 1 kg m/s2

Unbalanced forces is the case described by the 2nd Law of Newton.

0or 0 Forcesonaccelerati

FORCE IS A VECTOR !!!!

F

Just like acceleration, velocity and position.

+ + =1F

2F

3F

5 N3 N

1 N 3 N

netF

Two equal-magnitude horizontal forces act on a box. Is the

object accelerated horizontally?a) Yes.b) No.c) You can’t tell from

this diagram.

Since the two forces are equal in size, and are in opposite directions, they cancel each other out and there is no acceleration.

Is it possible that the box is moving, since the forces are equal in size but opposite in

direction?a) Yes, it is possible for

the object to be moving.b) No, it is impossible for

the object to be moving.

Even though there is no acceleration, it is possible the object is moving at constant speed.

Two equal forces act on an object in the directions shown.

If these are the only forces involved, will the object be

accelerated?a) Yes.b) No.c) It is impossible to determine

from this figure.

The vector sum of the two forces results in a force directed toward the upper right corner. The object will be accelerated toward the upper right corner.

A ball hangs from a string attached to the ceiling. What is the net force acting on the ball?

a) The net force is downward.

b) The net force is upward. c) The net force is zero.

Since the ball is hanging from the ceiling at rest, it is not accelerating so the net force is zero. There are two forces acting on the ball: tension from the string and force due to gravitation. They cancel each other.

Two masses connected by a string are placed on a fixed

frictionless pulley. If m2 is larger than m1, will the two masses

accelerate?a) Yes. b) No. c) You can’t tell

from this diagram.

The acceleration of the two masses will be equal and will cause m2 to fall and m1 to rise.

Quick Quizzes

1) An object can move even when no force acts on it.

2) If an object isn't moving, no external forces act on it.

3) If a single force acts on an object, the object accelerates.

4) If an object accelerates, a force is acting on it.

Answer True or False:

5) If an object isn’t accelerating, no external force is acting on it.

6) If the net force acting on an object is in the positive x-direction, the object moves only in the positive x-direction.

T

F

T

T

F

F

Quick Quiz

Consider the two situations shown in the figure below, in which there is no acceleration. In both cases the men pull with a force of magnitude F. How does the reading on the scale in part (i) of the figure relate to the reading on the scale in part (ii)?

1. The scale in situation (i) gives the greater reading.

2. The scale in situation (ii) gives the greater reading.

3. The scales in both situations give identical readings.

Quick Quiz

For the woman being pulled forward on the toboggan in the figure below, the magnitude of the normal force exerted by the ground on the toboggan is

1. equal to the total weight of the woman plus the toboggan.

2. greater than the total weight.

3. less than the total weight.

4. possibly greater than or less than the total weight, depending on the size of the weight relative to the tension in the rope.

Apparent weight and weightlessness

Quick Quizzes

1) Which has a greater value, a newton of gold won on Earth or a newton of gold won on the Moon?

1. the newton of gold on the Earth

2. the newton of gold on the moon

3. the value is the same, regardless.

2) No force of gravity acts on an astronaut in an orbiting space station.

3) At three Earth radii from the center of Earth, the acceleration of gravity is 1/9 its surface value.

4) One kilogram of gold would have greater value on Earth than on the Moon.

True or False?

F

T

F

Newton’s Third Law(“action/reaction”)

For every action (force),

there is an equal but opposite reaction

(force).

Third-Law Action/Reaction Pair

If the cart pulls back on the mule equal and opposite to the mule’s pull on the cart,

how does the cart ever move?

Third-Law Action/Reaction Pair

The car pushes against the road, and the road, in turn, pushes against the car.

Friction Tread designs channel water away from the bearing surfaces on wet roads to combat the tendency to skid.

Website about friction of cars:

http://hyperphysics.phy-astr.gsu.edu/hbase/Mechanics/frictire.html#c1

1) If you press a book flat against a vertical wall with your hand, in what direction is the friction force exerted by the wall on the book?

1. downward 2. upward 3. out from the wall 4. into the wall

Quick Quizzes

2) A crate is sitting in the center of a flatbed truck. As the truck accelerates to the east, the crate moves with it, not sliding on the bed of the truck. In what direction is the friction force exerted by the bed of the truck on the crate?

1. to the west 2. to the east 3. There is no friction force, because the crate is not sliding.

Suppose you're playing with your niece in the snow. She's sitting on a sled and asks you to move her across a flat, horizontal field. You have a choice of (a) pushing her from behind by applying a force downward on her shoulders at 30° below the horizontal (Fig. a), or (b) attaching a rope to the front of the sled and pulling with a force at 30° above the horizontal (Fig b). {image} Which option would be easier?

Suppose your friend is sitting on a sled and asks you to move her across a flat, horizontal field. You have a choice of (a) pushing her form behind by applying a force downward on her shoulders at 30° below the horizontal (figure a), or (b) attaching a rope to the front of the sled and pulling with a force at 30° above the horizontal (figure b). Which option would be easier?

1. situation (a)

2. situation (b)

Quick Quiz

Two types of friction forces:

Static friction, or also called starting friction: when the object is about to start moving.

Kinetic friction, or also called sliding friction: when the object is MOVING.

STATIC FRICTION

No motio

n yet!

KINETIC FRICTIONIt is moving!

fs is the force of static friction

fk is the force of kinetic friction

So, which force of friction is greater, in general?

Kinetic or peak static friction force?

Tire is

rolling: it is

safer.

Nkk Ff Nss Ff Static Friction Kinetic Friction

ts.coefficien are whileforces,friction are f

ks

Wheels are locked and tire is skidding.

v = 0

v = 2 vcm

v = vcm

Very blurred because at that point:

The image is sharper at the bottom because there:

Rolling motion of a wheel as a combination of purely rotational motion and purely translational motion.

(a) The purely rotational motion: all points on the wheel move with the same angular speed ω. Points on the outside edge of the wheel all move with the same linear speed v = vcm.

(b) The purely translational motion: all points on the wheel move to the right with the same linear velocity v as the center of the wheel.

(c) The rolling motion of the wheel is the combination of (a) and (b).

ROTATIONAL LINEAR

COMBINATION

FN

FN

FN

Cool website about Physics of cars:

http://hyperphysics.phy-astr.gsu.edu/hbase/Mechanics/frictire.html#c1