Phys141 Principles of Physical Science

Chapter 3 Force and Motion

Instructor: Li Ma

Office: NBC 126Phone: (713) 313-7028Email: malx@tsu.edu

Webpage: http://itscience.tsu.edu/ma

Department of Computer Science & PhysicsTexas Southern University, Houston

Sept. 15, 2004

Topics To Be Discussed

Force and Net Force Newton’s First Law of Motion Newton’s Second Law of Motion Newton’s Third Law of Motion Newton’s Law of Gravitation Momentum

Cause of Motion

A push causes something to move This push is the application of a force Force and Motion: Cause and Effect Galileo did experiments on moving objects Newton formulated the laws of motion and

explained the phenomena of moving objects on the Earth and the motions of planets

Force

Easy to describe force Define force in terms of what it does:

– A force can produce changes in motion– A force can produce a change in velocity (speed

and/or direction), or cause a acceleration– Observed motion is evidence of a force

A force is a quantity that is capable of producing motion or a change in motion

Net Force

A force’s capability may be balanced or canceled by other force(s): the net effect is then zero

More than one force acts on an object:– unbalanced/net force: tug of war

Forces are vector quantities Only net force can cause change in motion

Newton’s First Law of Motion

The natural state of motion:– Aristotle: the natural state of an object is being at

rest – no idea of friction– Galileo: objects could naturally remain in motion

rather than come to rest Newton’s first law of motion

– An object will remain at rest or in uniform motion in a straight line unless acted on by an external, unbalanced force

Newton’s First Law of Motion (cont)

External force: an applied force Internal force: can not change the state of

motion Friction and Gravity on the Earth make it

difficult to observe an object in a state of constant velocity

Motion and Inertia

Inertia: natural tendency of an object to remain in a state of rest or in uniform motion in a straight line - Galileo

Mass is a measure of inertia – Newton– The greater the mass of an object, the greater is

its inertia, the greater is its resistance to a change in motion

Newton’s first law: Law of inertia

Newton’s Assumptions of Acceleration

The acceleration produced by an unbalance force acting on an object (or mass) is directly proportional to the magnitude of the force (a ∞ F) and in the direction of the force

The acceleration of an object being acted on by an unbalance force is inversely proportional to the mass of the object (a ∞ 1/m)

Newton’s Assumptions of Acceleration (cont)

Combining these effects of force and mass on acceleration:

unbalanced forceacceleration ∞

mass

or

a ∞ (F / m)

Newton’s Second Law of Motion

a = F / m

or

F = m·a F is the net force m is the total mass Unit of the force is newton in metric system:

1 N = 1kg·m/s2

Example

Given:– m1=1.0kg, F1=-5.0N (left, negative direction);

– m2=1.0kg, F2=+8.0N (right, positive direction);

Wanted: a (acceleration)

Equation: a = (F1+F2)/(m1+m2)

a = (+8.0N-5.0N)/(1.0kg+1.0kg) = +1.5m/s2

m1

1.0 kg

m2

1.0 kg

aF1 = -5.0N F2 = +8.0N

Mass and Weight

Mass is the amount of matter an object contains, or a measure of inertia

Weight is related to the force of gravity (gravitational force acting on an object)

They are related:

weight = mass x acceleration due to gravity

w = m·g

Newton’s Third Law of Motion The law of action and reaction For every action there is an equal and

opposite reaction Whenever one object exerts a force on a

second object, the second object exerts an equal (in magnitude) and opposite (in direction) force on the first object

action = opposite reactionF1 = - F2

Comparing Newton’s Second & Third laws

Newton’s third law relates two equal and opposite forces acting on two different objects

Newton’s second law concerns how forces acting on a single object can cause an acceleration

Newton’s Law of Universal Gravitation

Gravity: a common fundamental force in nature

Every particle in the universe attracts every other particle with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them

F ∞ (m1m2 / r2)

Newton’s Law of Gravitation (cont)

F = (G m1m2 / r2)

G is the universal gravitational constant G = 6.67 x 10-11 N·m2/kg2

Why objects fall to the ground of the Earth, Earth doesn’t move?

Why we can’t feel attraction from book? Astronauts in space shuttle orbiting the Earth

are weightless?

Linear Momentum

Product of mass and velocity Linear momentum is a vector, in direction of

velocity If there is no external net force, linear

momentum is conserved

linear momentum = mass x velocity

p = m·v

Linear Momentum (cont)

Law of conservation of linear momentum:– The total linear momentum of an isolated system

remains the same if there is no external unbalanced force acting on the system

Example of the conservation of linear momentum: man jump out from the boat

Angular Momentum

Angular momentum arises when objects go in the paths around a fixed point

The angular momentum of a system can be changed by an external unbalanced torque

L = m·v·rr = distance of object from center of motion

Angular Momentum (cont)

A torque is a twisting effect caused by one or more forces

A torque tends to produce a rotational motion

r

F

T = F·r

v

Angular Momentum (cont)

Law of conservation of angular momentum:– The angular momentum of an object remains

constant if there is no external unbalanced torque acting on it

Example of the conservation of angular momentum: ice skaters spin on the ice