PHYSICS 50: Lecture 4.1

RICHARD CRAIG

Goals for Today

To visualize force as a vector To find the net force acting on a body and

apply Newton’s First Law Mass, acceleration, and their application

to Newton’s Second Law To calculate weight and compare/contrast

it with mass

Homework #3 Read Chapter 3: Sections 4,5 and

Chapter 4 Sections: 1,2,3 Exercises and Problems:3.31, 3.38,

3.40, 4.4 4.12. 4.19 Due Thursday, 2/14

What is a Force? A push or a pull

It has both a magnitude and a direction (vector)

It has units of N (Newton) = kgm/s2

There are four common types of forces

The normal force—When an object rests or pushes on a surface, the surface pushes back.

Frictional forces—In addition to the normal force, surfaces can resist motion along the surface.

There are four common types of forces II

Tension forces—When a force is exerted through a rope or cable, the force is transmitted through that rope or cable as a tension.

Weight—Gravity’s pull on an object. This force can act from large distances.

Mass, Weight and Gravity

• Your Mass is a function of the mass of the molecules that form your body

• Your Weight (a Force) is how that mass interacts with the Earth’s gravitational field

g, and hence weight, is only constant on earth, at sea level

On Earth, g depends on your altitude.

On other planets, gravity will likely have an entirely new value.

How to denote a force

Use a vector arrow to indicate magnitude and direction of the force.

Use the net (overall) force

• Several forces acting on a point have the same effect as their vector sum acting on the same point.

Decomposing a force into components

Fx and Fy are the parallel and perpendicular components of a force to a sloping surface.

Use F*Cosθ and F*Sinθ operations to find force components.

Newton’s First Law Simply stated—“objects at

rest tend to stay at rest, objects in motion stay in motion.”

More properly, “A body acted on by no net force moves with constant (or zero) velocity and zero acceleration.”

Examples of Newton’s First Law

Newton’s Second Law

An unbalanced force (or sum of forces) will cause a mass to accelerate.

Fnet= ma

The relationship of F, m, and a

Because a depends linearly on m and F, an acceleration will be inversely proportional to the object’s mass.

Using the Second Law—Example

What is the acceleration of the Box?How far will it go in 10 seconds?

No friction

Many have asked “how lethal is a coin dropped from atop a tall building”?

Urban legends have said that a penny dropped from the top of the Empire State Building can kill.

Cable TV has allowed those two science guys who test such “myths” to debunk this one.