Everyday ForcesChapter 4 – Section 4

St. Augustine Preparatory SchoolOctober 13, 2016

Weight

• Weight and mass are not the same thing• Mass is measured in kilograms, grams, etc.• Weight is measured in Newton’s and is a result

of gravity• To calculate the weight of something, we need

to find the force of gravity on it:Fg=ma,

where a = g = -9.81m/s2

Examples

1) The average mass of a male Asian elephant is 2300kg. What does the elephant weigh?

2) A book is set on a scale and the scale reads 3.12kg. What is the mass of the book? What is the weight of the book?

3) If the same book in question 2 is taken to the moon where the acceleration of gravity is only 1.6m/s2, what does the book weigh?

Answers

1) Fg =mg = (2300kg)(-9.81m/s2) = 22563N

Weight = 2.3x104 N2) Mass of the book is 3.12kg.Weight = Fg = mg = (3.12kg)(-9.81m/s2) = -30.607 N

Weight = 30.6 N3) Weight = Fg = mg = (3.12kg)(-1.6m/s2) = - 4.992 N

Weight = 5.0 N

Friction

• Friction opposes motion when it is present• It will be present when something slides or

moves while in contact with a physical surface(objects flying through the air do not

experience friction)• If an object moves left, friction is to the right.

If it moves right, friction is to the left.

Two Kinds of Friction

• There is Kinetic Friction and Static Friction• Kinetic Friction (Fk): Experienced with the

object is in motion• Static Friction (Fs): Experienced when the

object is at rest.

Static Friction

• Think about a book sitting on a desk. • Will a small force always move the book?• Remember, if a net force (not equal to 0) is

present, an object should move.• If the book does not move, then

Fs = -Fapp

As the applied force increases, so does the static friction. This happens until the object finally moves.

Kinetic Friction• Kinetic Friction is less than the maximum static

friction. • Even smooth surfaces are rough at the

microscopic level. As jagged surfaces slide on each other, the motion will be inhibited

Kinetic Friction

• If an object is sliding across a surface at a constant velocity (not accelerating) and air resistance is being ignored, then the Fk is equal to the Fapp

Friction

• In general, Friction is proportional to the Normal Force. Normal force increases with mass. So as mass increases, it is likely that friction will as well.

Formulas

Question

• A 24kg crate initially at rest on a horizontal floor requires a 75N horizontal force to set it in motion. Find the coefficient of static friction between the crate and the floor.

Answer

Practice Problems

• Page 137: Questions 1, 2, 3