Kinds of Forces

Lecturer: Professor Stephen T. Thornton

Reading Quiz: A hockey puck is sliding at constant velocity across a flat horizontal ice surface that is assumed to be frictionless. Which of these sketches is the correct free-body diagram for this puck?

A B C

Reading Quiz: A hockey puck is sliding at constant velocity across a flat horizontal ice surface that is assumed to be frictionless. Which of these sketches is the correct free-body diagram for this puck? No net force, because of constant velocity.

A B C

Last Time

Forces

Newton’s First Law

Newton’s Second Law

Newton’s Third Law

Today

Forces

Free body diagrams

Weight and mass

Normal force

Tension

Lots of conceptual quizzes

Conceptual QuizConceptual QuizA) the force pushing the stone forward

finally stopped pushing on it

B) no net force acted on the stone

C) a net force acted on it all along

D) the stone simply “ran out of steam”

E) the stone has a natural tendency to be at rest

You kick a smooth flat

stone out on a frozen

pond. The stone slides,

slows down, and

eventually stops. You

conclude that:

After the stone was kicked, no force was pushing

it along! However, there must have been some some

forceforce acting on the stone to slow it down and stop to slow it down and stop

itit. This would be friction!!

Conceptual QuizConceptual QuizA) the force pushing the stone forward

finally stopped pushing on it

B) no net force acted on the stone

C) a net force acted on it all along

D) the stone simply “ran out of steam”

E) the stone has a natural tendency to be at rest

You kick a smooth flat

stone out on a frozen

pond. The stone slides,

slows down, and

eventually stops. You

conclude that:

Short review from last time:

Forces on assistant.

Forces on sled.

Weight

Weight is a force

W = mg called gravitational force

Note that weight is not a mass!

W mg

Weight and Mass

The Normal Force May Equal the Weight

NF

Fg

Is the normal force always equal to the weight?

NO!

An Object on an Inclined Surface

W

Force pushing downhill

Weight—the Force of Gravity; and the Normal

Force

a) 0

b) 40.0 N 0

c) 40.0 N 0

y N

y N

y N

F F mg ma

F F mg

F F mg

What happens when a person pulls upward on the box in the previous example with a force greater than the box’s weight, say 100.0 N?

There is no normal force!

A. The normal force from the table.B. The gravitational force the apple exerts on the Earth.C. The gravitational force the apple exerts on the table.D. The normal force the apple exerts on the table.

Conceptual Quiz

B)

A. The normal force from the table.B. The gravitational force the apple exerts on the Earth.C. The gravitational force the apple exerts on the table.D. The normal force the apple exerts on the table.

Tension in a Heavy Rope

Heavy rope:

Light rope:

3 2 1T T T

3 2 1T T T

We usually consider light ropes.

g

= mg

mass m

A Pulley Changes the Direction of a Tension

Notice that the tension is constant throughout.

Elevator and counterweight (Atwood’s machine)

TI prefer T rather than F

E C

Magnitudes equal

a a

FT must be greater than mcg.

What is wrong with right diagram?

Tension in a String

Conceptual Quiz: We have a 1.0 kg mass hanging from the string. The string is wrapped around a pulley so the string is horizontal. If we separate the horizontal string and insert a spring scale, what will the scale read?

 

A) 0

B) 1 N

C) 5.9 N

D) 9.8 N

Answer: D

The gravitational force pulling on the string is (1 kg)(9.8 m/s2) = 9.8 N. The tension in the string must equal this, and it is constant throughout. We measure the tension by inserting the spring. It must measure 9.8 N.

Conceptual Quiz: We have 1.0 kg masses hanging from two pulleys. We unhook the horizontal string and insert a spring scale. What will the spring scale read for the tension in the horizontal string?

A) 0 N

B) 9.8 N

C) 19.6 N

Answer: B

It doesn’t matter whether the right hand string is attached to the pole or to a pulley with a hanging 1.0 kg mass.

Solving Problems with Newton’sLaws: Free-Body Diagrams

1. Draw a sketch.

2. For one object, draw a free-body diagram, showing all the forces acting on the object. Make the magnitudes and directions as accurate as you can. Label each force. If there are multiple objects, draw a separate diagram for each one.

3. Resolve vectors into components.

4. Apply Newton’s second law to each component.

5. Solve.

Let’s look more carefully at Free-Body Diagrams

Free-Body Diagrams

Box slides down an incline.

Hanging Object. An object is hanging by a string from your rearview mirror. While you are accelerating at a constant rate from rest to 28 m/s in 6.0 s, what angle does the string make with the vertical?

Pulley. Suppose the pulley in the figure is suspended by a cord C. Determine the tension in this cord after the masses are released and before one hits the ground. Ignore the mass of the pulley and cords.