demonstration: what force stops a climber on a rope from falling?

Demonstration: What force stops a climber on a rope from falling? • The more two objects are pressed together,

the greater the friction.

• This is the force that saves a falling climber.

First Questions

• In what direction is friction?

• What would walking be like, without friction?

• What is friction?

What affects friction?

• Materials?

• Smoothness?

• Surface area?

• Amount two objects are pressed together?

What makes friction?

• Molecules of one object attract molecules of the _ _ _ _ _ object.

• At its core, friction is the attraction between _ _ _ _ _ _ons (negatively charged) and

_ _ _ _ons (positively charged).

What makes friction?

• Molecules of one object attract molecules of the other object.

• At its core, friction is the attraction between _ _ _ _ _ _ons (negatively charged) and

_ _ _ _ons (positively charged).

What makes friction?

• Molecules of one object attract molecules of the other object.

• At its core, friction is the attraction between electrons (negatively charged) and protons (positively charged).

Quantifying Friction

• Consider a bag of groceries on a table

• You pull the bag with 0.1 N. What does it do?

• Draw all the forces on the bag.

You pull toward the right. Draw all the forces.

Groceries

Pull

PullFriction

PullFriction

Table

Weight

More about the grocery bag.

• If you double your force, the bag remains stationary . How much is the friction force now? ____ Newtons.

• In fact, even if you pull with 8 N, the bag does not budge.

• Only if you pull with more than 8 N does the bag move.

Make a chart of your pulling force (P) & the friction force (f).

P (Newtons) f (Newtons)

0.0

0.1

0.2

4.0

8.0

Make a chart of your pulling force (P) & the friction force (f).

P (Newtons) f (Newtons)

0.0

0.1

0.2

4.0

8.0

0.0

0.1

0.2

4.0

8.0

Graph friction vs. Pull

f

P

Graph friction vs. Pull

f

P

Aside: Quantifying Materialsand Friction

• Each pair of materials can have a different amount of friction between them

• Think of a pair of materials that has an unusually large amount of friction.

• The greek letter ‘’ (“myoo”) is the coefficient that describes the amount of friction between two materials.

• The greater , the more the friction.

Friction and materials

static kinetic

Concrete

and tire 1.0 0.8

Metal on

oiled metal 0.15 0.06

When P > 8 N, the bag moves.

• As the bag begins to slide, the friction force _ _ _ _ _ _ _ _ _!

When P > 8 N, the bag moves.

• As the bag begins to slide, the friction force decreases!

When P > 8 N, the bag moves.

• As the bag begins to slide, the friction force decreases!

• Before sliding, f = P. The friction would vary and have a maximum: fMAX = sN.

When P > 8 N, the bag moves.

• As the bag begins to slide, the friction force decreases!

• Before sliding, f = P. The friction would vary and have a maximum: fMAX = sN.

• But during sliding, f has one value, the “kinetic” value. f = kN.

When P > 8 N, the bag moves.

• As the bag begins to slide, the friction force decreases!

• Before sliding, f = P. The friction would vary and have a maximum: fMAX = sN.

• But during sliding, f has one value, the “kinetic” value. f = kN.

• During sliding, it takes _ _ _ _ force to pull the bag than it did to get it to start to slide.

When P > 8 N, the bag moves.

• As the bag begins to slide, the friction force decreases!

• Before sliding, f = P. The friction would vary and have a maximum: fMAX = sN.

• But during sliding, f has one value, the “kinetic” value. f = kN.

• During sliding, it takes less force to pull the bag than it did to get it to start to slide.

When P > 8 N, the bag moves.

• As the bag begins to slide, the friction force decreases!

• Before sliding, f = P. The friction would vary and have a maximum: fMAX = sN.

• But during sliding, f has one value, the “kinetic” value. f = kN.

• During sliding, it takes less force to pull the bag than it did to get it to start to slide. Try to include this in your graph. (Hmm…)

Pull or Push (Newtons)f (N)Motion f=kN No Motionf = Pf max = sN

“I thought weight (or mass) ought to fit into the amount of

friction, but it’s not on the graph?

So, does it matter?” …Yes, more weight leads to a greater

Normal, which leads to more friction.

Example: Pull a crate with a force with 80 N. What happens?• The mass of the crate is 10 kg

• The s = 0.9

• The k = 0.7

• Draw all the forces that act on the crate.

You pull toward the right. Draw all the forces.

The forces

PNfW = Mg

How much is the friction?

• If it is moving, f=kN

• If it is not moving, the most the friction could be is fmax = sN

• Either way, we need to know the value of the Normal force.

• How can we get that value?

How to get the Normal

• It is not accelerating vertically, so Fy = ?

• N - Mg = 0

• N = Mg = (10)(9.8) = 98 Newtons

How much is the friction?

• If it is moving, f=kN = (0.7)(98) = 68.6 N• If it is not moving, the most the friction

could be is fmax = sN = (0.9)(98)=88 N• If you pull with 80 N, how much friction is

there?• So, what happens?• What would happen if instead you pulled

with 90 N?

What happens if youpull with 90 N?

• a = F / M

• a = / 10

• a = Newtons / 10 kg

• a = m/s2

Last Example: Moving a crate.

• You are taller than the crate. So when you push it, you end up pushing sideways, and down. Draw the forces.

• When you pull it (maybe by a handle, maybe with a rope) you end up pulling sideways and up. Draw the forces.

Pushing the crate (one)

Push23 degreeshorizontal line

Pushing the crate (two)

P

N

f

W

23 deg

Pulling the crate (one)

Pull23 degrees

Pulling the crate (two)

PN

f

W

23 deg

Which case will have the greater acceleration?

The Crate, with numbers:

• The crate has a mass of 100 kg (weight of 980 Newtons).

• The Push or Pull will be 23˚ off the horizontal, with a magnitude of 180 N.

• The coefficient of kinetic friction between the crate and the floor is 0.1

• What is the acceleration of the crate in both cases?

Push (Sideways and Down)

ax = Fx / M (Newton’s Second Law)

ax = [ Px - f ] ÷ M

Uh Oh, What is the value of f ?

f = kN

But now we need the value of the Normal.

How do we get N? …

Push (Sideways and Down)

ay = 0, so Fy = 0

N - Psin - Mg = 0

N = Psin + Mg

N = 180sin(23˚) + 980

N = 70 N + 980 N

N = 1050 Newtons

Push (Sideways and Down)

ax = Fx / M

ax = [ Px - f ] / M

ax = [ Pcos - kN ] ÷ M

ax = [ 180cos(23˚) - 1050) ] ÷ 100

ax = [166 - 105 ] ÷ 100

ax = 0.61 m/s2 {Done with the Push}

Pull (Sideways and Up)

ax = Fx / M

ax = [ Px - f ] / M

To get the friction, we need the normal force.

How will the normal compare to the normal for the Push?

Get the Normal, for the Pull

Fy = 0

N + Psin - Mg = 0

N = -Psin + Mg

N = -180sin(23˚) + 980

N = -70 N + 980 N

N = 910 Newtons

Pull (Sideways and Up)

ax = Fx / M

ax = [ Px - f ] / M

ax = [ Pcos - kN ] ÷ M

ax = [ 180cos(23˚) - 910) ] ÷ 100

ax = [166 - 91] ÷ 100

ax = 0.75 m/s2 {Done with the Pull}

Compare the Pull with the Push.

The acceleration is greater if you don’t press the objects together.