# Homework Help: Interactions Questions

1. Sep 3, 2007

### eraemia

1. The problem statement, all variables and given/known data

Each part of this problem describes an object in a specific physical context in which it interacts with other objects. In each case, use the macroscopic interaction categories to list the ways that the object interacts with its surroundings; and for each listed interaction, state the other object involved. Remember that an interaction, if unopposed, will act to change the object’s velocity, but two opposed interactions may cancel each other’s effect on the velocity. Do the best you can; some of these are hard! >_<

0) A hockey puck sliding on rough ice

a) A person sits on a chair.

b) A leaf falls from a tree. (Consider the leaf after it has left the tree

c) A magnet sticks to the side of a refrigerator.

d) A tiny bit of paper jumps from a tabletop toward an electrically charged comb. (Consider the bit after it has left the tabletop but before it reaches the comb.)

e) Difficult: A child hangs on to a playground merry-go-round that is rotating so rapidly that the child is suspended above the ground. (ignore air friction)

f) Difficult: A car accelerates down a road. We will see later in this unit that processes internal to an object can never change the motion of the object as a whole: only an interaction with something external can do this. If this is so, what interaction increases the car’s speed? Would the acceleration be possible if the road were very slippery?

2. Relevant equations

Macroscopic Interactions (2 categories)

A. Long-range interactions—that allow objects to influence one another over significant distance
1) Electrostatic interactions—involve at least one electrically
charged object
2) Magnetic interactions—involve at least one magnet
3) Gravitational interactions
B. Contact interactions—arise only when objects come into direct physical contact
1) Friction interactions—oppose the relative motion of the
objects in contact
2) Compression interactions—seek to keep them from merging
3) Tension interactions—oppose their separation

3. The attempt at a solution

a) A person sits on a chair.
Long-range: Gravitational interaction: Between the earth and both the chair and the person
Contact: Compression interaction: Between the chair and the person

Is there any friction or electrostatic interaction occurring here?

b) A leaf falls from a tree. (Consider the leaf after it has left the tree but before it hits the ground.)
Long range: Gravitational interaction: Between earth and leaf
Contact: Friction interaction: Between air and leaf
Contact: Compression interaction: Between air and leaf

c) A magnet sticks to the side of a refrigerator.
Long-range: Magnetic interaction: Between magnet and refrigerator
Long-range: Gravitational interaction: Between earth and both magnet and refrigerator
Contact: ?

d) A tiny bit of paper jumps from a tabletop toward an electrically charged comb. (Consider the bit after it has left the tabletop but before it reaches the comb.)

Long-range: Electrostatic interaction
Long-range: Gravitational
?

e) Difficult: A child hangs on to a playground merry-go-round that is rotating so rapidly that the child is suspended above the ground. (ignore air friction)

Long-range: Gravitational

f) Difficult: A car accelerates down a road. We will see later in this unit that processes internal to an object can never change the motion of the object as a whole: only an interaction with something external can do this. If this is so, what interaction increases the car’s speed? Would the acceleration be possible if the road were very slippery?

Long-range: Gravitational
I don't understand this one at all...

Thanks for the help, everyone.

2. Sep 3, 2007

### Astronuc

Staff Emeritus
The car accelerates by virtue of the torque applied by the engine, through the transmission to the wheels. The tire get 'traction' by virtue of friction between the surface of the tire and the surface of the road. Friction is proportional to the weight of the car, which is the product of acceleration of gravity and mass of the car, and the proportionality constant is the coefficient of friction.

What keeps the magnet from sliding down the side of the refigerator?

Gravity is pulling down, but what causes the child to be suspended? What is the force opposing gravity? Certainly the child is hanging on, so there must be ___________ in the arms.

What about the hockey puck on rough ice?

Electrostatic attraction is a relatively short range phenomenon, and so is magnetic attraction.

Last edited: Sep 3, 2007
3. Sep 3, 2007

### eraemia

Whoa, that's complicated. So are you saying that there is friction interaction between tire and road, and gravitational interaction between car and earth?

I'm very confused about these interactions. Could you explain to me what compression and friction and tension interactions are? Doesn't the magnetic interaction keep the magnet from sliding off?

Certainly the child is hanging on, so there must be tension interaction? in the arms.

Well, there is compression friction holding the hockey up on the ice, against gravity, which pulls it down. I don't think there's tension interaction or electrostatic or magnetic here.

Thank you for our help, Astronuc.

4. Sep 3, 2007

### Astronuc

Staff Emeritus
Tension is a pulling force. If one pulls on a coil spring, it extends (lengthens). It is under tension.

Compression is a pushing force. If one pushes on a spring, it contracts (shortens). It is under compression.

Friction is a shear force acting perpendicular to the normal force. In the case of gravity, the weight is a function of mass and acceleration due to gravity. The weight of the car pushes down on the road surface, which is pushing back. The weight acts downward, normal to the horizontal.

Friction operates parallel with the interaction surfaces, and is proptional to the compressive normal force acting at that surface.

In the case of the magnet on the refrigerator, and magnetic field applies a magnetic force and the magnet is pushed against the surface of the refrigerator. Friction then acts in proportion to the compressive force (magnet pushing against refrigerator).

The child must have tension in the arms because the child's mass wants to travel tangent to the merry-go-round. Think centripetal force.

5. Sep 4, 2007

### eraemia

What keeps the child from falling off though? Compression with the air? I don't understand how the air can lift the weight of the child, though.