Physics Exam 4

What is a collision?

A short-duration impact between two objects.

What does the duration of a collision depend on?

It depends on the materials of the object. A hard object will have a short contact time while a soft object will have a long one.

What is an impulsive force?

It is a large force exerted during a short time interval.

What is the formula for impulse based on a force vs time curve?

J = average force x change in time 

Momentum points in the same direction as —-?

Velocity

The average force needed to stop an object is —— to the time of the collision.

Inversely proportional. The greater the collision time, the less the force of impact.

What is the formula for impulse and its relation to momentum?

J = average force x change in time

J = p final - p initial 

J = mv final - mv initial 

What is an impulse approximation?

It is when smaller forces are ignored during the brief time of the impulsive force.

What does the area under the curve of a force vs time graph represent?

Impulse/ change in momentum

The forces acting on two balls during a collision are an —/— pair. They have — magnitude in — direction.

action/reaction, equal, opposite 

If an object is only subject to internal forces, what will happen to the total momentum of the system?

There is no change.

What is the law of conservation of momentum?

The total momentum of an isolated system is a constant. Interactions within the system do not change the system’s total momentum.

What conditions must be met for the total momentum of a system to be conserved?

There can be no external forces acting on the system.

What is conserved in an inelastic collision?

momentum, some kinetic energy is transformed

T/F: Energy is not created or destroyed.

True

What is total energy?

The sum of different energies present in a system.

What is an energy transfer?

An exchange of energy between the system and the environment.

What is kinetic, gravitational potential, elastic/spring, thermal, chemical, and nuclear energy?

Kinetic (K): motion

Gravitational Potential (Ug): stored energy associated with the object’s height above ground

Elastic/spring (Us): energy stored when an object is stretched (→ kinetic)

Thermal (E thermal): sum of microscopic kinetic and potential energies of the molecules in an object

Chemical (E chem): energy in atomic bonds

Nuclear (E nuclear): forces that hold particles in the nucleus together (→ kin → thermal)

What is heat?

The nontechnical transfer of energy due to a temperature difference.

Describe work.

It is the mechanical transfer of energy through pushing or pulling. The environment applies a force while the system undergoes a displacement. Energy is only transferred when the system moves while the force acts. 

T/F: The total energy of an isolated system is conserved.

True

What forces can change the energy of the system?

Only external forces

A —— displacement or force causes greater work to be done.

Greater

When does force do the most work?

When it is in the same direction of the object’s displacement (parallel).

When does a force not do work?

1. The object undergoes no displacement while the force acts.

2. Force is completely perpendicular to the system.

3. The part of the object that the force acts on has no displacement.

What is the equation that related force to work?

W = Fd (J/Nm)

What is the equation that relates energy to work?

Change in E = W

When is negative work done?

The angle between the force and r is greater than 90 degrees.

Is the kinetic energy of a rolling object greater/less than the kinetic energy of a non rotating object?

It is always greater because of the sum of translational and rotational kinetic energy. 

What does Ug depend on?

The height of the object in relation to the reference point. It does not depend on the path to get there.

When does thermal energy occur?

When there is friction between objects.

Describe thermal energy.

When heated, atoms jiggle around their average position more. Each atom has a higher kinetic energy because they are moving faster. Each atom has higher potential energy because they are straying further from their equilibrium position. 

Why is the line for energy in a potential energy curve always horizontal?

The sum of kinetic and potential energy is the same at every point.

What is photodissociation?

It is the breaking of molecule bonds by absorbing light.

What is kinetic speed dependent on? What is potential energy dependent on?

Speed; position

What is stable vs unstable equilibrium?

Stable: small disturbances only cause small oscillations around the equilibrium point

Unstable: small disturbances drive the particle from equilibrium 

Where are unstable & stable equilibriums located on a PE curve?

Unstable: local max

Stable: local min 

What is a fluid?

It is a substance that flows and takes the shape of its container.

Will a connected liquid in hydrostatic equilibrium rise to the same height in all open regions of the container?

Yes

In hydrostatic equilibrium, the pressure is the — at all points on a horizontal line through a connected liquid of a single kind.

Same

If pressure at one point in an incompressible fluid is changed, what will happen to the pressure at every other point?

It changes by the same amount.

What is a buoyant force?

An upward force of the fluid.

T/F: The magnitude of a buoyant force = the weight of the fluid displaced by the object

True

What is neutral buoyancy?

The densities are equal and the object and fluid are in static equilibrium.