Unit 2: energy & momentum

Question 1

work

Answer 1

done on an object when a force acts on the object, causing the object to move in the direction of the force

Question 2

what happens if the force and displacement are not in the same direction

Answer 2

the component of the force that is in the direction of the
displacement must be used.

Question 3

positive work

Answer 3

• applied force acts in the same direction as the motion (θ lies between 0 ̊ and 90 ̊)
• energy is added to an object
• final energy is greater then inital energy

Question 4

negative work

Answer 4

• applied force acts opposite to the direction as the motion (θ lies between 90 ̊ and 180 ̊)
• force causes the object to lose energy
• final energy is less then inital energy

Question 5

zero work (ex.1)

Answer 5

When the applied force is perpendicular (90 ̊) to the
displacement of the object.
Ex) carrying a textbook across the room

Question 6

zero work (ex.2)

Answer 6

When the applied force is equal to zero.
Ex) on object travelling through space at a constant
velocity

Question 7

zero work (ex.3)

Answer 7

When the displacement of the object is zero.
Ex) pushing on a wall

Question 8

zero work (ex.4)

Answer 8

When an applied force creates Circular Motion.
Ex) keys spinning in a circle (F is perpendicular to the
motion)

Question 9

Work – Energy Theorem

Answer 9

the work done to an object by an external force is equal to the change in its kinetic energy

Question 10

Isolated Systems (Closed)

Answer 10

a system that cannot interact or exchange energy with external systems (its surroundings)

Question 11

Open Systems

Answer 11

a system that can interact with another external system and exchange energy with its surroundings

Question 12

Elastic Object

Answer 12

an object is said to be elastic if it returns to its original or normal
shape after a deforming force is removed
ex) a sling shot, elastic band, trampoline, strut on a car

Question 13

Inelastic Object

Answer 13

object doesn’t return to its original shape

Question 14

Hooke’s Law

Answer 14

the amount of force exerted by a spring is directly proportional to
the displacement of the spring

Question 15

Perpetual Motion Machines

Answer 15

• this is a device that can operate for an unlimited amount of time without outside help (restarting or refueling)
• these machines cannot be created in the real world as some of the mechanical energy will always be lost from the system as thermal energy, sound energy, or other forms of energy.

Question 16

Damping

Answer 16

the effect of friction acting on the SHM. The motion may stop before one complete oscillation or after many oscillations.

Question 17

Types of Damping

Answer 17

1) overdamping: the oscillation ceases and the mass moves extremely slowly back to the equilibrium
2) Critical damping: the displacement falls to zero as quickly as possible without moving past the equilibrium position
3) Underdamping: the system undergoes many oscillations before damping brings it to rest

Question 18

What if the force is NOT constant?

Answer 18

then find the area between the line of motion and the time axis

Question 19

closed or isolated system

Answer 19

the term given when there is no external unbalanced force acting on the system

Question 20

Perfectly Elastic Collisions

Answer 20

Here the momentum and energy of the system are both conserved

Question 21

Perfectly Inelastic Collisions

Answer 21

• momentum is conserved (because momentum is always conserved)
• kinetic energy is no longer conserved