Work and energy

Question 1

Energy

Answer 1

Describes the state of an object or a system When you transfer energy to an object, you do work to that object.

Question 2

Work and energy can describe…

Answer 2

interaction between object; not only physical, but also thermal and chemical

Question 3

Work

Answer 3

W= F*s

Work describes the strength of the interaction and the distance in which the interaction took place.

Symbol: W

Question 4

Work in angle

Answer 4

W = Fs cos alpha

When the force acting on an object is at an angle with the direction of movement, then we only take the projection of the force vector along the displacement.

Question 5

SI unit of Work

Answer 5

Joule (1 N*m)

Question 6

Is work a vector or a scalar quantity?

Answer 6

Work is a scalar quantity and has no direction.

Question 7

When is the work zero?

Answer 7

When the direction of force is perpendicular to the direction of displacement, that is, alpha is 90 and cos is zero.

Question 8

Power

Answer 8

the rate of doing work

P=W/t

Unit: Watt (1 J/s)
Symbol: P

Question 9

Energy

Answer 9

Symbol: E

The ability of a system to perform work, when work is done on a system its energy will either decrease or increase.

Unit: Joule (same as work)

Question 10

Kinetic energy

Answer 10

the energy of an object due to it’s motion

Ekin= 1/2 *m *v^2

We assume uniform acceleration from rest and that the average velocity is half of the final velocity.

Question 11

Kinetic energy

Answer 11

the energy of an object due to it’s motion

Ekin= 1/2 *m *v^2

We assume uniform acceleration from rest and that the average velocity is half of the final velocity.

Question 12

Potential energy

Answer 12

Energy that results from a position or configuration, it may be gravitational, electric or magnetic.

An example of configuration-dependent energy is the elastic energy stored in an object undergoing elastic deformation.

Question 13

Gravitational potential energy

Answer 13

capacity for doing work as a result of the object’s position in a gravitational field

Epot= mgh

In order to lift up a mass (m), from the reference level to a height (h), we have to exert force of mg on the object along a displacement of h. The work during elevation is also mgh.

Question 14

Elastic potential energy

Answer 14

The energy stored as a result of deformation of an elastic object, such as a stretched spring:

Eelastic = 1/2 k*s^2

To stretch the spring we have to exert a uniformly increasing force. Work done (0,5ks^2) is stored in the spring as elastic potential energy.

Question 15

Conservation of energy

Answer 15

the total amount of energy in an isolated system remains constant.

E(kin) + E(pot) + E(kin) = Constant.

Question 16

(PP) What is the power, if the work done during half a minute is 120 kj?
A) 120 kW
B) 240 W
C) 4 kW
D) 240 kW

Answer 16

C

Question 17

(PP) On a 10 km cross-country running race the finish is located at an altitude 1500 meters higher than the start. What is the mechanical work done by a 70 kg runner when running the field from start to finish? (g=10m/s^2)
A) 1050 kJ
B) 1050 N
C) 105 kJ
D) 105 N

Answer 17

A????

Used Epot= mgh