Statics and dynamics

Question 1

Force

Answer 1

describes strength of interactions between objects
F=ma
Unit: Newton, kgm/s^2
vector quantity: it’s direction equals direction of acceleration

Question 2

Newton’s first law

Answer 2

Law of inertia
When viewed in an inertial reference frame, an object either remains at rest or continues to move linearly at constant velocity, unless acted upon by net force.

Change in motion can always be blamed on another object

Question 3

Inertia

Answer 3

An objects resistance to change in it’s velocity

(In our case: our reference frame doesn’t accelerate.

Question 4

Newton’s second law

Answer 4

The acceleration of an object and the force acting on the it are proportional to each other. F = m*a

If more forces acts on the object at the same time, they are all summed to have the acceleration proportional to the forces.

Question 5

Newton’s third law

Answer 5

Law of equal action and reaction describes the symmetry of interactions. When object A exerts a force on object B, then object B exerts a force with same magnitude but opposite direction to object A.

Forces always appear in pairs

Question 6

Equilibrium

Answer 6

Refers to the law of inertia. An object will be in equilibrium when net force acting on it is zero.

Question 7

Law of universal gravitation

Answer 7

A particle attracts every other particle in the universe with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them

F= G ((m1*m2)/r^2)

G is the gravitational constant, G= 6,6710^-11 m^3/(kgs^2)

Increasing distance decays interaction inversely with the square of the distance.

Question 8

Gravity

Answer 8

The force that accelerates an object falling freely towards the Earth. Calculated from the law of universal gravitation.

Fgravity = m*g gives the gravity of earth acting on an object

Question 9

Weight

Answer 9

The force with which an object is pressing or pulling the surface that holds it.

An object in equilibrium with only gravity and normal force acting on it has a weight that equals gravity.

Question 10

Weightlessness

Answer 10

When an object isn’t supported by any surface and it’s weight is zero. Example is a free falling object.

Question 11

Hooke’s law

Answer 11

When a spring is extended a restoring force develops which is proportional to the extension of the spring.

F= -k*s

k is the spring constant (N/m) that shows what force a spring can exert if we extend it by one meter, it depends on the materials properties

Question 12

Why is there a minus sign in Hooke’s law?

Answer 12

Needed because the elastic restoring force developed in the spring points in a direction opposite to extension.

In compression, the k will be positive.

Question 13

(PP) Which one of the phrases is included in Newton’s second law?
A… until another object will compel to it’s change in motion
B… acceleration of an object ant the force acting on the object are proportional to each other
C… forces always appear in pairs
D…object remains in equilibrium if the net force…

Answer 13

B.

Question 14

(PP)Base units of Newton

Answer 14

1kg*m/s^2

Question 15

(PP) We throw a ball up in the air then it falls back into our hands. Neglecting air drag, what can be said about the ball while it’s in the air?
A) There is no force acting on it
B) There are two equal, butt oppositely directed forces acting on it: a force an a counter force.
C) There is one force acting on it, which first decreases until it becomes zero for a moment, then increases.
D) There is one constant force acting on it.

Answer 15

C ?

Question 16

(PP) A pear falls from a tree and in 0,65 seconds it reaches the ground. What was the height of the branch from where it fell? 
A) 414,5 cm
B) 6,377 cm
C) 637,7 cm
D) 207,2 cm

Answer 16

A) 414,5 cm?

Question 17

(PP) According to the law of universal gravitation…
A) the heavier the object, the faster it falls
B) none of the answers is correct
C) mass can be converted into energy according to the following equation: E=mc^2
D) objects with greater mass are attracted by stronger forces to each other.

Answer 17

B ??

Question 18

(PP) A race car's mass is 700 kg. It reaches a speed of 100 km/h in 6 seconds from rest. Calculate the force acting on the accelerating car.
A) 42 kN
B) 151,2 N 
C) 3241 N
D) 11,67 kN

Answer 18

C

REMEMBER TO CONVERT KM/H TO M/S!!!!!

Question 19

(PP) Let us consider the achilles tendon as a simple spring. If it is pulled by 0,5 kN force, it extends by one millimeter. Find it's spring constant.
A)  2000 mm/N
B) 5000 N/m
C) 2 mm/kN
D) 500 000 N/M

Answer 19

D

Question 20

(PP) According to the law of universal gravitation …
A) the attractive force acting on a body is proportional to its mass
B) none of the other three answers are correct
C) the gravitational acceleration is 9,81m/s^2 everywhere
D) The heavier an object, the faster it falls

Answer 20

A

Question 21

(PP) An elevator accelerates, moves with a constant speed, decelerates, and then stops as it moves from the 2nd floor to the third floor. When is the elevator in mechanical equilibrium?
A) When it accelerates, moves with constant speed, decelerates and when it does not move (all the time)
B) Only when it does not move
C) Only when it moves with a constant speed and when it does not move
D) Never, because gravity is always acting on it

Answer 21

My intuition says C but it could be D.

Question 22

(PP) A truck's mass is 3800 kg. A 7,6kN force accelerates it from rest. What will be it's velocity after 10 seconds?
A) 20 km/h
B) 72 km/h
C) 2,888 m/s
D) 5 m/s

Answer 22

A

Question 23

(PP) A ball is falling freely in the air. When air drag is neglected, which statement is true?
A) There is no force acting on it
B) There are two equal, but oppositely directed forces acting on it: a force and a counter force.
C) The ball is strongly attracted by the Earth but the ball only weakly attracts the earth
D) There is one constant force acting on it.

Answer 23

D

Question 24

A car is going straight along a roas with a constant speed of 45 km/h. Choose the correct statement.
A) There are no forces acting on this car
B) There is exactly one force acting on this car
C) The car is in equillibrium
D) The car is not in equilibrium because it Is in motion

Answer 24

C)