Hilda pushes her sofa across the room with a force of 40 n. it accelerates at 2 m/s/s. what is the mass of the sofa?
2 kg
20 kg
40 kg
80 kg

keiko is riding a skateboard 2 m/s, west. how long will it take her to reach a velocity of 8 m/s, west if her acceleration is 2 m/s2?
3 seconds
6 seconds
8 seconds
10 seconds

The correct answer of this question is : A) 20 Kg and B) 3 seconds.

EXPLANATION :

As per the first question, the force applied on sofa F = 40 N.

The acceleration of the sofa a = .

We are asked to calculate the mass of the sofa.

From Newton's second law, we know that external unbalanced force is equal to the product of mass with acceleration of the body.

Mathematically force F = ma.

Hence, mass of the sofa is calculated as -

                              mass of sofa  m =

                                                         =

                                                         = 20 Kg.

As per the second question, initial velocity u = 2 m/s due west.

The final velocity of the skateboard v = 8 m/s.

The acceleration of the skate board a =

The time taken [ t ] to reach that velocity is calculated as -

From equation of kinematics, we know that -

                                           v = u + at

                                          ⇒8 = 2 + 2t

                                         ⇒ 2t = 6

                                         ⇒  t =

                                           ⇒t = 3 s.            [ans]