2004PPp1q4 2010SPp1q4 Dynamics (3rd law)

Answer: Option D

Key concept here is Newton’s 3rd Law. Action equals reaction. Both trolleys are connected by the spring, hence the tension force experienced by each trolley is equal and opposite.

Fx    =    Fy

ma       =    ma2

m x 2    =    m x a

a        =   4 m/s2

Another important concept to know. In Fig A, a box of m is pulled with a force of 40 N. Given that the frictional force is 10 N. The resultant force is 30N. Box is accelerating.

In Fig B, if another identical box m is added on top of the existing box (no change to the floor), the total mass is 2m, the frictional force now will double = 20 N. Hence resultant force now will be 20N. The acceleration of the box will be lower than previously.

Bar magnet near a current-carrying rectangular coil (attraction / repulsion)

Basic terms in an electric circuit