Kinematics – Swimmer in river with current, find distance downstream

Solutions: Option B

Without the current in the river, the time taken for the swimmer to reach Q is 50s. This is based on the vertical constant speed of 1.2 m/s and veritcal (width of river) distance.

With the current, which is constant 1.0 m/s, the swimmer will be drifted downstream. But the time taken to reach R from P is still 50s. So with the same 50s, you can calculate the horizontal distance based on the horizontal current speed of 1.0 m/s.

Vector Diagram – Parallelogram Method for 4 different kinds of questions

For Combined Science Physics Olevel, when constructing vector diagram, a method called Parallelogram Method is commonly used.

Parallelogram Method is used when 2 forces are known, and the resultant (net) force R, can be found.

Below show 5 different variations of questions when can be solved by using Parallelogram Method:

(A) Body has acceleration, forces are unbalanced, there is a resultant force R (Newton’s 2nd law)

(B) Body at rest, all forces are balanced, there is no resultant force R (Newton’s 1st law)(C) Body moving at constant velocity, all forces are balanced, there is no resultant force (Newton’s 1st law)

(D) Finding unknown force (with resultant force R  + another known force are given)(E) Vector diagram is applicable to all vector quantities (with magnitude and direction). Not just for forces.

Coppers Surprising Reaction to Strong Magnets Force Field Motion Dampening

If the circuit is open, i.e. there two ends of the solenoid is open, there is induced emf produced, but no induced current produced. That is why the magnet can fall through normal with free fall acceleration 10 m/s2.

Only when the circuit is closed, there is an induced current flowing through. According to Lenz's law, the direction of the induced current is such that the magnetic effect opposses the change producing it, hence there is a repulsion force when the magnet is falling through the solenoid.