$(a)$ Define force and state three possible effects of force on an object.
$(b)$ $A$ motorcycle is moving with a velocity of $90 \, km \, h^{-1}$ and it takes $5 \, s$ to stop after the brakes are applied. Calculate the force exerted by the brakes on the motorcycle if its mass along with the rider is $200 \, kg$.

(N/A) Force is an external agent that produces or tends to produce acceleration in an object. Three possible effects of force are:
$(i)$ It can set a stationary object into motion.
$(ii)$ It can stop a moving object.
$(iii)$ It can change the speed or direction of motion of an object.
$(b)$ Given:
Initial velocity $u = 90 \, km \, h^{-1} = 90 \times \frac{5}{18} = 25 \, m \, s^{-1}$
Final velocity $v = 0 \, m \, s^{-1}$
Time $t = 5 \, s$
Mass $m = 200 \, kg$
Using the first equation of motion $v = u + at$:
$0 = 25 + a(5)$
$5a = -25$
$a = -5 \, m \, s^{-2}$
Using Newton's second law of motion $F = ma$:
$F = 200 \times (-5) = -1000 \, N$
The magnitude of the force exerted by the brakes is $1000 \, N$ in the direction opposite to the motion.