Velocity

[please note: if the the velocities of objects A and B are vₐ and vᵦ with respect to a stationary observer (neutral observer) on the ground, then the velocity of A relative to B is vₐ - vᵦ. this is as if B were at rest and only A were moving. here B is the truck and A the ball]

let velocity of the ball relative to a stationary observer by the side of the road = vₐ km/h

given velocity of the truck relative to a stationary observer by the side of the road = vᵦ km/h = 50 km/h

velocity of A relative to B is vₐ - vᵦ = 15 km/h [truck's direction being +tive and as if the truck were at rest and only the ball moving]

=> vₐ - 50 = 15 or vₐ = 65 km/h


ii) here A is the ball and B is the driver of a car moving in the same direction as the truck at a speed of 75km/h

therefore velocity of the ball relative to the driver = vₐ - vᵦ = 65 - 75 = - 10 km/h [ie: 10 km/h speed in the opposite direction of car and truck movement considering the car driver to be at rest as it were]