18. Exercises

As a car moves along a straight rod the distance, s metres, of a car from the origin at time t seconds is given by:

s=3t3−t460 for 0t10.

a) By differentiating, find an expression for the velocity of the car at time t.

b) Find an expression for the acceleration of the car at time t.

c) Find the times when the acceleration of the car is zero.

A particle, of mass 4 kg, accelerates from rest so that the distance that it has travelled in t seconds is s where s=5t2−6t.

a) Find the velocity and acceleration of the particle.

b) Find the time when the velocity is zero.

c) Find the magnitude of the resultant force on the particle at this time.

A crane lifts a load from ground level. The height, s m, of the lift at time t seconds is given by s=503t2−t3250 for 0t10.

a) Show that the velocity of the load is zero when t= 10.

b) Find the height of the load at this time.

c) Find the time when the acceleration of the load is zero.

d) Find the height of the load at this time.

The distance, s m, travelled by a dragster at time t seconds is given by,

s=36t2−2t3

This expression only applies until the acceleration of the dragster becomes zero for the first time.

a) Find the time when the acceleration of the dragster is zero.

b) Find the speed of the dragster at this time.

c) Find the maximum acceleration of the dragster.

A particle moves so that its displacement, s m, at time t seconds is given by s=kt2−35t3.

a) Show that the particle is at rest when t = 0.

b) Find k, if the particle comes to rest when t = 20.

c) Sketch an acceleration-time graph for the particle.

d) Find the time when the acceleration of the particle is zero.

A particle, of mass 5 kg, moves so that its position vector, r metres, at time t seconds is given by:

r=(12−2t2)i+(10−5t2)j

The unit vectors i and j are perpendicular.

a) Find the position of the particle when t= 10.

b) Find the speed of the particle when t= 2.

c) Find the magnitude of the resultant force on the particle when t= 6.

An aeroplane moves so that at time t seconds, its position vectors, r metres, is given by:

r=40ti+3t2+20tj  

The unit vectors i and j are that are directed horizontally and vertically respectively. The aeroplane is initially at ground level.

a) Find the time when the height of the aeroplane is 500 metres.

b) Find the speed of the aeroplane at this time.

c) Show that the acceleration of the aeroplane is constant and state its magnitude.