Question

The position vector of an object of mass 0.5 kg is given by r 2.0t2i+ 3.0tj m where t represents time. Which ONE of the following statements is FALSE when a time of t 2.0 s has elapsed. a) The object has moved a distance of 10 m. b) The momentum of the object is 41+1.5/ kg.m.s c) The kinetic energy of the object is 18 J d) The force on the object acts perpendicular to the direction of the unit vectorj e) The magnitude of the force acting on the object is 4 N

Answer 1

Answer:

Statement E is false.

Explanation:

Statement a says that the object would have moved a distance of 10 m.

At t = 0

x0  = 0

y0 = 0

At t= 2

x2 = 2 * 2^2 = 8

y2 = 3 * 2 = 6

$d2 = \sqrt{x2^2 + y2^2}$

$d2 = \sqrt{8^2 + 6^2} = 10 m$

Statement A is true.

Statement B says that the momentum of the object would be 4*i+1.5*j

To test this we need to know the speed.

The speed is the first derivative by time:

v = 4*t*i + 3*j

At t = 2 the speed is

v2 = 8*i + 3*j

The momentum is

P = m * v

P2 = 0.5 * (8*i + 3*j) = (4*i+1.5*j)

This statement is true.

Statement C says that the kinetic energy of the object would be 18 J.

The magnitude of the speed is

$v = \sqrt{8^2 + 3^2} = 8.54 m/s$

The kinetic energy is

Ek = 1/2 * m * v^2

Ek = 1/2 * 0.5 * 8.54^2 = 18 J

Statement C is true

Statement D says that the force acts perpenducular to the direction of the unit vector j.

The acceleration is the second derivative respect of time

a = 4*i + 0*j

Since there is no acceleration in the direction of j we can conclude that the force is perpendicular to the direction of j.

Statement D is true.

Statement E says that the force acting on the object is of 4 N

f = m * a

f = 0.5 * 4 = 2 N

Statement E is false.