Question

A block of mass 15kg is moving with a constant velocity of 4m/sec then calculate the force on the block

Answer 1

Explanation:

Actually, if you read the question carefully, it's been mentioned that there body is moving with a CONSTANT velocity of 4 m/s in some direction.

=> acceleration in that direction = 0 m/s^2

Since F = ma

=> F = 15 x 0

= 0 N

The force in that direction = 0 N

Hence, the only force acting on the block is the force of gravity.

F = mg

Taking g = 9.8 m/s^2 ,

F = 147 N

Hence the total force acting on the block is 147 N

Answer 2

Net force on block is zero.

Explanation:

• Given data

        Mass of block (M) = 15 kg

        Velocity of block (V) $\mathbf{=4 \ \frac{m}{s}}$ = Constant

• From Newton's second law

        $\mathbf{\sum F=M\frac{V-u}{t}}$

        Where

        $\mathbf{\sum F}$= Net force acting on object

        $\mathbf{\frac{V-u}{t}}$ = Rate of change of velocity with respect to time

• But here it is given that velocity is constant, So

       $\mathbf{\frac{V-u}{t}}$ = 0

• Now come to Newton's second law

        $\mathbf{\sum F=M\frac{V-u}{t}}$

        $\mathbf{\sum F=15\times \frac{4-4}{t}=15\times \frac{0}{t}=0 \ N}$

       Means net force on block is zero.