Question

Emily pulls a 5-kg block across a rough horizontal surface, exerting a constant force on the block. The magnitude of the force is initially 5 N, and the block moves at a constant velocity of 2 m/s. While the block is moving, Emily instantly increases the force to 10 N. How will the block move now? Andrew says: The block will move at a constant velocity of 4 m/s because it was initially moving with constant velocity and the final force is two times larger than the initial force. Jasit says: The block will move at a constant acceleration of 2 m/s2 because the sum of the forces exerted on the block is 10 N and the mass of the block is 5 kg. Mary says: The block will move at a constant acceleration of 1 m/s2 because the sum of the forces exerted on the block is 5 N and the mass of the block is 5 kg. Part A Which student is correct?

Answer 1

Answer:

Mary

Explanation:

The block will move at a constant acceleration of 1 m/s2 because the sum of the forces exerted on the block is 5 N and the mass of the block is 5 kg.

Answer 2

Answer:

Mary is correct.

Explanation:

When the block is being pulled with a 5N force, it is moving at a constant velocity. Constant velocity means no acceleration, therefore the net force is zero (this by Newton's second law: $\sum F=ma$). When applying an additional 5N to the block its net force will become 5N. Using Newton's second law we can obtain its new acceleration:

$5N=(5kg)a$,

solving for the acceleration

$a=(5N/5kg)$,

$a=1(m/s^2)$.