Question

You push a hockey puck that is initially at rest on slick ice by applying a constant force until the puck reaches a final velocity of 1 m/s. On the second attempt, you want the hockey puck to reach the same final velocity by applying a force that is twice as large.
1. Therefore, you must exert the force for a time interval that is
A. shorter than the time interval of your first attempt.
B. longer than the time interval of your first attempt.
C. the same as the time interval of your first attempt.
2. After the hockey puck has reached the final velocity, you suddenly stop pushing it. The hockey puck:
A. stops abruptly
B. reduces speed gradually
C. continues at constant velocity
D. increases speed gradually
E. reduces speed abruptly

Answer 1

Answer:

1. A

2. B or C

Explanation:

1.

F=ma, meaning that if you use two times more force on a constant mass, the acceleration must double. Acceleration is change in velocity, which means that if you are aiming for the same final velocity the change must happen in half of the time. Therefore, the correct answer is choice A.

2.

By Newton's first law, an object in motion will stay in motion unless an external force acts on it. Since there is nothing pushing the puck in the other direction, the puck will either keep on going for at a constant velocity or will reduce its speed gradually, depending on whether or not this ice is considered to be frictionless. Hope this helps!

Answer 2

(1) You must exert the force for a time interval that is  shorter than the time interval of your first attempt.

(2) The hockey puck reduces speed abruptly.

According to Newton's second law of motion; the force applied to an object is directly proportional to the mass and acceleration of the object.

F = ma

$F = \frac{mv}{t}$

• The force applied to an object is directly proportional to the velocity of the object and inversely proportional to the time of motion of the object.

• To double the force, you must halve the time interval.

Thus, to apply a force that is twice as large as the first while maintaining the same velocity, you must exert the force for a time interval that is  shorter than the time interval of your first attempt.

(2) The force applied to an object is direct directly proportional to the velocity of the object. Once you stop applying force to the hockey puck, it moves for a short with initial momentum gained before it will stop.

Thus, the magnitude of the velocity (speed) will drop sharply once the force on the object is removed.

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