Question

homework # 15 – newton’s first law
handout problems:

1. how does the law of inertia explain each of the following:

(a) passengers standing in a train are thrown backward when the train suddenly moves forward.
(b) they are thrown forward when the train suddenly stops.
(c) they are thrown to one side when the train rounds a curve.

1. which person has the greatest inertia? explain.

(a) a 110-kg wrestler resting on a mat
(b) a 90-kg man walking at 2 m/s
(c) a 70-kg runner traveling at 5 m/s
(d) a 50-kg girl spinning at 10 m/s

1. a highly polished block of wood is given a push that sets it sliding across a long polished floor. (a) what path does it take? (b) why does it gradually slow down? (c) what change would take place in the motion of the block of wood if the friction between it and the floor were increased?

Explanation:

For question 1:

• (a) Initially, passengers and the train are at rest. When the train moves forward, the lower part of passengers' bodies (in contact with the train) moves with the train due to friction, but the upper part tends to stay at rest (inertia), so they are thrown backward.
• (b) When the train stops, the lower part stops with the train, but the upper part tends to keep moving forward (inertia), so they are thrown forward.
• (c) When the train rounds a curve, the train's path changes. Passengers' bodies tend to move in a straight line (inertia), so they are thrown to the side.

For question 2: Inertia depends on mass (more mass means more inertia). The 110 - kg wrestler has the greatest mass, so the 110 - kg wrestler (option a) has the greatest inertia.

For question 3:

• (a) In the absence of friction (ideal case), it would take a straight - line path (due to inertia, it tends to move in a straight line at constant velocity). With minimal friction, it takes a nearly straight - line path initially, but friction causes it to slow.
• (b) It gradually slows down because of the frictional force between the block and the floor. Friction opposes the motion, acting as a net force that causes deceleration (Newton's second law, but related to inertia as without friction, it would keep moving).
• (c) If friction is increased, the deceleration (rate of slowing down) will be greater. The block will slow down more quickly, and its path might be slightly affected (but still approximately straight) as the frictional force is a retarding force.

Answer:

1. (a) Explained by inertia (upper body resists motion change). (b) Explained by inertia (upper body resists motion change). (c) Explained by inertia (body resists path change).
2. a. a 110 - kg wrestler resting on a mat
3. (a) Straight - line (nearly) path. (b) Friction opposes motion. (c) Slows down faster.