1.1 Force - ReactInk

Gravity, Impulse, Inertia, and Momentum

1 Motion

Nature has patterns.

We can call them principle or rules. But by any name, they say what we expect will happen. If I toss a ball up, I expect it will fall down. Gravity. This section introduces four rules related to motion. One of the rules is gravity.

The rules are simplified. For instance, unless otherwise noted, ignore air resistance and friction. The rules are also rounded and ignore some exceptions to avoid math since the focus is on critical thinking, not calculations.

Gravity

Gravity. Objects with mass attract (pull on) one another. More mass, more attraction. The closer the objects, the greater their attraction.

Impulse

Impulse is force times time. Increasing the time of contact reduces the force of contact. That’s why a baseball catcher’s mitt is padded thickly, to slow the blow.

Inertia

Objects at rest stay at rest and objects in motion stay in motion unless acted on by a net force. The more the mass, the more the tendency to stay as they are.

Momentum

Momentum is mass times velocity. A train has a lot, even moving slowly, given its large mass. If speed or mass increase, then the momentum increases.

Court of Nature.

Next are twenty assertions about the way things work. Two are false. They’re close to the truth, but not correct. Identify the imposters. For each of the rest, identify which of the above principles best explains how or why it works as it does.

Imagine this is the court of nature. You have twenty cases to judge. Two to find out and throw out. For each of the rest, decide which of the four laws of nature apply.

Which rule?

A fast bullet doesn’t have to be very massive to have a big impact.

Momentum. As speed increases, momentum increases

A pebble falls just as fast as a stone ten times more massive.

Inertia. The stone is pulled by gravity with a force 10 times that of the pebble, but it also has 10 times the inertia (reluctance to change its state of motion).

A rope is hung over a pulley. At one end is a weight which exactly balances a monkey at the other end. As the monkey climbs, the weight will rise by the same amount.

Inertia. To move up, the monkey pulls down on the rope; down on the monkey’s side is up on the weight’s side. The two are balanced when the money is stationary and move as the monkey moves.

After one ice skater collides with another stationary skater with the same mass, they come to an abrupt stop on the ice.

False. Not quite an example of linear momentum because. The mass doubles (two skaters), so the speed is half. They don’t come to an abrupt halt, but slide together in the direction of the first skater, but at half of what was his speed

At a grocery, it is more effective to snap-jerk a plastic bag off the large roll than to slowly pull the bag off along the perforated line.

Inertia. The inertia of the large roll at rest resists the rapid acceleration of the snap-jerk so the plastic bag tears off evenly, easily. Pulling slowly, the large roll begins to unwind and the plastic stretches or tears unevenly.

Automobiles have padded dashboards and flexible bumpers to cushion impact.

Impulse. Cushioning spreads the impact over a longer time, reducing the force.

Basketball courts are hardwood over a concrete foundation to makes the ball bounce more lively.

False. The hardwood gives the floor a bit of bounce so it is easier on the legs of the players, like a trampoline effect. This makes the ball bounce less (not more), however.

Boxers tend to roll with the punch to increase the time of glove-to-face contact.

Impulse. Rolling with the punch spreads the impact over a longer time, reducing the force.

Harold hit the brakes to avoid hitting a skunk and his groceries tipped over in back seat.

Inertia. When the vehicle stops, the groceries — once in motion — will stay in motion at the same velocity.

It is easier to balance a longer pole than a shorter pole on one finger.

Inertia. More mass, more inertia — which means more resistance to motion. In this case the motion is falling rotation about the axis of a finger tip