Place the lead brick gently on your fingers and strike the lead brick sharply with the hammer. The inertia of the lead brick prevents damage to your fingers.

Engagement Suggestion

Engagement Suggestion:

Background:

Newton’s First Law of Motion states that an object’s velocity is constant unless there is a net force acting on it. What this means is that if an object is not moving (at rest), it will not start moving until there is a force pushing or pulling on it. If an object is moving at a constant speed and direction, it will keep going with that same speed and direction unless a force pushes or pulls on it to change that. When the pen is sitting on top of the hoop, the force of gravity is pulling it down, but the normal force of the hoop is exactly equal to the gravitational force and holds it up. If another force suddenly affects the pen (such as if you walk up and tap on its side, or jiggle the hoop up and down), that force could cause it to move, and probably fall.

But if the hoop is snatched sideways quickly and smoothly, it does not give any force to the pen. Now the only force acting on the pen is gravity, and the pen falls straight down into the bottle.

If a plastic straw (like those from McDonald's) is pushed slowly into a raw potato, the rigid potato will cause the softer plastic straw to bend and break. However, if a significant velocity is given to the straw it can be pushed clean through the potato. This is what often happens in a tornado, where apparently softer objects are accelerated to high speeds by the wind and driven through apparently harder objects.

CAUTION: Be sure that the hose fitting is securely attached to the tube and that the plastic shield is in place before firing. The shield should be latched in place, with no debris blocking its edge from meeting the baseplate

Engagement Suggestions

• • Before using, encourage your students to predict what will happen to the pencil.
• • For advanced students, discuss the energy involved in the problem and where the kinetic energy of the pencil went after the collision.

Background

This demonstration can be presented in multiple ways. It has been offered classically as an illustration of the principle of inertia – the pencil is in motion at a high velocity, and continues in motion despite the intervening wood until arrested by a greater force. Alternatively, consider the high velocity and high momentum of the pencil. The abrupt deceleration at the plywood means a high impulse. The pointed pencil has a very small cross-sectional area, resulting in force applied over a small area leading to a high momentary pressure.

Linked below is a slow-motion video of the collision, shot at 600 frames per second. A fun class activity could be to use the video to measure the motion of the pencil and estimate its momentum and kinetic energy, based on what you see in the video and by measuring typical lengths and masses for wooden pencils.

Donated to the Facility by Tom Senior of the Physics Instructional Resource Association.

Load the arms with equal masses at the same or different distances from the center, and observe what happens when the cart is accelerated by hand along the track. Alternatively, load the arms with masses in the ratio of 10:1 which look the same, and ask students to account for the behavior of the apparatus. By lifting one end of the track, show that when a force (gravity) is allowed to act uniformly on all parts of the apparatus the crossarm will not rotate regardless of how it is loaded.

A simple demonstration sequence is to place more mass on one side (at front in pictures above) and accelerate the cart with your hand to illustrate inertial mass, then let the cart accelerate down the inclined track to illustrate gravitational mass.