Summary
Overview
Forces, which we might instinctively describe as pushes and pulls, are acting on us at all times, but we cannot always see them. This hands-on lesson offers a fun opportunity to explore "invisible" forces like gravity and air resistance. Students will build parachutes and investigate how they allow skydivers to safely land.
Learning Objectives
- Use an arrow to represent a force.
- Make a schematic drawing of a falling object indicating gravity, air resistance, and speed.
- Predict how and explain why changing the air resistance of an object affects how fast it falls.
- Explain how parachutes can create safer landings.
NGSS Alignment
This lesson helps students prepare for these Next Generation Science Standards Performance Expectations:- 3-PS2-1. Plan and conduct an investigation to provide evidence of the effects of balanced and unbalanced forces on the motion of an object.
| Science & Engineering Practices | Disciplinary Core Ideas | Crosscutting Concepts | |||
| Science & Engineering Practices | Planning and Carrying out Investigations.
Make observations and/or measurements to produce data to serve as the basis for evidence for an explanation of a phenomenon or test a
design solution.
Analyzing and Interpreting Data. Compare and contrast data collected by different groups in order to discuss similarities and differences in their findings. |
Disciplinary Core Ideas | PS2.A: Forces and Motion.
Each force acts on one particular object and has both strength and a direction. An object at rest typically has multiple forces acting on it, but they add to give zero net force on the object. Forces that do not sum to zero can cause changes in the object's speed or direction of motion. |
Crosscutting Concepts | Cause and Effect.
Cause and effect relationships are routinely identified, tested, and used to explain change. |
Materials
For each group of 2 students, you will need:
- Tissue paper or a plastic bag
- Scissors
- Ruler
- Tape
- Hole punch
- Twine
- Small action figure or miniature doll that may be dropped on the floor. If you do not have an action figure, use any small, dense object such as a piece of clay, an eraser, a measuring spoon, etc.
Background Information for Teachers
This section contains a quick review for teachers of the science and concepts covered in this lesson.Scientists refer to a push or pull as a force. Forces can change the movement of an object (its speed and/or direction), but they do not always do so. Imagine a grocery cart standing still. You can push on the handlebar to make it move (Figure 1, top left). If it is already moving, and you push it, you can make it move faster (Figure 1, top right).
Figure 1. Pushing on a shopping cart can change its motion. Red lines indicate the cart's speed, and blue arrows indicate a push. A push can make a cart move (top left), and an additional push can make it speed up (top right). Pushes in opposite directions can cancel each other out (bottom).
It gets a little more complicated when more than one force acts on an object. Imagine pushing the grocery cart again, but this time, another person is pushing equally hard on the opposite side of the cart. The two opposing forces would cancel each other out, and the cart's movement would not change (Figure 1, bottom). Most often, objects have many forces acting on them. We often do not realize this because some forces are canceled out by others and thus do not affect the motion of the object. For example, the shopping cart has mass, so gravity pulls it down—but the cart does not fall, because the ground pushes back with equal strength in the opposite direction.
In this lesson, students will study how forces can affect the speed of a falling object by looking at a skydiver. Without an open parachute, the skydiver is in free fall. Gravity pulls him or her down, and almost nothing is pushing back up to slow down or prevent the fall. The situation changes when the parachute opens. Suddenly, a lot of air particles need to move out of the way to let the open parachute pass. The air pushes the parachute—and the skydiver hanging from it—up, as shown in Figure 2. This push, or force, is called air resistance or drag. It has a direction opposite to the movement. In this case, this force acts in the opposite direction to gravity. As a result, the force of gravity is partially cancelled out and the skydiver does not gain speed as quickly. The skydiver falls at a slower pace and is able to safely land.
Figure 2. Forces acting on a skydiver coming down with a parachute.
As shown in Figure 2, scientists represent forces with directional arrows. The arrow points to where the force pulls or pushes. Gravity always pulls objects down, and air resistance always points against the motion of the object it acts upon.
