Forces and Motion

In this lesson students build on their knowledge of forces as they explore the relationship between the amount of force needed to change an object’s motion and the mass of the object. They create a model energy system to observe how the amount of force needed to move a tray changes as the tray’s mass increases.

• For an object to change its motion, a force has to transfer energy to or from the object.
• An object’s motion will change according to its mass, as well as the sum of the forces acting on it.

• What has to happen to cause a stationary object to start moving?
• Why does an object need an outside force to get it to move?
• Why does a force applied to an object cause its motion to change?
• Why does a ball rolling on the ground eventually come to a stop?

Misconception: Energy is used up, and then it disappears.

Fact: Energy is never created or destroyed. Instead it transforms from one form to another. remain in motion unless acted on by an outside force.

Misconception: Sustaining motion requires a continued force.

Fact: An object in motion will remain in motion unless acted on by an outside force. 

Acceleration : an increase in speed over time; measured in meters per second squared (m/s2)

Gold Medal in Bobsledding

Vonetta Flowers used to dream of competing in the Summer Olympics in track and field. In 2000, she tried out for the Olympic Trials but didn’t earn a spot on the team. She felt discouraged and thought about giving up on her Olympic dreams.

Two days after the Olympic Trials, Vonetta’s husband saw a flier urging track and field athletes to try out for the U.S. bobsled team. Vonetta didn’t know much about bobsledding, but her husband, also a track and field athlete, convinced her to try out with him.

She did, and one year later, Vonetta won a gold medal at the 2002 Winter Olympics in the two-person bobsledding event. With her gold medal, Vonetta broke several records. She was the first person from Alabama to win a gold medal in the Winter Olympics, and the first person of African descent from any country to win a gold medal in the Winter Olympics.

The Initial Push

In bobsledding, a two- or four-person team drives a specially designed sled down an ice track. In the first five seconds of the course, the bobsledders sprint while pushing the sled forward. They push with as much force as they can to get the sled moving. They then jump into the back of the sled, behind the driver, to move down the track. They can reach speeds of up to 135 kilometers (80 miles) per hour.

Forces and Interactions

Once the bobsled begins to move over the track, the force of gravity takes over. Remember that gravity is the attractive pull between all matter. Earth’s gravity pulls down on all objects, including the bobsled as it moves over the hill.

BAs they begin to move down the hill, the primary goal of the bobsledders is to reduce the forces that act on them that might slow them down. This is because forces can transfer energy out of a system as well. The bobsled is carefully designed to minimize the forces that transfer energy out of the system.

Forces that Transfer Energy Out of a System

Friction is one way that energy is transferred out of a system. Friction is a force that slows motion when two objects rub against each other. Friction slows motion because it causes some of the energy of the moving object to change into heat. Friction is why your hands feel hot after you rub them together.

Ice is much smoother than other surfaces, so there is less friction that occurs when the sled moves over the ice compared to rougher surfaces such as grass. The low amount of friction explains why it is difficult to walk across ice without slipping and falling. However, some friction still occurs. To counter this, fast bobsleds are made with smooth runners that are as thin as possible. This is to reduce the amount of surface area that comes into contact with the ice, and therefore reduce the amount of friction.

Drag, also called air resistance, is another force that transfers energy out of a system. Drag is similar to friction, but it occurs between a solid substance and a fluid such as air. Air moving around the bobsled produces drag, which transfers energy out of the system. The shape of the bobsled and the materials it is made from are designed to reduce the amount of drag. The bobsledders themselves also wear skin-tight clothing to reduce drag.

Relationship between Force and Mass

There are rules about how much mass a bobsled can have when empty and when the bobsledders are on it. Remember that mass is the amount of matter in an object. An object’s motion will change according to its mass, as well as the sum of the forces acting on it.

This is because of another law of motion, which states that force equals mass times acceleration. Acceleration is an increase in speed over time. It is measured in meters per second squared (m/s^2). This law says that the amount of force needed to move an object any distance depends on its mass. An object with greater mass needs more force to accelerate than an object with less mass. In other words, a more massive sled will need more force to cause it to accelerate a certain amount than a less massive sled would need.

A Moving Sled

Once the sled starts moving down the hill, a more massive sled will go faster than a less massive sled. We’ll explore this idea more in the next section because it has to do with the relationship between an object’s mass and the amount of kinetic energy it has.

In this lesson students conduct an investigation to explore the relationship between the amount of force needed to change an object’s motion and the mass of the object. Students engage as scientists as they figure out the phenomena of how the amount of force needed to pull an object changes depending on the mass of the object.