Comparing Forces

In this lesson, students apply their knowledge of matter, mass, and gravity to carry out an experiment and analyze the effects of unbalanced forces on the motion of a straw rocket.They discuss how unbalanced forces cause movement and how forces of different strengths affect the movement of a straw rocket.

• All motion is caused by unbalanced forces.
• Gravity is a force that can cause motion.
• A rocket needs an unbalanced force that is strong enough to overcome the force of Earth’s gravity to cause it to launch (move) into space.
  
  

• Why is gravity a force?
• How does the force of gravity change the direction of a ball that is thrown up in the air?
• Since gravity pulls on all objects, why don’t we all sink into the ground?
• Why are the forces acting on you balanced when you are standing still on the ground?
• What has to happen to cause a stationary soccer ball to move?
• What will most likely happen if you kick the soccer ball harder?
• Why do space rockets have engines that push hot gas downward?
• How does the hot gas pushing downward cause a rocket to move upward?
• What would happen if a weak force is applied to a rocket?
  
  

Misconception: Only Earth has gravity.

Fact: All matter has gravity. All objects near Earth’s surface are pulled on by Earth’s gravity because Earth is the most massive object near us.

Misconception: Earth doesn't move in the solar system. 

Fact: The sun’s gravity keeps Earth and the other planets in orbit.

Misconception: If an object is at rest, no forces are acting on it.

Fact: Forces are constantly acting on objects both at rest and in motion. 

Cause and Effect : a relationship between events or things, where one is the result of the other

Force : a push or pull that acts on an object, changing its speed, direction, or shape

Gravity : a force of attraction between all matter

Mass : a measure of the amount of matter that makes up an object; a property of matter

Matter : everything that has mass and takes up space

Orbit: to travel in a circle around an object

Pattern : something that happens in a regular and repeated way

Planet : a body that orbits the sun, is massive enough for its own gravity to make it round, and has cleared out smaller objects around its orbit; there are eight known planets in our solar system

Scale :the size, extent, or importance (magnitude) of something relative to something else

Weight : the gravitational force exerted on an object by a planet or moon

Overcoming Gravity

Scientists sent Curiosity to Mars on a rocket in November 2011. The rocket launched from Cape Canaveral in Florida. More than thirteen thousand people watched it take off.

As scientists counted down, there was a loud noise. A trail of fiery gas seemed to push the rocket upward into the sky.

When a rocket takes off, an engine pushes hot gas down toward the ground. In reaction, the rocket moves up through the atmosphere.

The huge upward force is important. There has to be enough force to overcome the pull of Earth’s gravity. A force is a push or pull that acts on an object, changing its speed, direction, or shape.

Balanced & Unbalanced Forces

Forces are all around us. There are forces acting on you right now, even if you can’t feel them. Gravity is a force because it is pulling down on you.

Forces always come in pairs. For example, when you stand on the ground, gravity pulls you down toward the center of Earth. In reaction, the ground has its own force that pushes back up. This keeps you from sinking into the ground.

Forces can also be unbalanced. If the wall is not strong enough, the force of you leaning against the wall will cause the wall to collapse. This is because you applied a greater force to the wall than the wall returned.

The same thing happens when you lean against a wall. You apply a force to the wall. The wall applies the same amount of force in return. In this case, the forces are balanced.

In this lesson, students conduct an experiment using a rocket launcher to see how high a straw rocket lifts off the ground when pushed on with strong and weak forces. In the experiment, students collect and analyze data on how far the straw rocket traveled upward when pushed with a strong force compared to a weak force. Students use their data to support the argument that unbalanced forces cause objects to move, while balanced forces do not change an object’s motion. They also discover that a straw rocket lifts higher off the ground when pushed by a strong force compared to a weak force.