Exploring Forces · Lesson 4 of 7
Gravity and Vertical Motion
“Use falling and upward-moving objects to understand gravity as an always-attractive non-contact force.”
• Define gravitational force and explain why it is a non-contact force. • Describe gravity as an always-attractive force. • Explain how gravity affects an object thrown vertically upward. • Describe changes in speed and direction during vertical motion. • Compare gravitational force with magnetic and electrostatic forces.
Why Objects Return to the Ground
Throw a ball straight upward. Even if it is thrown harder, it eventually returns to the ground. The same pattern is seen with many objects: after rising or moving away from the ground, they are pulled back toward Earth.
This tells us that Earth exerts an attractive force on objects. The force acts even though Earth is not physically touching the object, so gravity is a non-contact force.
The force with which Earth attracts objects toward itself is called gravitational force. It is also called the force of gravity or simply gravity.
Gravity is always attractive. This differs from magnetic and electrostatic forces, which may either attract or repel depending on poles or charges.
| Force | Can attract? | Can repel? | Needs contact? |
|---|---|---|---|
| Gravitational force | Yes | No | No |
| Magnetic force | Yes | Yes | No |
| Electrostatic force | Yes | Yes | No |
Vertical Motion Under Gravity
When an object is dropped, it moves downward toward Earth. When it is thrown vertically upward, it first moves upward, slows down, stops momentarily at its highest point, reverses direction, and then speeds up as it falls.
While the ball moves upward, gravity acts downward, opposite to the direction of motion, so the ball's speed decreases. At the highest point, its speed is momentarily zero, but gravity still acts. After the direction reverses, gravity and motion are both downward, so the ball speeds up.
Motion in a vertical direction under the influence of gravitational force is called vertical motion.
At the top of its path, a ball may have zero speed for an instant, but gravitational force is still acting downward. That force causes the ball to begin moving downward.
Problem
A ball thrown vertically upward reaches its highest point and is momentarily at rest. Is gravity still acting?
- 1.The ball's speed is zero only for an instant.
- 2.Earth still attracts the ball even when it is momentarily at rest.
- 3.Therefore, gravity continues to act downward and changes the ball's motion from upward to downward.
Connecting Gravity with Everyday Motion
The same gravitational pull helps explain why bicycles accelerate while moving downhill and why dropped objects fall toward the ground. Gravity continuously influences motion even when no person is pushing or pulling the object.
Quiz
Why is gravity called a non-contact force?
Which statement about gravity is correct?
What happens to the speed of a ball moving vertically upward?
At the highest point of a vertically thrown ball, gravity is
How is gravity different from magnetic force?
Practice Problems
- Explain why a ball thrown upward always comes back down.
- Describe the motion of a vertically thrown ball from release to return.
- What force acts on the ball at the highest point of its path? Explain.
- Compare gravitational force with magnetic force in terms of attraction and repulsion.
- Explain why a bicycle moving downhill can speed up even without pedalling.
- Imagine gravity suddenly disappeared. Describe two immediate effects on everyday life.
Key Takeaways
• Gravity is the attractive force with which Earth pulls objects toward itself. • Gravitational force is a non-contact force and is always attractive. • A vertically thrown object slows while rising, stops momentarily, then falls and speeds up. • Gravity acts downward throughout the motion, including at the highest point. • Vertical motion provides a clear example of force changing both speed and direction.