Gravity — Answer Key
Part A: Fix the Sentence
Each sentence has an error. Rewrite it correctly on the line.
1. Fix the sentence:
Gravity can either push or pull any object that is near a planet.
Corrected: Gravity can only pull objects; it never pushes them away from a planet.
Gravity is purely attractive between masses; it does not push objects apart.
2. Fix the sentence:
When a book sits still on a desk, the forces on it is unbalanced.
Corrected: When a book sits still on a desk, the forces on it are balanced.
The desk pushes up with the same force that gravity pulls down, so forces are balanced.
3. Fix the sentence:
A falling apple have unbalanced forces because gravity pull it down with no support.
Corrected: A falling apple has unbalanced forces because gravity pulls it down with no support.
With no upward force from a stem or hand, gravity wins and the apple accelerates downward.
Part B: Fill in the Blank
Write the missing word or number on each line.
1. Gravity is an attractive force, which means it only pulls objects together.
Gravity acts only as a pulling force between any two masses.
2. Forces that cancel each other and cause no motion are called balanced forces.
Balanced forces produce no change in motion because they sum to zero.
3. Forces that do not cancel and cause an object to speed up are called unbalanced forces.
Unbalanced forces create acceleration in the direction of the larger force.
4. Gravity does not have an opposite force; it only causes attraction between masses.
Gravity is one-way attraction; there is no known repulsive twin force.
Part C: Short Answer
Answer each question in one or two complete sentences.
1. Why is a Grade 5 student standing still an example of balanced forces?
Sample answer: Gravity pulls the student downward, but the floor pushes upward with an equal force. Because the two forces cancel out, the student does not move up or down. The forces are balanced.
Equal and opposite forces produce zero net force, so motion does not change.
2. Explain why gravity is called a one-way force instead of having a push opposite.
Sample answer: Gravity always pulls masses toward each other and never pushes them apart. There is no opposite force that repels objects with mass, so we describe gravity as one-way attraction acting between any two objects.
Scientists have observed gravity only as attraction, not repulsion, between masses.