Forces: Pushes and Pulls - Second Grade Science
Nothing in our universe moves on its own without a reason. When a soccer ball sits motionless in the green grass, it will remain there forever unless an outside force kicks it into the goal net. When a wagon rolls down the sidewalk, you must grab its handle to pull it to a stop. In physics, any action that causes an object to start moving, speed up, slow down, change direction, or stop completely is called a force. The two fundamental types of forces are pushes and pulls.
What is a Force?
A force is a push or a pull acting upon an object. Forces have both strength (magnitude) and direction.
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| THE TWO BASIC FORCES |
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| A PUSH A PULL |
| Moves an object AWAY from you! Moves an object TOWARD you! |
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| [ YOU ] ====> [ BOX ] [ YOU ] <==== [ WAGON ] |
| (Pushing a swing, throwing a ball) (Pulling a sled, opening a door)|
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Examples of Everyday Pushes
A push moves an object away from the person or thing applying the force: Kicking a soccer ball across a playground. Pushing a lawnmower across the grass. Tapping the keys on a computer keyboard. Pressing a doorbell with your index finger.
Examples of Everyday Pulls
A pull moves an object closer to the source of the force: Pulling open a heavy refrigerator door. Tugging on a kite string in the wind. Reeling in a fishing line with a hooked fish. Pulling your winter boots onto your feet.
Balanced vs. Unbalanced Forces
What happens when multiple forces act on the same object at the exact same time? The outcome depends on whether the forces are balanced or unbalanced.
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| BALANCED VS. UNBALANCED FORCES |
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| BALANCED FORCES (Equal Strength, Opposite Direction) |
| [ Person A: 50 N ] ---> [ BOX ] <--- [ Person B: 50 N ] |
| Result: Net force = 0! The box DOES NOT MOVE! |
| |
| UNBALANCED FORCES (Unequal Strength) |
| [ Person A: 100 N ] ========> [ BOX ] <--- [ Person B: 30 N ] |
| Result: Net force to the right! The box ACCELERATES to the right! |
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Balanced Forces: Staying at Rest
When two equal forces act in opposite directions, they cancel each other out. The net force is zero. If two students pull on opposite ends of a tug-of-war rope with identical strength, the rope will not move an inch. Balanced forces do not change an object's motion.
Unbalanced Forces: Causing Motion
When one force is stronger than the opposing force, the forces become unbalanced. Unbalanced forces cause objects to accelerate, decelerate, or change their direction of travel.
How Forces Change Motion
Forces can alter the movement of objects in several distinct ways.
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| HOW FORCES AFFECT MOTION |
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| 1. START MOTION ----> Kicking a stationary ball rolls it forward. |
| 2. SPEED UP ----> Giving a moving skateboard an extra push. |
| 3. SLOW DOWN ----> Dragging your sneakers to stop a bicycle. |
| 4. CHANGE DIR. ----> A baseball bat hits a pitch out to field. |
| 5. STOP MOTION ----> A catcher's mitt catches a speeding ball. |
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Force, Mass, and Acceleration
The mass (weight) of an object strongly influences how much force is required to move it.
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| FORCE AND MASS RULE |
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| LIGHTWEIGHT PING-PONG BALL HEAVY SOLID BOWLING BALL |
| Tiny mass (2.7 grams) Huge mass (6,000 grams) |
| A tiny puff of air sends it flying! Requires a powerful push to |
| roll down the alley! |
| |
| Heavier objects require GREATER force to move or stop! |
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More Mass Needs More Force
If you push an empty shopping cart, a gentle push sends it rolling easily. If that same cart is filled with heavy gallons of milk and canned food, you must lean forward and push with all your muscle power to get it moving. Similarly, stopping a heavy moving object requires far more braking force than stopping a light one.
Chapter Review Exercises
Question 1: Define what a force is in science, and describe the difference between a push and a pull.
Question 2: What happens to the movement of an object when two equal forces push against it from opposite directions?
Question 3: List three different ways that an unbalanced force can change the motion of a rolling toy car.
Question 4: Why does a heavy solid metal bowling ball require a much stronger push to start rolling than a lightweight foam ball?
Solutions and Explanations
Solution 1: A force is a push or a pull acting upon an object that can change its motion or shape. A push is a force that moves an object away from the person or thing applying the force, whereas a pull is a force that moves an object closer.
Solution 2: When two equal forces push from opposite directions, the forces are balanced and cancel each other out. As a result, the net force is zero, and the object remains completely still without moving.
Solution 3: An unbalanced force can cause the rolling toy car to speed up (accelerate forward), slow down and come to a stop, or change its direction by deflecting it to the left or right.
Solution 4: A heavy bowling ball has significantly more mass than a lightweight foam ball. According to the laws of motion, objects with greater mass have greater inertia (resistance to changes in motion), which means they require a much larger force to start moving or stop.