Force and Motion

November 24, 2012 2:00 pm

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Force and Motion
You make forces all the time. Forces press on you, and you use them to press on objects. Pushing your foot against your bicycle pedal makes a force. The force sets the bike in motion. Every time something starts moving or stops moving, a force is responsible.
You use force to throw a ball in the air. The force of Earth’s gravity pulls the ball back down. Every step you take creates a force on your foot and on the ground. Forces and motion are part of everyday life.
EVERYTHING IS LAZY
Why do you have to push a shopping cart to get it started? Objects that are sitting still stay that way until something makes them move. They have something called inertia. Inertia makes objects keep on doing whatever they are doing. Everything made of matter has inertia, even you!
If a shopping cart is standing still, inertia keeps it standing still. If the cart is moving, inertia keeps it moving. You have to apply a force to overcome inertia. The cart needs a push or pull to start moving down the grocery aisle. To stop the cart by your favorite cereal, you need to apply another force.
CHANGING SPEED AND TURNING
Imagine that you’ve been shopping for a while and the store is about to close. You start pushing your shopping cart faster. You turn your cart when you head for a different aisle. Scientists call any change in motion acceleration. Acceleration can be a change in speed, direction, or both.
By now, your shopping cart is full. It’s a lot heavier than when you first started to push it. You have to push harder to get the cart moving. You have to push harder to make the cart go faster. Your cart needs more force because it has more mass or stuff in it. Scientists would tell you that the force needed to accelerate your cart or any other object by a given amount depends on the object’s mass (amount of matter). The more mass an object has, the more force you have to use to get it moving or slow it down.
PROBLEMS WITH FRICTION
As if inertia and mass were not enough, you have to overcome friction to make objects move. Friction is also a force. Friction resists one object moving against another. Friction tries to keep your shopping-cart wheels from turning.
What if you had all your groceries in a big box? You would have to slide the box over the floor. Friction between the box and the floor comes from tiny bumps. The bumps on the box and the bumps on the floor get stuck against each other. You need force to move the bumps past each other. The rougher the floor the harder it is to slide the box. You could pour oil on the floor. Oil would lessen the friction. Oil would make the floor smooth and slippery. It would be easier to slide the box over the bumps on the floor.
Moving groceries in a cart with wheels is easier than sliding them in a box. There is still friction between the floor and the wheels. But it takes less force to roll the wheels over the floor.
A WORLD WITHOUT FRICTION
What if there were no friction? Wouldn’t that make life easier?
Life would be impossible without friction. Friction makes your shoes stick to the floor. Without friction you would slip and fall and not be able to stand up. You wouldn’t be able to pick anything up either. Friction allows your hands to grip things. Cars wouldn’t be able to stop without friction. Lids would pop off of containers. Anything that moves would never stop moving without friction.
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