Category Science

Dr. Mothbold’s automatic polka-dotting machine can paint polka dots on anything – shirts, socks, pancakes, and even pretzels!

Best of all, the polka-dotter is a perpetual motion machine. Perpetual motion is motion, or movement, that goes on forever.

All the parts of the polka-dotter create forces that make the other parts work. As each part gets a push or pull, it gives a push or pull to another part. So, says Dr. Mothbold proudly, this machine will never stop! Is the doctor right?

No. Although pushes and pulls will keep the machine going for a while, other things will make it slow down. Friction is one of these things. The rubbing of wheels against the belt, the brushes rubbing as they paint and the bumping of the brushes on the paint lever will slow down the machine. Finally, the automatic polka-dotter will stop.

People have tried to make all kinds of perpetual motion machines. But the friction that slows the machines is always a little stronger than the forces the machines make to keep running. That is why perpetual motion does not work.

Picture Credit : Google

The light turns green, and the car starts to move quickly. You feel as if a big, invisible hand pushes you back in the car seat. When the car stops quickly, the same “hand” seems to push you forwards.

What is this invisible “something” that pushes you when a car starts and stops? It’s called inertia.

Inertia is the name for the way things resist a change in movement. When something is stopped, it stays stopped. It starts to move only when a force – a push or a pull – makes it move. And when something is moving, it tries to keep moving. It won’t stop until a force stops it.

When a car starts to move, your body tries to stay stopped. So you feel yourself pressing back as the car seat moves forwards. And when the car stops, your body tries to keep moving. Inertia “pushes” you forwards. Your seat belt is there to hold you back.

Use a whirling egg to see the force of inertia. Let a fresh egg warm up to room temperature. Then place the egg in a bowl. Gently spin the egg by pushing it with your finger. Stop the egg from spinning by touching it lightly on the top. Then quickly lift your finger from the egg. What happens?

The egg starts spinning again. Why? When you start the egg spinning, the liquid inside the shell spins, too. When you stop the egg, inertia makes the liquid inside keep spinning. When you let go of the egg, the spinning liquid makes the egg start spinning again.

There’s a big branch on the pavement. Slow down your bike! Then you can go around the branch safely.

Your bike has brakes to help it stop. When you squeeze the levers on the handlebars, the brake pads rub against the wheels and stop them from turning. The rubbing that stops the wheels is called friction.

Friction happens because all things have a rough surface. Even things that look shiny and polished have very tiny areas that are not smooth. When one object slides across another, the rough spots rub against one another. This rubbing, or friction, makes things move more and more slowly, until finally they stop.

Friction is useful when it helps you stop your bike. But sometimes we want things to keep moving smoothly. Then we need to lessen friction. We can do this with a slippery substance, such as oil or grease. For example, the grease on your bicycle chain lessens friction so that you can pedal easily and smoothly.

Suppose someone asked you to lift a hippopotamus! It sounds impossible, but with a simple machine called a pulley, you could do it.

A pulley is a special kind of wheel and axle. A rope or steel cable fits around the rim of the wheel. When one end of the rope is pulled down, the rope slides over the wheel, which turns on the axle. Then the load at the other end moves up.

With one pulley, the load moves up as far as you pull the rope down. You work just as hard to pull the rope as you would to pick up the load, but you can pull in a direction that is easier for you.

With two pulleys, you can make lifting even easier. The second pulley is attached to the thing you lift. Each part of the rope between the pulleys holds half the weight, so you pull only half as hard to move the load. But the load is held up by twice as much rope. So you will have to pull the rope twice as far as the distance you want the load to move.

The more pulleys you use, the easier it is to lift a load. But you will have to pull more and more rope. You might be able to lift that hippopotamus with a great number of pulleys, but you will have to pull a lot of rope!

Wheels don’t eat. But some wheels use teeth to help them do work, work that other wheels can’t do.

A wheel with teeth is called a gear. It is a wheel that makes other wheels move. If you look at an eggbeater, you can see three gears. The big one with the handle is the gear you turn. The teeth on this gear fit into the spaces between the teeth on the two smaller gears. When you turn the handle, the teeth on the big gear push against the teeth on the small gears and make these gears turn. One turn of the big gear makes the small gears turn many times. So the beater blades move very quickly.

First, second, third, fourth – these are the different-sized gears that fit together in a car. The engine turns a set of gears connected to a rod called a crankshaft. The crankshaft turns the axle and the wheels. The lowest gear is first gear. It allows the crankshaft to turn much more quickly than the wheels. This makes more power for starting and for climbing steep hills. Fourth gear allows the wheels to travel around more quickly while using less power. Fourth gear is used for cruising along at the same speed without stopping and starting. It’s the gear that is used on a motorway.

Picture Credit : Google

Back and forth, up and down. What makes a swing fly up in the air?

Most of the time you do the work. You “pump” with your legs and body. Sometimes a friend pushes you. And sometimes the wind pushes the empty swing. But something always has to push it. The swing can’t move by itself.

Any time an object moves, something makes it move. The thing that pushes or pulls it is called a force. Ready to find out how forces make things move?

Forces make things speed up (or accelerate). When a force pushes or pulls the object, the object will move in the direction of the force. The bigger the force, and the lighter the object, the greater the acceleration. It can also make something slow down, speed up or change direction.

Every action has an equal and opposite reaction. When a force acts in one direction it creates an equal force in the opposite direction.