Now when we know Newton’s laws of motion, let’s explore the forces that can start and stop motion.

Are you still not sure what is Newton’s laws about? Check bellow:

Newton’s first law – see what it says.

Newton’s second law – reed more.

Newton’s third law – see what it says.

Do you remember what strength is?
Yes. This is the ability to push or pull an object. An example of such a force that we encounter every day is gravity. Another force that we constantly encounter is friction.

Gravity and friction are the main factors that affect the movement of an object.

👀 Observe
Rub your palms vigorously against each other for a few minutes. What happened? Why?

They warmed up because of the friction.

The energy of the movement with which you rubbed them into each other turned into thermal energy.

Have you ever had calluses on your feet from uncomfortable shoes? Yes? Do you know why? Because the shoe rubs at your feet while walking, and in the place where the shoe was uncomfortable there was the greatest friction, calluses come out. You already know that the force of friction can cause damage.

Whenever we try to move something, we face the force of friction. It impedes movement. For example, when we push (slide) a book on the table, the friction invisibly pulls the book in the opposite direction. The rougher the surface of the table, the greater the friction is and we have to apply more force to push the book.

Scientists define the force of friction as a force that impedes the movement of an object. The force of friction not only stops the movement but also works in the opposite direction of the movement. In other words, the force of friction is a force opposite to the direction of motion.

Without friction, our lives would be very difficult. For example, if there was no friction between the tires of the wheel and the road, we would not be able to stop.

Why is the road sanded in winter? To increase the friction between the car tires and the frozen road.

Have you tried to grab and hold a wet soap in your hand? Yes? It always slips away, doesn’t it? While a dry soap is much easier to grab and hold in your hand. Why?
A dry soap rubs more against your hands than the wet one.

Do you know how it works at the atomic level? Molecules in a liquid move more freely than those in a solid. The molecules on the wet surface slide easily next to each other. When dry objects rub against each other, the friction is stronger.

Water and oil, for example, can greatly reduce friction on an object.

Rough surfaces have a much stronger friction force than smooth surfaces. The asphalt of the road is rougher than the ice in winter. But in some cases, we may be surprised by this statement.

When a car slips on the ice in the winter or gets stuck in the mud, we say that there is no grip. The grip is the friction between two objects that help them not to slide against each other.

How can you most easily get the car out of the mud? By gathering a group of strong people to push it out of the mud? Or add dry sticks, stones and soil under the wheels and push the car lightly so that the wheels stand on the sticks and stones?

Have you noticed that goalkeepers wear gloves? Do you know why?

Precisely because of the friction. The surface of the gloves is made of a special material (polymers) with fine grooves that increase traction, so they can more easily catch and hold in their hands the ball that is thrown at them.

Adhesion
Adhesion is another form of friction. Adhesion means bonding two different materials together. There is a great similarity between adhesion and cohesion. But cohesion means a material sticking to itself. If you take a piece of duct tape and fold its edges so that they stick to each other – this is cohesion.
Adhesive tape wound patches and other sticky substances are called adhesives. An object is an adhesive if it can hold the materials together through surface contact.
The glue is also an adhesive. If the adhesive is very strong, it permanently stops the movement.

Adhesion is similar to friction. And it also stops the movement. How do adhesives work?

Most scientists believe that dispersion adhesion is caused by so-called forces of Van der Waals. This Dutch scientist has discovered that bonding occurs at the molecular level. Polar molecules can have a positive and a negative side, so they can attach to almost any surface. This explains why geckos can walk on almost any surface, holding on to one little finger. The sticky legs of the gecko use the forces of Van der Waals. The molecules on the gecko’s legs are strongly attracted to the molecules on the surface on which it climbs. That is why understanding atomic physics is very important for studying physics.

✅🧑‍🎓 Check your skills:

1. What is strength?
2. What is friction?
3. The objects became warmer or colder because of the friction?
4. Say 3 examples about force of friction around you.
5. Why does the wet surface slide more easily next to each other then the dry surface?
6. What is difference between adhesion and cohesion?
7. Can you say an example of adhesion and cohesion around you?
8. How do adhesives work?
9. Does adhesion stop the movement?

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