Review
In the section on Moving, we began by introducing some new words.
We learned that there are many kinds of forces: pushing, pulling, weight (or gravity), friction and air resistance. When many act at once, they combine to form the net force. It is the net force that determines motion.
Friction is important in our everyday lives. It keeps you from slipping when you walk. Your foot pushes back against the ground and the frictional force of the ground pushes back against your foot.
There are actually lots of forces at work when we walk. These forces act to shorten certain muscles to make our bones move. It is complicated, for sure! But even though there are many forces involved, they add up to only one net force, which acts to propel us forward as long as there is enough friction on the walking surface. If you could put on a space suit and take a walk in space, you wouldn't go anywhere because you wouldn't have that walking surface to oppose your motion!
Friction depends on mu, the coefficient of friction, and N, the normal (or supporting) force. Sometimes mu is a good thing to have.
Sometimes it's not such a good thing - especially if you want to skate fast on ice!
Friction acts in a direction that is opposite the direction of motion (and the forces that drive the motion). It combines with other forces to create the net force, which is always less than the driving force. Friction can therefore be used to control motion. You can slow down your bike when you go down a hill (where the driving force is gravity) by applying just a little friction with a small squeeze on your brakes. If you are sliding down a pole, your weight (gravity) is the driving force that pulls you to the ground. Friction exists between the pole and your body and acts to reduce the net force acting on you. If you want to go more slowly, you need to hold on tighter to increase the friction and decrease the net force.
The force of gravity always pulls us toward the earth. That means that if we are not attached to the earth, we soon will be! Objects not attached to the earth in any way - objects flying through the air, for example - are called projectiles.
When a projectile moves through the air, the path is a shape called a parabola.
Another new term we learned is momentum. While momentum is just the combination of mass and velocity, it is most interesting because the total amount of momentum doesn't change when a number of objects interact.
One of the most important things we learned is that Isaac Newton figured out three laws of motion. These can be used to describe everything from hockey pucks on ice to rockets in space. Newton's First Law says that if we apply a force to something, its velocity will change.
Newton's Second Law tells us how much the velocity will change, given the force acting on an object and its mass. We saw that if you apply a large force to a baseball, you will generate a large acceleration and if you apply a small force to the same baseball, you will generate a small acceleration. If you apply a force to a bow with a single arrow, the arrow will experience an acceleration before it is released.
If you double the mass of the arrow and apply the same force, the acceleration will be half as much.
There's another way to think about this example. That is, you would have to apply twice the force to the arrow that has twice the mass to generate the same acceleration as the single arrow.
Newton's Third Law says that for every force, there is an equal and opposite force. This law comes in handy when you lean against a wall, sit on a chair, or lie in a hammock. If the wall, chair or hammock can't support your weight, you will fall to the ground and the equal and opposite force to your weight will be the normal force of the ground. Plus, you might get hurt!
Whether you are walking, riding in a car, sledding down a hill, or just dozing off in the shade, physics is all around you. All you need to do is think about it whenever you look at something. If you do, it will make your world a much more interesting place!