Sliding Down the Hill

SCIENCE CONCEPT:

To make anything start moving, go faster or stop, a force in needed. A person needs a 'force' (a push from a friend) to start sliding down the grassy hill on cardboard. Once the person starts down the hill they would continue forever unless a force acted upon them. This is Newton's First Law of Motion. Friction is important here. Friction is a force that happens when two things rub together. Friction opposes motion and makes things slow down. The person accelerates slowly up to a certain speed and also takes a little time to decelerate again. The time taken to change speed (accelerate or decelerate) depends on the size of the person. A bigger person would take longer. This resistance to change of movement is called 'inertia'. The bigger the person the more inertia they have.

STUDENT OBJECTIVE:

The student will be pushed down a grassy hill riding a piece of cardboard. They will experience, first hand, the dynamic of Newton's First Law of Motion.

OVERVIEW:

In this activity, the student will use a piece of cardboard to ride down a grassy hill. Each student will take a turn being pushed by a classmate to start their ride down the hill. In the second ride the students will ride in pairs and be pushed by two classmates to start their ride. In this way they will experience the dynamic of movement, acceleration, friction, and inertia.

PREPARATION TIME:

Variable

LESSON TIME:

45 minutes.

TEACHER PREP:

Locate a grassy hill where the children can safely 'ride' down the hill on a piece of cardboard.

WORDS TO KNOW:

force

friction

static

kinetic

inertia

accelerate

decelerate

TEACHER TEXT:

Friction is a secondary force that is present when two different objects or materials touch each other. Friction never causes any motion. It always acts against any attempt to move the surface of one object over another. Friction is treated like any other force that may be pulling or pushing on an object. Friction always has a direction opposite to that of any movement. But it is present within the dynamic of Newton's First Law of Motion. It is safe to say that we couldn't live without friction.

Without friction, every surface would be perfectly slippery, far slippery than the smoothest sheet of ice. We wouldn't be able to stand without falling. Our hands would be nearly useless for grasping things. The only way we could hold something would be to cup our hands around it. Squeezing an object would only make it shoot out of our fingers.

We take friction for granted, but without it our world would be very different. Without friction, most of our homes would fall apart. It is friction that holds the nails in boards. Without it, all of the nails would just slide back out again and the boards would separate. We need friction for just about everything, but there are a few cases were we would like to make it much less. To motors and machinery of all kinds, friction is an enemy. Any place where moving surfaces touch, friction causes wear. Without friction, cars, airplanes, and toys would last many times longer than they do today. We need friction to live, but there are times and places where we would like to be able to reduce it.

There are two types of friction. Static friction and kinetic or sliding friction. Static friction is the force between two surfaces that are not moving relative to each other. A book resting on top of a table is an example of static friction because there is no movement between the book and the table. Kinetic friction is the force between surfaces where there is movement between them. If you pushed on the book hard enough to start it sliding, the force between the moving book and the tale is kinetic friction.

The strength of static friction is the amount of applied force that is needed to just start an object moving. The strength of kinetic friction is the amount of applied force that is needed to keep the object moving at a constant speed.

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Last modified: Tue Mar 24 15:43:58 PST 1998