Investigating Motion Science Chapter B2 Text Pages B43-B73

Lesson 2 What Affects Motion? Text Pages B49-B53

Vocabulary inertia: the tendency of an object to stay at rest or to remain in motion unless an outside force causes a change

Inertia An object that is moving will stay moving until something makes it stop. A baseball flying through the air A marble rolling across the floor An object that is sitting still will stay sitting still until something makes it move. A marker sitting on a table A soccer ball sitting on the field Every object has inertia!

Inertia A force is needed to overcome inertia. A push or a pull What forces could overcome the inertia of the following items? A baseball flying through the air A marble rolling across the floor A marker sitting on a table A soccer ball sitting on the field

Inertia and Vehicles When we travel in a vehicle, we feel inertia. Our bodies seem to move backward when a vehicle takes off suddenly. Inertia tries to keep them in the same place. Our bodies keep moving forward when a vehicle stops suddenly. Inertia tries to keep them moving forward.

Seat Belts Save Lives! A seat belt is a force that can overcome inertia. Seat belts keep our bodies from moving forward when a vehicle stops suddenly.

Inertia and Bicycles Inertia must be overcome when we ride a bicycle. A force is needed to make the bicycle go. We push on the pedals to make the bicycle start moving. A force is needed to make the bicycle stop. We apply the brakes to make the bicycle stop moving.

Invisible Forces Some forces cannot be seen. Air molecules bump against moving object and slow them down. Gravity pulls objects toward the earth. Friction works against the movement of objects.

Missing Forces! In outer space, there are no air molecules to push against moving objects. When spacecraft get far from the surface of the earth, Earth’s gravity is too week to overcome their gravity. Once a spacecraft gets moving, it can keep going because there is nothing to stop it.

Pairs of Forces Forces always work in pairs. One force pushes in one direction while another pushes with equal strength in the opposite direction. One force is called the action force. The other force is called the reaction force.

Pairs of Forces A jumping frog is a good example of pairs of forces. When the frog hops forward, the lily pad moves backward. Which is the action force, and which is the reaction force?

The Earth vs. You We experience action and reaction forces when we walk. When we take a step, we push against the earth in order to move forward. This is the action force. We don’t feel it, but the earth also pushes against our feet when we walk. This is the reaction force. Why doesn’t the earth move like the lily pad?

Put it all together! How does inertia affect an object’s motion? Name some forces that affect motion. How do forces work?