Motion and Force

• How do we decide if something is moving? For example, if you are in the car with your friends...are your friends moving?

• Sure, their hands are going up and down, they are blinking, and most of us would say they are moving.

• What determines if something is moving or not?

• If an object is in motion, then its position changes when compared to another object.

• If you use yourself as a reference point for motion, then you might say that your friends are not moving. Or if you use the seat in the car as the reference point, again, you might say they are not moving.

• What if you used the stop sign on the corner of the road as the reference point? Are they moving now?

Because motion is determined by a reference point, and that reference point can change...we say that motion is relative.

So, if you are in a car and you use the trees on the side of the road as your reference point, then you are moving.

What happens if you change the reference point to your seat in the car? Are you still moving?

• When you move your chair, type on your keyboard, or sit at your desk you are applying a force.

• A force is a push or pull.

• A force is described by using an arrow pointing in the direction of the force. The longer the arrow, the stronger the force.

• The SI unit for force is a newton, named after Sir Isaac Newton.

• If you push a box across the floor, you are applying a contact force. The friction that works against you is also a contact force.

• The gravity holding you to the Earth is a noncontact force. Magnetism and electric forces are other examples of noncontact forces.