Motion & Forces Notes

Describing and Measuring Forces An object is said to be in motion if it changes position relative to a reference point. Motion means moving. To find out if an object is in motion, you MUST compare it to another object or place. This is known as relativity. An object is in motion if its distance from another object or place is changing.

A reference point is an object or place that you can use to tell if an object is in motion. A tree, a sign, or a building make good reference points. Describing and Measuring Forces

Have you ever been in a bus or car sitting beside another one? Suddenly, you think that your vehicle is moving backward. Upon closer inspection, you actually find that it’s the other vehicle that is moving forward. Because you were using a moving object as a reference point, your sense of motion was inverted. Now, imagine astronomers when they saw the sun. Because they thought that Earth was the stationary reference point, they mistakenly thought that it was the universe that revolved, and not Earth. Describing and Measuring Forces

Describing and Measuring Motion Motion is measured as distance. A meter is a unit of length. This can be modified by adding a metric prefix such as kilo-, nano-, cent-, etc. 1mm =.001 meters 1nm = meters 1km = 1000 meters

Calculating Speed If you know the distance an object travels in a certain amount of time, you can calculate the speed of an object. Speed is a rate. It tells how far something moves in a certain amount of time. For example, 1 meter per second is speed.

Calculating Speed D ST D = DistanceT =Time S = Speed

Calculating Speed On a car ride to school, you speed up and slow down. Average speed tells the total distance you rode divided by the total time it took. Instantaneous speed is the speed you were moving at an instant in time. during the car ride.

It’s ten miles from 79th Street to 5300N. It takes your school bus 20 minutes to drive this distance. What is it’s average speed? But note the railroad crossings. At each crossing, the bus is required to stop. Therefore, as the bus approaches each crossing, it must slow down, and it’s instantaneous speed will decrease. Another way to determine instantaneous speed is to just look at the speedometer.

Calculating Speed Suppose that a cyclist travels 32 kilometers in the first 2 hours of her trek. She then travels another 13 kilometers during the next hour. What is her average speed?

Describing Velocity To completely describe the motion of an object, you need to not only know its speed, but also its direction. When you know both speed AND direction, then you can calculate velocity Velocity is speed in a given direction.

Describing Velocity For example, you hear that a thunderstorm is traveling at 25 km/h. Should you fear for your life? Well, that depends on its direction of motion. If you’re an air traffic controller, if you just give the speeds of the other planes to the pilots, then they still might crash into each other. The planes’ directions are vitally important.

24 hours of United States Air Traffic

What is a Force? Like velocity, force is described by its strength and by the direction in which it acts. A force is a push or pull.

What is a Force? To tell about a force, you must tell how strong the force is. The SI unit for strength of a force is the newton. To tell about a force, you must also tell the direction the force is pushing or pulling. Arrows can be used to show forces. The point of the arrow shows the direction of the force. The length of the arrow shows how strong the force is.

Combining Forces Unbalanced forces acting on an object result in a net force and cause a change in the object’s motion. Balanced forces acting on an object do not change the object’s motion. Often there is more than once force acting on an object. The total of all the forces acting on an object is called the net force.

Combining Forces Sometimes the net force on an object is 0. This means there are balanced forces acting on the object. The object’s motion does not change. In space, this means it could be stationary, or it’s traveling at constant velocity.

Combining Forces Sometimes, the net force does not equal 0. This means there are unbalanced forces acting on the object. The object’s motion changes. In space, this means the object will curve, speed up, or slow down. In other words, it’s velocity will change.