Balanced & Unbalanced Forces

Transcription

1 3 rd Grade Force in Motion An object's motion changes because of force. Pushing and Pulling are Kinds of Forces Motion is movement that changes an object's position. Pushing or pulling forces can be used to change the motion of an object. When force is applied, the object can start moving, stop moving, change speed, or change direction. Effect of Force Strength on Motion The motion of an object acted on by a force depends partly on the strength of the push or pull. The stronger the push or pull, the faster the object will move. For example, the father in the picture below is pushing his daughter in a swing. When he uses more force to push, the swing moves higher. Balanced & Unbalanced Forces An object's motion can be changed by unbalanced forces. Balanced forces do not change the motion of an object. Balanced Forces

2 The motion of an object will not change if the forces pushing or pulling the object are balanced. An object that is sitting still will stay still if the forces acting on it are balanced. If an object is in motion, it will keep moving in the same direction and at the same speed if the forces acting on it are balanced. Look at Diagram A, shown below. Diagram A: Balanced Forces These forces are balanced, so the motion of the object stays the same. The object in the diagram could be sitting still, or it could be moving. But its motion is not changing. This is because the forces acting on it from opposite sides are exactly the same size. Unbalanced Forces An unbalanced force can change an object's motion. An unbalanced force acting on a still object could make the object start moving. An unbalanced force acting on a moving object could make the object change direction, change speed, or stop moving. Look at Diagram B, shown below. Diagram B: Unbalanced Forces These forces are unbalanced, so the object moves to the right. In Diagram B, Force 1 is a large force acting on the object by pushing the object toward the right. Force 2 is smaller, and it is pushing on the object toward the left. Some of Force 1 acts to stop Force 2 from moving the object. Since Force 1 is larger, there is still

3 some force left over. This leftover force makes the object start moving to the right. The red arrow shows the direction of the object's new motion. Let's look at another example. In the picture below, Henry and Ben are playing tug of war. Henry is much stronger than Ben. Both boys pull on the rope, but Henry pulls with a greater force. How does the rope move? The rope moves toward Henry since Henry is stronger than Ben, and Henry is pulling on the rope with a greater force. Tracing Object Motion The path that an object follows when it moves is called its path of motion. An object's path of motion can be traced by drawing arrows. There are different types of motion. An object might slide, roll, spin, swing, or fall. The path of motion depends on the type of movement and the reason the object is moving. Sliding Sliding is when two things rub against each other as they move. A flat block could slide down a steep ramp. The ramp and the block would rub against each other as the block moved. Objects usually slide in a straight line unless they are pushed or bumped in a different direction.

4 A sled slides down a snowy hill in a straight line. The path of the sled can be shown with an arrow. At the bottom of the hill, the sled will keep going for a short distance. An object sliding on a slanted surface, like a ramp, will slide downward. This is because Earth's gravity pulls everything on or near the Earth's surface down toward the planet's center. Rolling Things that are round, such as balls and wheels, can move by rolling. When a wheel is rolling, a different part of the wheel touches the ground at different times. This happens because the wheel turns as it rolls. A ball rolling down a ramp does not change direction. It moves in a straight line. The ball also turns in a circle as it moves down the ramp. A curly arrow shows the path taken by one point on the surface of the ball as it rolls.

5 Some shapes do not roll in a straight line. The cup in the picture below is wider at the top than at the bottom, so this cup will roll in a circle. Spinning A wheel can also move by spinning. If the edge of the wheel is not touching the ground, the wheel can turn, but it does not travel along the ground. This motion is called spinning. A merry-go-round moves by spinning. If you look down at a merry-go-round from up above it, the path of the merry-go-round is a circle. A toy top is another example of an object that spins. When a person turns a top and lets go of it, the top stands on its tip and moves in a circle.

6 These tops are spinning in a circle. Swinging Some objects move back-and-forth, which creates a swinging motion. With this kind of motion, the object moves first in one direction, and then it moves back the other way along the same path. A person on a swing has this kind of motion. A person swinging in a swing is moving back-and-forth. The swing first travels up in one direction. Then, it changes directions and travels in the opposite direction. A ball rolling in a bowl can also show a swinging type of motion. It may move up one side of the bowl, then down toward the bottom of the bowl, and then up the other side of the bowl until gravity brings the ball to rest at the bottom of the bowl.

7 There is a pattern to the motion of swinging objects or objects that are moving backand-forth. For example, look at the metronome below. The arm on the metronome is swinging back and forth in a pattern. Metronomes are tools that musicians use to make sure they are playing at the right speed. The pattern is that the arm moves from one side to the center, then over to the other side and back again. This pattern lets us predict what the metronome arm will do next. The pattern can even be measured. It is possible to time how long it takes for the metronome arm to move from one side to the other or from one side to the other and back again. Then, this measurement can be used to predict how long the next movement will take. Falling or Being Thrown Objects dropped or thrown into the air will eventually fall down to Earth. An object that is dropped may fall in a straight line. But an object that is thrown or kicked travels in a curved path that can be described as a parabola. A parabola is a shape that looks a bit like an upside-down bowl. Objects that are dropped from a fixed position fall straight down toward the Earth.

8 Objects that are thrown, kicked, or propelled into the air will return to Earth's surface along a parabola-shaped path.