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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
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
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.
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.
There is a pattern to the motion of swinging objects or objects that are moving back- and-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.
Objects that are thrown, kicked, or propelled into the air will return to Earth's surface along a parabola-shaped path.
