Motion

Motion

Middle School

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Learning Objectives

• Define motion and explain the importance of a frame of reference.

• Explain the difference between distance and displacement, and speed and velocity.

• Describe acceleration as a change in velocity, like speeding up or slowing down.

• Interpret graphs of motion and identify different types of motion.

Key Vocabulary

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Motion

A change in the position, direction, or location of an object over a period of time.

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Frame of Reference

A non-moving object or location used to detect and describe the motion of another object.

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Displacement

The shortest distance an object has moved from its starting point, indicating a change in position.

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Velocity

A measure of an object's speed combined with its specific direction of motion, making it a vector quantity.

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Acceleration

The rate at which an object’s velocity changes over time, either in speed or direction.

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What Is Motion?

• Motion is a change in an object's position over time.

• How we observe motion depends on our frame of reference.

• A reference point is a fixed spot used to detect motion.

• Position is described by its distance and direction from a reference point.

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Multiple Choice

Why does a passenger on a moving bus not appear to be in motion to the other people on the bus?

• Because the other passengers are their frame of reference.

• Because the bus is moving at a constant speed.

• Because the seats prevent them from changing position.

• Because motion can only be detected from outside the bus.

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Distance vs. Displacement

• Both distance and displacement measure an object's change in position.

• Distance is a scalar, meaning it is the total path traveled.

• Displacement is a vector, showing the change from start to end.

• In a round trip, your displacement is zero but the distance is not.

Multiple Choice

A runner completes one full lap around a 400-meter circular track. What is their distance and displacement?

• Distance is 400 meters, and displacement is 0 meters.

• Distance is 400 meters, and displacement is 400 meters.

• Distance is 0 meters, and displacement is 400 meters.

• Distance is 0 meters, and displacement is 0 meters.

Speed vs. Velocity

• Speed is the measure of how fast an object is moving.

• Velocity is an object's speed combined with its direction of motion.

• Objects can have the same speed but different velocities.

Multiple Choice

Two cars are driving at 60 mph. Car A is going North and Car B is going South. Which statement is true?

• They have the same speed but different velocities.

• They have the same velocity but different speeds.

• They have different speeds and different velocities.

• They have the same speed and the same velocity.

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Acceleration

Speeding Up

• An object speeds up when its acceleration is in the same direction as its ongoing motion.

• This type of acceleration causes the object’s velocity and its overall speed to constantly increase over time.

• The push or pull on the object helps it move faster in its current direction of travel.

Slowing Down

• An object slows down when its acceleration is in the opposite direction of its ongoing motion.

• This form of acceleration, also known as deceleration, causes the object’s velocity to constantly decrease.

• A push or pull acts against the object's motion, causing it to reduce its overall speed.

Multiple Choice

A car is traveling east and the driver applies the brakes. In which direction is the car's acceleration?

• North

• There is no acceleration.

• East

• West

Types of Motion

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Linear Motion

• This is when an object moves in a straight line.

• Uniform motion has a constant velocity, without any acceleration.

• Non-uniform motion involves a change in the object's acceleration.

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Projectile Motion

• This is the curved path an object takes when thrown.

• It combines horizontal speed with vertical acceleration from gravity.

• A kicked soccer ball is a great example of this.

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Free Fall

• This describes an object moving only under gravity's influence.

• The downward acceleration is constant, even when moving up.

• At its highest point, the object's vertical velocity is zero.

Multiple Choice

Which of these best describes an object in free fall at its maximum height?

• Its velocity is downward and its acceleration is zero.

• Its velocity is zero, but its acceleration is constant and downward.

• Both its velocity and acceleration are zero.

• Its acceleration is zero, but its velocity is constant.

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Introduction to Force

• A force is a push or a pull on an object.

• Forces can change an object's speed, or its direction of motion.

• The net force is the sum of all forces on an object.

• Examples are gravity, friction, air resistance, applied force, and tension.

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Multiple Choice

What is the primary role of a net force acting on an object?

• To increase the object's mass.

• To change the object's motion.

• To maintain the object's current position.

• To eliminate friction.

Graphs of Motion

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Distance vs. Time Graph

• The slope of the line on this graph represents the object's speed.

• A straight line shows constant speed, and a curved line shows changing speed.

• A horizontal line means the object is at rest and not moving.

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Velocity vs. Time Graph

• The slope of the line on this graph represents the object's acceleration.

• A horizontal line means the object is moving at a constant velocity.

• The area under the graph is equal to the object's total displacement.

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Acceleration vs. Time

• This graph shows how an object's acceleration changes over a period of time.

• For motion with constant acceleration, the graph is a horizontal line.

• The area under the curve shows the change in the object's velocity.

Multiple Choice

On a velocity-time graph, what does a straight horizontal line above the x-axis represent?

• Constant acceleration and changing velocity.

• Constant velocity and zero acceleration.

• The object is returning to its starting point.

• The object is at rest.

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Equations of Motion

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Multiple Choice

Which variable is NOT present in the equation $$v^2 = v_0^2 + 2a\Delta x$$ ?

• acceleration (a)

• time (t)

• displacement (Δx)

• initial velocity (v₀)

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Common Misconceptions

Misconception	Correction
Zero velocity means zero acceleration.	A ball at its peak has zero velocity but still accelerates due to gravity.
Speed and velocity are the same.	Speed is how fast; velocity includes direction.
Negative acceleration always means slowing down.	It can also mean speeding up in the negative direction.
Constant speed means no acceleration.	Turning at a constant speed is a change in velocity, which is acceleration.

Multiple Choice

A car drives 5 km north, then 3 km east, and finally 5 km south. How does the total distance traveled compare to the magnitude of the final displacement?

• The displacement is greater than the distance.

• The distance and displacement are equal.

• The distance is greater than the displacement.

• The displacement is zero.

Multiple Choice

Why can a person sitting in a moving train feel like they are not moving?

• Because the other seats and passengers on the train are their frame of reference.

• Because inertia prevents them from feeling the motion of the train.

• Because the train is accelerating too slowly to be perceived.

• Because their eyes are closed and they cannot see outside.

Multiple Choice

Imagine a velocity-time graph showing a straight line starting at the origin and sloping upwards. What does this indicate about the object's motion and how would you calculate its displacement?

• The object is undergoing constant positive acceleration; displacement is the area of the triangle under the line.

• The object is at rest; displacement is zero.

• The object is accelerating and then decelerating; displacement cannot be determined from this graph.

• The object is moving at a constant velocity; displacement is the final velocity value.

Multiple Choice

A rocket is launched straight up. After its fuel runs out, it continues to move upward before falling back to Earth. Analyze the forces acting on the rocket and its resulting acceleration at the very peak of its flight.

• The upward force from the launch is equal to the force of gravity, resulting in zero acceleration.

• The only significant force is gravity, causing a constant downward acceleration even though its velocity is momentarily zero.

• The force of gravity is zero at the peak, causing the rocket to begin falling.

• There are no forces at the peak, so its acceleration is zero.

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Summary

• Motion is a change in position compared to a reference point.

• Distance is the total path, while displacement is the change from start to end.

• Velocity is speed in a specific direction, and acceleration is a change in velocity.

• Motion can be described and calculated using graphs and key equations.

Poll

On a scale of 1-4, how confident are you about the concepts covered in today's review?

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• 4