Speed, Velocity, and Acceleration

Speed, Velocity, and Acceleration

Middle School

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Learning Objectives

• Define speed, velocity, and acceleration to describe an object's motion.

• Interpret distance-versus-time graphs to analyze an object's movement.

• Explain how unbalanced forces and mass affect an object's resulting acceleration.

• Understand the importance of a frame of reference when describing motion.

Key Vocabulary

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Speed

The measure of how fast an object is moving along a path or distance.

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Velocity

Describes an object's speed combined with its specific direction of motion, like north or south.

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Acceleration

The rate at which an object's velocity, including its speed and direction, changes over time.

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Force

A push or a pull acting upon an object, which can cause a change in motion.

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Net Force

The sum of all the individual forces that are currently acting on a single object.

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Frame of Reference

A point of view or a set of coordinates from which motion is described and measured.

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Calculating Speed

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Multiple Choice

A car trip to a city 200 miles away takes 4 hours. The car speeds up on the highway and slows down in towns. How would you find the average speed for the entire trip?

• By dividing the total distance (200 miles) by the total time (4 hours).

• By looking at the speedometer at the exact halfway point of the trip.

• By using the fastest speed the car traveled on the highway.

• By adding the fastest and slowest speeds and dividing by two.

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Graphing Speed

• A distance-versus-time graph plots an object's motion over a period of time.

• The vertical axis shows distance, and the horizontal axis shows time.

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

• A steeper slope indicates a greater speed, while a shallower slope means a slower speed.

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Multiple Choice

On a distance-versus-time graph, what does the slope of the line represent?

• The speed of the object

• The acceleration of the object

• The distance the object has traveled

• The time the object has been in motion

Velocity and Acceleration

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What is Velocity?

• ​Velocity describes motion with both speed and a specific direction.

• ​​For example, 25 km/h eastward is a velocity, not just speed.

• ​If direction changes, velocity changes, even if speed does not.

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What is Acceleration?

• ​Acceleration is the rate an object's velocity changes over time.

• ​​An object accelerates if its speed, direction, or both change.

• ​A change in direction is acceleration, even with constant speed.

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Types of Acceleration

• ​Speeding up is a type of acceleration called positive acceleration.

• ​​Slowing down is also known as negative acceleration or deceleration.

• ​A change in the direction of motion is also acceleration.

Multiple Choice

According to the definitions provided, how can an object accelerate if its speed remains constant?

• By changing its direction of motion.

• An object cannot accelerate with constant speed.

• By increasing its mass.

• By moving in a perfectly straight line.

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How Forces Affect Motion

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

• Forces can cause an object to accelerate, which changes its motion.

• Unbalanced forces change an object's motion; balanced forces do not.

• A force has a magnitude (in Newtons) and a specific direction.

Multiple Choice

What is the primary effect of an unbalanced force acting on an object?

• It causes a change in the object's motion.

• It has no effect on a moving object.

• It only changes the direction of an object.

• It always makes an object stop moving.

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Balanced vs. Unbalanced Forces

Balanced Forces

• The net force is zero because all forces acting on the object are balanced.

• This results in no change in the object's current state of motion.

• The object either remains at rest or moves at a constant velocity.

Unbalanced Forces

• The net force is not zero because the forces acting on the object are unequal.

• The presence of a net force causes the object to accelerate.

• The object's motion changes by speeding up, slowing down, or changing direction.

Multiple Choice

What is the key difference that determines if the forces acting on an object are balanced or unbalanced?

• The total net force acting on the object.

• The speed at which the object is moving.

• The direction the object is traveling.

• The object's overall mass.

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Force, Mass, and Acceleration

Multiple Choice

What are the two factors that determine an object's acceleration?

• Its mass and the net force applied to it.

• Its speed and its direction.

• Its size and its shape.

• Its weight and its height from the ground.

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What is a Frame of Reference?

• A viewpoint from which we describe and measure an object's motion.

• Motion is relative and depends on your specific frame of reference.

• On a moving train, a book next to you appears to be still.

• To an observer on the platform, that same book is clearly moving.

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Multiple Choice

Why can a book on a train seem still to a passenger but appear to be moving to an observer on the platform?

• They are observing the book from different frames of reference.

• The train is accelerating, which changes the book's speed.

• Only one person is measuring the book's motion accurately.

• The passenger is closer to the book than the observer.

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Common Misconceptions

Misconception	Correction
Acceleration only means speeding up.	Acceleration is any change in velocity, including slowing down or changing direction.
If an object isn't moving, no forces are acting on it.	Forces on a still object are balanced, but they are still present.
An object moving at a constant speed cannot be accelerating.	Changing direction is a form of acceleration, even if the speed remains constant.

Multiple Choice

A satellite orbits Earth at a constant speed. Which statement best explains why it is accelerating?

• Because its direction of motion is constantly changing.

• Because its speed is greater than zero.

• Because there are no forces acting on it.

• Because its mass is changing as it orbits.

Multiple Choice

If two people push a large crate in opposite directions, one with a force of 100 N and the other with 80 N, what will happen to the crate?

• The crate will accelerate in the direction of the 100 N force.

• The crate will not move because the forces are balanced.

• The crate will accelerate in the direction of the 80 N force.

• The crate will move at a constant velocity.

Multiple Choice

Imagine two identical-looking boxes, but one is full of feathers and the other is full of rocks. If you apply the same pushing force to both, how will their acceleration compare?

• The box of feathers will accelerate more because it has less mass.

• The box of rocks will accelerate more because it is heavier.

• They will both accelerate at the same rate because the force is the same.

• Neither box will accelerate.

Multiple Choice

You are in a car moving at a constant 50 mph. You toss a ball straight up into the air. From your frame of reference, the ball goes up and comes straight back down. What would an observer on the sidewalk see?

• The ball travels forward in an arc, moving with the car.

• The ball goes straight up and lands behind the car.

• The ball goes straight up and comes straight back down.

• The ball appears to stand still in the air.

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Summary

• Speed is how fast an object moves; velocity also includes its direction.

• Acceleration is any change in an object's speed or its direction of motion.

• Unbalanced forces cause acceleration, which depends on the object's mass and the net force.

• All motion is relative and must be described from a frame of reference.

Poll

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

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