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Detailed Explanation of Describing Motion (Speed, Velocity, Acceleration) 🏃♂️🚗⚡
Speed: What Is It? 🏎️💨
Speed is a measure of how fast an object is moving. It tells us the distance travelled per unit time but does not include the direction of movement. In other words, speed is a scalar quantity.
- Definition: Speed = distance ÷ time
- Units: The common unit for speed is metres per second (m/s) or kilometres per hour (km/h).
Example: If a car travels 100 metres in 5 seconds, its speed is:
Speed = 100 m ÷ 5 s = 20 m/s.
Velocity: Speed with Direction ↔️📍
Unlike speed, velocity is a vector quantity. This means it includes both how fast an object is moving and the direction it is moving in.
- Definition: Velocity = displacement ÷ time
- Displacement: This is the straight-line distance from the starting point to the finishing point, including direction.
- Units: Velocity is also measured in m/s or km/h.
Key Difference Between Speed and Velocity:
- Speed only shows how fast something moves, but velocity shows speed and direction.
- For example, if a runner runs 100 metres east in 10 seconds, the velocity is 10 m/s east. If the runner returns to the start, the displacement is zero, so the velocity is zero, but speed is not zero because the runner still moved.
Acceleration: How Velocity Changes ⚙️📈
Acceleration describes how quickly the velocity of an object changes. It can involve changes in speed or changes in direction (or both).
- Definition: Acceleration = change in velocity ÷ time taken
- Units: metres per second squared (m/s²).
Acceleration is a vector quantity because velocity is a vector.
Types of acceleration:
- Positive acceleration: When an object speeds up.
- Negative acceleration (deceleration): When an object slows down.
- Centripetal acceleration: When an object changes direction, like a car turning around a bend.
Example: If a car increases its velocity from 0 m/s to 20 m/s in 5 seconds, its acceleration is:
Acceleration = (20 m/s – 0 m/s) ÷ 5 s = 4 m/s².
Summary Tables for Quick Reference 📊
| Quantity | Formula | Type | Units |
|---|---|---|---|
| Speed | distance ÷ time | Scalar | m/s or km/h |
| Velocity | displacement ÷ time | Vector | m/s or km/h |
| Acceleration | change in velocity ÷ time | Vector | m/s² |
Practical Tips for Understanding Motion 🧠✨
- Always identify whether the question refers to distance or displacement. Distance is total length travelled; displacement is straight-line distance with direction.
- Draw motion diagrams or graphs to visualise speed, velocity, and acceleration.
- Remember velocity requires direction, so two objects with the same speed but opposite directions have different velocities.
- Practice calculating acceleration with different scenarios (speeding up, slowing down, turning).
By understanding these concepts, you build a strong foundation for exploring more complex physics topics like forces, graphs of motion, and Newton’s laws. Keep practising with real-life examples, such as cars, runners, or falling objects, to see how speed, velocity, and acceleration describe different kinds of motion around you.
10 Examination-Style 1-Mark Questions on Describing Motion ✍️✅
- What term describes how fast an object is moving without regard to direction?
Answer: Speed - What is the term for speed with a specified direction?
Answer: Velocity - What word describes the change in velocity over time?
Answer: Acceleration - What is the term for an object moving at a constant speed in a straight line?
Answer: Uniform - What is the name given to the rate at which speed changes?
Answer: Acceleration - What unit is commonly used to measure speed in the metric system?
Answer: Metres per second - What term describes velocity that changes due to a change in direction only?
Answer: Acceleration - What do we call a decrease in speed?
Answer: Deceleration - What is the term for the speed of an object in a specific direction at a given time?
Answer: Instantaneous - What do we call the change in speed or direction during motion?
Answer: Acceleration
10 Examination-Style 2-Mark Questions on Describing Motion 📝🕑
- Define speed in terms of distance and time.
Answer: Speed is the distance travelled per unit time. - What is the difference between speed and velocity?
Answer: Velocity includes direction, while speed does not. - How do you calculate average velocity from displacement and time?
Answer: Average velocity is displacement divided by time taken. - What does a negative acceleration indicate about an object’s motion?
Answer: The object is slowing down or decelerating. - Describe how acceleration can be positive but speed still decreases.
Answer: If velocity is negative, positive acceleration increases the velocity towards zero, reducing speed. - What unit is used to measure acceleration in the SI system?
Answer: Metres per second squared (m/s²). - Give an example of a situation where speed is constant but velocity changes.
Answer: An object moving at constant speed around a circular track. - How is instantaneous speed different from average speed?
Answer: Instantaneous speed is speed at a specific moment, average speed is over a time interval. - Calculate the acceleration of a car that changes its velocity from 0 m/s to 20 m/s in 5 seconds.
Answer: Acceleration = (20 – 0) ÷ 5 = 4 m/s². - Explain what a velocity-time graph tells you about motion.
Answer: It shows how velocity changes over time and can be used to find acceleration.
10 Examination-Style 4-Mark Questions on Describing Motion (Speed, Velocity, Acceleration) 🧮📚
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Question: Define speed and velocity. How do they differ from each other?
Answer: Speed is a scalar quantity that measures how fast an object is moving, regardless of direction. It is calculated by dividing the distance travelled by the time taken. Velocity is a vector quantity that includes both speed and direction. The main difference is that velocity specifies direction, so two objects moving at the same speed can have different velocities if they move in different directions. Because velocity considers direction, it can change even if speed remains constant. -
Question: An object travels 50 m north in 4 seconds and then 30 m south in 2 seconds. Calculate the average velocity of the object.
Answer: First, find the total displacement: 50 m north – 30 m south = 20 m north. The total time taken is 4 + 2 = 6 seconds. Average velocity = total displacement ÷ total time = 20 m ÷ 6 s ≈ 3.33 m/s north. This shows that while the object moved a total distance of 80 m, its average velocity depends on the net displacement and direction. -
Question: What is acceleration and how is it related to velocity?
Answer: Acceleration is the rate of change of velocity over time. It is a vector quantity, which means it has both magnitude and direction. If an object changes its speed or direction, it is accelerating. For example, speeding up, slowing down, or changing direction all involve acceleration. The formula for acceleration is acceleration = change in velocity ÷ time taken. This means any change in velocity produces acceleration. -
Question: A car increases its velocity from 10 m/s to 30 m/s in 5 seconds. Calculate its acceleration.
Answer: Use the formula acceleration = (final velocity – initial velocity) ÷ time. Here, final velocity = 30 m/s, initial velocity = 10 m/s, and time = 5 s. So, acceleration = (30 – 10) ÷ 5 = 20 ÷ 5 = 4 m/s². The car’s velocity increases by 4 metres per second every second, meaning it is accelerating uniformly. -
Question: Explain why an object moving in a circle at a constant speed is accelerating.
Answer: Even if the speed stays the same, the direction of velocity is continuously changing when an object moves in a circle. Since velocity includes direction, any change in direction means the velocity changes, which means the object is accelerating. This type of acceleration is called centripetal acceleration and points towards the centre of the circle. So, constant speed does not mean zero acceleration if the direction changes. -
Question: Calculate the average speed of a runner who completes a 400 m track in 50 seconds.
Answer: Average speed is total distance ÷ total time. The distance covered is 400 m, and the time is 50 s. So, average speed = 400 ÷ 50 = 8 m/s. This means the runner covers 8 metres every second. Average speed does not consider the direction of motion. -
Question: Describe how you could use a distance-time graph to find an object’s speed.
Answer: On a distance-time graph, distance is plotted on the vertical axis and time on the horizontal axis. The speed of the object is the gradient (slope) of the graph. A steeper slope means higher speed. To find the speed, pick two points on the graph, find the difference in distance (change in y) and time (change in x), then divide distance by time. This gives the average speed between those points. -
Question: A cyclist slows down uniformly from 15 m/s to 5 m/s over 4 seconds. Calculate the acceleration and describe its direction.
Answer: Acceleration = (final velocity – initial velocity) ÷ time = (5 – 15) ÷ 4 = -10 ÷ 4 = -2.5 m/s². The negative sign means the acceleration is opposite to the direction of motion, so the cyclist is decelerating. Deceleration is just acceleration in the direction opposite to velocity. -
Question: If a toy car moves with a velocity of 3 m/s east and then changes to 4 m/s east in 2 seconds, what is its acceleration?
Answer: Acceleration = (final velocity – initial velocity) ÷ time = (4 m/s – 3 m/s) ÷ 2 s = 1 ÷ 2 = 0.5 m/s². Since the velocity increases eastward, the acceleration is also in the east direction. This means the toy car is speeding up. -
Question: Explain the difference between instantaneous speed and average speed.
Answer: Instantaneous speed is the speed of an object at a specific moment in time. It could vary from one instant to another if the object’s speed changes. Average speed is calculated over a period of time by dividing total distance by total time. It gives a general idea of speed for the whole journey, even if the speed varied during the trip. Instantaneous speed can be found using speedometers or from the slope of a tangent on a distance-time graph.
10 Examination-Style 6-Mark Questions on Describing Motion (Speed, Velocity, Acceleration) 🎯💡
Question 1:
A car travels 120 km in 2 hours. Calculate its average speed and state what this tells you about the car’s motion.
Answer:
To find the average speed, use the formula speed = distance ÷ time. The distance travelled is 120 km and the time taken is 2 hours. So, average speed = 120 km ÷ 2 h = 60 km/h. This means the car travels 60 kilometres every hour on average. Average speed does not tell us about changes in speed or direction during the trip, just the overall rate of motion.
Question 2:
Explain the difference between speed and velocity, and give an example where they would have different values.
Answer:
Speed is defined as the distance travelled per unit time and is a scalar quantity (only has magnitude). Velocity is speed in a given direction, so it is a vector quantity. For example, if a car travels 60 km/h east and then returns west at the same speed, its speed is always 60 km/h but its velocity changes because the direction changes. Velocity considers both magnitude and direction, while speed does not.
Question 3:
A runner accelerates from 0 m/s to 8 m/s in 4 seconds. Calculate the acceleration and explain what it means.
Answer:
Acceleration = change in velocity ÷ time taken. The change in velocity is 8 m/s – 0 m/s = 8 m/s. Time taken is 4 seconds. So, acceleration = 8 m/s ÷ 4 s = 2 m/s². This means the runner’s speed increases by 2 metres per second every second. Positive acceleration shows the runner is speeding up.
Question 4:
Describe what is meant by negative acceleration, and provide a scenario where it occurs.
Answer:
Negative acceleration, often called deceleration, means an object’s velocity decreases over time. This happens when the acceleration vector points opposite to the direction of motion. For example, when a car brakes to stop at a traffic light, it slows down, so it has negative acceleration.
Question 5:
A cyclist changes velocity from 12 m/s north to 12 m/s east in 3 seconds. Explain how to find the acceleration.
Answer:
Since velocity is a vector, we consider both magnitude and direction. The cyclist changes direction, so the velocity change isn’t zero even though speed is constant. Use vector subtraction to find change in velocity: from 12 m/s north to 12 m/s east forms a right angle, so change in velocity = √(12² + 12²) = √288 = 16.97 m/s. Acceleration = change in velocity ÷ time = 16.97 ÷ 3 ≈ 5.66 m/s². This acceleration is due to changing direction.
Question 6:
Explain how a velocity–time graph shows acceleration and describe what a horizontal line and a sloping line represent.
Answer:
A velocity–time graph shows how velocity changes over time. Acceleration is the gradient (slope) of the graph. A horizontal line means velocity is constant (zero acceleration). A sloping line means velocity is changing; a positive slope indicates acceleration, while a negative slope indicates deceleration.
Question 7:
A ball is thrown upwards with a velocity of 15 m/s. Describe its motion until it comes back down and explain the acceleration throughout.
Answer:
The ball rises, slows down until it reaches the highest point where velocity is zero, then falls back down, speeding up. The acceleration throughout is constant and directed downwards due to gravity, about 9.8 m/s². Going up, acceleration is opposite to motion, causing deceleration. Coming down, acceleration is in the same direction as motion, causing acceleration.
Question 8:
Define instantaneous speed and explain how it differs from average speed during a journey.
Answer:
Instantaneous speed is the speed at a specific moment in time. Average speed is total distance divided by total time for the entire journey. Instantaneous speed can vary during the trip, while average speed is one overall value. A driver might have varying instantaneous speeds during a trip, but a single average speed.
Question 9:
How can acceleration be zero even if an object is moving? Give an example.
Answer:
Acceleration is zero when velocity doesn’t change; this means speed and direction remain constant. For example, a car cruising at 50 mph on a straight motorway has zero acceleration because its velocity is constant. It is moving but not speeding up, slowing down, or changing direction.
Question 10:
Describe how you would calculate the acceleration of a vehicle from a velocity–time graph.
Answer:
Acceleration on a velocity–time graph is the gradient of the line. To calculate this, pick two points on the line. Find the change in velocity (vertical difference) and the change in time (horizontal difference). Divide change in velocity by change in time to get acceleration (a = Δv/Δt). A steeper slope means greater acceleration.
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