5.6.1 - Describing Motion Along a Line

Does a distance quantity require a specific direction? i.e. is it a scalar or vector quantity?

No specific direction is required so it’s a scalar quantity.

If an object moves 3 meters to the left and then 3 meters back to its initial position, what is the object’s total displacement?

• The object has zero displacement.

• Displacement is a vector quantity so it also involves direction.

• The object starts and ends at the same point.

State a typical value for the speed of sound.

330 m/s

What is a typical value for human walking speed?

1.5 m/s

What is a typical value for human running speed?

3 m/s

What is a typical value for human cycling speed?

6 m/s

State the equation linking distance, speed, and time. Give appropriate units.

• distance = speed × time

• distance (m), speed (m/s), time (s)

Why can an object travelling at a constant speed in a circle not have a constant velocity?

• Speed is a scalar quantity.

• Velocity is a vector quantity which means it can only be constant if the direction is constant.

• In a circular motion, the direction is continuously changing.

How can speed be calculated from a distance-time graph?

The speed is equal to the gradient of the graph.

What must be done to calculate speed at a given time from a distance-time graph for an accelerating object?

• Drawing a tangent to the curve at the required time.

• Calculating the gradient of the tangent.

State the equation for the average acceleration of an object. Give appropriate units.

• acceleration = change in velocity ÷ time taken

• acceleration (m/s²), velocity (m/s), time (s)

How can the distance travelled by an object be calculated from a velocity-time graph?

It’s equal to the area under the graph.

Give an approximate value for the acceleration of an object in free fall under gravity near the Earth’s surface.

9.8 m/s²

What can be said about the resultant force acting on an object when it’s falling at terminal velocity?

• The resultant force is zero.

• When at terminal velocity, the object is moving at a constant speed and so isn’t accelerating.