Question

the following motions describe objects that are not accelerating:

• a bird flies at a constant speed of 10 m/s in a constant direction of north
• a train travels at a constant velocity of 45 mi/h south
• a car travels along a straight section of highway at a constant 100 km/h

the following motions describe objects that are accelerating:

• a bird, flying at a speed of 5 m/s, speeds up to 10 m/s in a constant direction of north
• a person runs at a constant 3 mi/h while turning from the north to the east
• a car travels along a curve while slowing down to 60 km/h from 100 km/h

1. below are several descriptions of motion. for each one, state whether or not the object is accelerating and explain your answer.

a) a sprinter runs a race on a straight track starting from rest and reaches a top speed of 10 m/s
b) a race car travels along a straight section of a racetrack at a constant 90 m/s
c) a cyclist travels around a curve at a constant 13 m/s
d) a train moving at 20 km/h comes to a stop at a station along a straight section of track.

Explanation:

Step1: Recall acceleration definition

Acceleration is change in velocity. Velocity is speed with direction.

Step2: Analyze a)

The sprinter starts from rest and reaches a top - speed. Since speed changes, there is acceleration.

Step3: Analyze b)

The race car has a constant speed and moves in a straight line (constant direction), so no change in velocity, no acceleration.

Step4: Analyze c)

The cyclist moves around a curve. Direction changes, so velocity changes, there is acceleration.

Step5: Analyze d)

The train's speed changes from 20 km/h to 0 km/h, so there is acceleration.

Answer:

a) Accelerating. Reason: Speed changes from 0 to 10 m/s.
b) Not accelerating. Reason: Constant speed and direction.
c) Accelerating. Reason: Direction changes.
d) Accelerating. Reason: Speed changes from 20 km/h to 0 km/h.