# Worksheet: Relative Motion

In this worksheet, we will practice calculating the motion of objects relative to each other using the Galilean transformation.

**Q1: **

A man standing still at a train station watches a train move past him eastward at 20 m/s. Two boys inside the train throw a baseball. The ball is thrown westward, at a speed of 5.0 m/s relative to the boy that throws the ball. What is the velocity of the ball as measured by the man in the train station? Assume that eastward corresponds to positive values of displacement.

**Q5: **

A truck is traveling eastward at 80 km/h. At an intersection 32 km east of the truck’s position, a car is traveling north at 50 km/h. At an instant , after the car passes the intersection, the vehicles have positions which are less distant from each other than are any other positions that the vehicles subsequently have.

How much time elapses between the car passing the intersection and the instant ?

What is the distance between the vehicles at the instant ?

**Q6: **

Two speedboats, boat A and boat B, are traveling in opposite directions to each other along a flowing river, moving toward and away from the flow direction respectively. If the river was not flowing, then the boats would move at the same speed as each other. An observer on the riverbank sees boat A move at 6.0 m/s and boat B move at 7.5 m/s.

What is the speed of boat A relative to the river’s flow?

What is the speed of boat B relative to the river’s flow?

The river’s banks are parallel to the river. What is the speed of the river’s flow relative its banks?

**Q7: **

A boat can be rowed at 8.0 km/h in still water.

How much time is required to row 1.50 km downstream in a river moving 3.0 km/h relative to the shore?

How much time is required for the return trip?

In what direction must the boat be aimed to row straight across the river?

Suppose the river is 0.80 km wide. How much time is required for the boat to get to the opposite shore?

Suppose the river is 0.80 km wide and the boat is aimed straight across the river. How much time is required to get to the opposite shore?

**Q9: **

A river flows parallel to its banks, eastward, at 3.2 m/s. A boat starts out from a dock on a bank of the river, heading north of west at 5.8 m/s. The river is 1,560 m wide.

What is the magnitude of the velocity of the boat with respect to Earth?

How much time does it take the boat to reach the opposite bank of the river?

**Q10: **

Raindrops fall vertically at 4.5 m/s relative to the earth. An observer in a car moves through the rain at 17.9 m/s in a straight-line. The observer measures the velocity of the raindrops relative to the car.

What speed does the observer measure the raindrops to move at?

At what angle below the horizontal does the observer measure as the direction of the raindrops’ velocity?

**Q11: **

A flight attendant pushes a cart down the aisle of a plane in flight. In determining the acceleration of the cart relative to the plane, which factor do you not need to consider?

- AThe mass of the plane
- BThe force with which the flight attendant’s feet push on the floor
- CThe friction of the cart’s wheels
- DThe velocity of the plane
- EThe mass of the items in the cart

**Q13: **

Near the end of a marathon, the first two runners are separated by a distance of 45.0 m. The front runner has a velocity of 3.50 m/s and the second a velocity of 4.20 m/s. The front runner is 250 m away from the finish line.

What is the velocity of the second runner relative to the first?

Who will win the race, assuming they run at a constant velocity?

- Athe first runner
- Bthe second runner

What distance ahead will the winner be when she crosses the finish line?

- A3.89 m
- B4.71 m
- C4.17 m
- D2.65 m
- E6.71 m

**Q14: **

A pilot must fly his plane due north to reach his destination. At full power, the plane can fly at 300 km/h in still air. A wind is blowing southwest at 90 km/h and the plane is flying at full power.

What is the speed of the plane relative to the ground?

- A260 km/h
- B270 km/h
- C210 km/h
- D240 km/h
- E220 km/h

At what angle east of north must the plane be directed to fly due north?

**Q15: **

A truck is traveling south at a speed of 70.0 km/h toward an intersection. A car is traveling east toward the intersection at a speed of 80.0 km/h, as shown in the diagram.

What is the magnitude of the velocity of the car relative to the truck?

At what angle north of east is the velocity of the car relative to the truck?

**Q16: **

A man is running on a straight road that makes an angle of with a train track. The man is running away from the track at a speed of 15.0 m/s. A train is moving at a speed of 40.0 m/s with respect to the track. What is the speed of the man with respect to a passenger sitting on the train?

- A31.2 m/s
- B15.0 m/s
- C40.0 m/s
- D55.0 m/s
- E25.0 m/s

**Q17: **

On a certain space–time diagram, time is shown on the diagram’s vertical axis.

Which of the following describes how the world line of an object that remains at rest at a specific position would be displayed on the diagram?

- AAn inclined line making an angle of with the time axis
- BA vertical line shifted to the right
- CA parabola
- DA vertical line coinciding with the time axis
- EA horizontal line coinciding with the position axis

Which of the following describes how the world line of an object that moves at a constant velocity would be displayed on the diagram?

- AA vertical line
- BA horizontal line
- CA parabola
- DAn inclined line making an angle of with the time axis
- EA straight line with an angle of with the time axis

Which of the following describes how the world line of an object that begins at rest and accelerates at a constant rate would be displayed on the diagram?

- AAn inclined line with an angle of to the position axis
- BA parabola symmetric about the positive position axis
- CA parabola symmetric about the negative position axis
- DA parabola symmetric about the positive time axis
- EA parabola symmetric about the negative time axis

**Q19: **

A ship sets sail due north with a speed of 5.13 m/s relative to the water. The water velocity is 1.26 m/s in a direction north of east.

What is the speed of the ship relative to the Earth’s surface?

In what direction does the ship move?

- A east of north
- B east of north
- C east of north
- D east of north
- E east of north