Forces in two dimensions (including resolution of a force) | Exit Quiz | Oak National Academy

A helicopter is in the air in a state of equilibrium. Which of the following motions are possible?

• It is moving upwards at constant speed.

• It is falling with decreasing speed.

• It is travelling in a circle.

• It is stationary.

Answer explanation

In equilibrium, the helicopter does not accelerate so its velocity is constant. This means that it either moves in one direction at a constant speed or it stays in one place so has a constant velocity of zero.

A duck is swimming in a straight line across the surface of a lake at a steady speed. Which of the following free body diagrams correctly shows the forces acting on the duck?

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Answer explanation

The duck is not changing its velocity, so there is no resultant force acting on it – it is in equilibrium. The forces to left and right (drag and thrust) must be equal, and the forces upwards and downwards (upthrust and weight) must be equal.

An aeroplane and its free body diagram are shown. Which of the following are possible descriptions of the aeroplane’s motion?

• Moving to the left at decreasing speed.

• Moving downwards at increasing speed.

• Moving up at increasing speed, while also moving steadily to the right.

• Moving to the left at decreasing speed, while also moving steadily upwards.

• Moving to the left at decreasing speed, while also moving steadily downwards.

Answer explanation

There is no vertical resultant force on the aeroplane so it either has no vertical motion or it is moving up or down at constant speed. There is a horizontal resultant force to the right, so it could be moving left while decelerating.

A spaceship in space uses two rockets to produce forces acting in two directions at the same time. Which of the following vector diagram shows how to find the resultant force?

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Answer explanation

Draw either of the force arrows, and then draw the tail of the other arrow starting at the head of the first. The resultant force is the arrow joining the tail of the first arrow to the head of the second arrow.

The vector diagram shows the addition of three forces to find the resultant force. What is the magnitude of the resultant force?

• 40 N

• 60 N

• 63 N

• 72 N

Answer explanation

The vectors showing the 72 N, 60 N and 63 N forces are all joined from head to tail. The 40 N vector joins the tail of the first to the head of the last and shows the resultant of the other three forces.

The vector diagram shows a force on a grid of 1 cm squares. Which of the following is the correct description of the horizontal component of the force?

• 30 N upwards

• 30 N downwards

• 40 N to the right

• 40 N to the left

• 50 N upwards and to the left

Answer explanation

The force shown can be made by adding a 40 N force to the left and a 30 N force upwards, therefore the horizontal component is 40 N to the left.