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Lesson: Forces that Resist Motion Physics

In this lesson, we will learn how to compare friction and drag forces, and also explain how the variable drag force on an object results in a terminal velocity.

Lesson Plan

Students will be able to

recognize that frictional forces can act between surfaces that are in contact,
recognize that frictional forces act in the opposite direction to the motion of a moving object,
recognize that frictional forces can act on a stationary object that is in contact with a surface,
recognize that frictional forces acting on a stationary object on a surface occur only if some force acts on the object that would make the object move along the surface if no friction existed,
recognize that frictional forces acting on a stationary object on a surface act in the opposite direction to forces that would make the object move along the surface if no friction existed,
recognize that the motion of an object within a fluid produces a force on the object in the opposite direction to the motion of the object called drag force,
recognize that the magnitude of the drag force on an object is related to the mass of fluid displaced by the object per unit time and hence increases when an object’s velocity within a fluid increases,
recognize that the magnitude of the drag force on an object can equal the magnitude of the forces that accelerate the object to move within the fluid and that when this occurs the object moves at a constant velocity called terminal velocity,
qualitatively describe how the shape of an object affects the drag force acting on it.

Lesson Video

Lesson Playlist

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Lesson Worksheet

Q1:

A ball bearing of mass 150 g is dropped into a tube filled with oil. The ball bearing hits the oil’s surface with a vertically downward velocity of 1.5 m/s and after falling through the oil for 1.5 s, the ball bearing’s velocity becomes 6.3 m/s. What vertically upward average drag force does the oil apply to the ball bearing?

Q2:

The shapes (A), (B), (C), (D), and (E) shown in the diagram are the shapes of the surfaces of objects that move through a fluid. The front of each object has the shape shown. All the objects move at the same speed through the same fluid. Which object has the largest drag force acting on it?

Q3:

A wheel falls off an airplane that is flying horizontally. The vertically downward speed of the wheel changes with time as shown in the diagram.

How much time after the wheel detaches from the airplane does the upward force applied by the air on the wheel equal the weight of the wheel?

What is the magnitude of the upward force applied by the air on the wheel at the instant that the wheel detaches from the airplane?

At what time would the net force on the wheel be equal to the wheel’s weight?

Is the acceleration of the wheel increasing, decreasing, or constant in the first 2 seconds after the wheel detaches from the airplane?

Is the acceleration of the wheel increasing, decreasing, or constant between 4 and 6 seconds after the wheel detaches from the airplane?

Is the velocity of the wheel increasing, decreasing, or constant between 2 and 4 seconds after the wheel detaches from the airplane?

Is the velocity of the wheel increasing, decreasing, or constant between 6 and 8 seconds after the wheel detaches from the airplane?