Topic questions Forces Viscosity and viscous drag
5 questions · 43 marks · about 55 minutes
A small ball bearing falls slowly through a liquid at terminal velocity. The temperature of the liquid is increased.
Which row of the table shows the change in viscosity of the liquid and the change in
An object moving through a fluid experiences a drag force due to the viscosity of the fluid.
Some liquid chocolate is poured into a mould. Within the chocolate a bubble of air is formed. The bubble has negligible weight and moves upwards at a constant speed, as shown.
Calculate the constant speed at which the air bubble moves upwards in the liquid chocolate.
radius of air bubble = 6.8 
10–3m density of liquid chocolate = 925 kg m–3 viscosity of liquid chocolate = 52.6 Pa s
The viscosity of a gas increases with temperature.
It is suggested that the relationship between viscosity η and temperature T is of the form
where A and B are constants.
Explain why a graph of log η against logT would give a straight line.
The table shows data for the viscosity of hydrogen gas over a range
Plot a graph of log η against logT. You should use the columns provided to show any processed data.
Determine values for A and B. You are not required to state the units of A.
Chocolate is a solid at room temperature, but melts just below body temperature.
A student investigated the viscosity of some melted chocolate using a falling-ball method. Since chocolate is opaque, a thin rod was attached to the ball so that the movement of the ball could be monitored. The apparatus is shown in the diagram.
The chocolate was maintained at a constant temperature during the investigation.
The student used a stopwatch to measure the time t for the ball to fall 22.5 cm whilst travelling at its terminal velocity v.
Her results are shown in the table.
| t1 / s | t2 / s | t3 / s |
| 9.6 | 9.9 | 9.6 |
v is given by the formula
where r is the radius of the ball
The graph is taken from a commercial website. It shows how, at the temperature of the experiment, η depends on the percentage of cocoa in the chocolate.
The chocolate wrapper indicated that the chocolate had a 35% cocoa content.
Assess whether the student’s timing data supports this percentage cocoa content.
r = 4.25×10–3m
Explain one reason why the student’s data may have led to an inaccurate conclusion about the cocoa content.
One type of chocolate melts at a temperature of 32
C.
The energy released when 65 g of this chocolate is digested is 345 kcal.
It is suggested that the energy used to melt a piece of this chocolate is at least 15% of the energy released when the chocolate is digested.
Assess the accuracy of this suggestion.
initial temperature of chocolate = 15
C specific heat capacity of chocolate = 3.9
103 J kg–1K–1 specific latent heat of chocolate = 1.50
105J kg–1 1 kcal = 4200 J
A student carried out an experiment to determine the viscosity of some honey. He filled a tall glass cylinder with honey as shown, and timed a ball bearing as it fell through the honey.
The student placed rubber bands near the top and bottom of the cylinder. He started a stopwatch when the ball bearing passed the first band and stopped the stopwatch when the ball bearing passed the second band. He repeated this several times to determine a mean time.
Criticise the student’s method.
The time t for the sphere to fall through a distance of 25.0 cm is shown in the table.
| t | / s | ||
| 6.40 | 6.35 | 6.36 | 6.38 |
Show that the mean velocity v of the ball bearing is about 0.04 m s−1.
The student had three different types of honey available.
Viscosity η is given by the following expression
radius r of ball bearing = 5.50 × 10−3m density of ball bearing ρB = 7750 kg m−3
| Viscosity (at 20 °C) / Pa s | ||
| Honey A | Honey B | Honey C |
| 10.6 | 12.5 | 13.6 |
Deduce which honey the student used.
A student investigated the terminal velocity of steel spheres falling through oil.
The student obtained the following results.
radius of steel sphere = 1.50 mm volume of steel sphere = 1.41 
10−8m3 mass of steel sphere = 1.10 
10−4kg maximum speed of sphere = 0.849 m s−1
The student had the following table.
| Type of oil | Density at 26 °C / kg m−3 | Viscosity at 26 °C / Pa s |
| Corn | 918 | 0.0447 |
| Hazelnut | 918 | 0.0504 |
| Sunflower | 918 | 0.0414 |
Identify which type of oil the student used.
The values in the table are for oil at 26 °C.
Explain the effect of carrying out the investigation with oil at a lower temperature.
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