Chapter 5 Uniform Circular Motion and Gravitation

# 5.3 Centripetal Force

### Summary

- Calculate friction on a car tire moving in a circle

Any force or combination of forces can cause a centripetal or radial acceleration. Just a few examples are the tension in the rope on a tether ball, the force of Earth’s gravity on the Moon, friction between roller skates and a rink floor, a banked roadway’s force on a car, and forces on the tube of a spinning centrifuge.

Any net force causing uniform circular motion is called a **centripetal force**. The direction of a centripetal force is toward the center of curvature, the same as the direction of centripetal acceleration. According to Newton’s second law of motion, net force is mass times acceleration: net ** F = ma**. For uniform circular motion, the acceleration is the centripetal acceleration—

**. Thus, the magnitude of centripetal force**

*a*=*a*_{c}**is**

*F*_{c}_{centripetal}= F

_{c}=m a

_{centripetal}= m a

_{c}= m v

^{2}/r = m ω

^{2}r

You may use whichever expression for centripetal force is more convenient. Centripetal force ** F_{c}** is always perpendicular to the path and pointing to the centre of curvature, because

**is perpendicular to the velocity and pointing to the centre of curvature.**

*a*_{c}Note that if you solve the first expression for * r*, you get

This implies that for a given mass and velocity, a large centripetal force causes a small radius of curvature—that is, a tight curve.

### Example 1: What Coefficient of Friction Do Care Tires Need on a Flat Curve?

(a) Calculate the centripetal force exerted on a 900 kg car that negotiates a 500 m radius curve at 25.0 m/s.

(b) Assuming an unbanked curve, find the minimum force of friction , also called traction, due to the tires and the road.

**Strategy and Solution for (a)**

We know that [latex]\boldsymbol{{F}_{\textbf{c}}=\frac{mv^2}{r}}.[/latex]Thus,

**Strategy for (b)**

Figure 2 shows the forces acting on the car on an unbanked (level ground) curve. Friction is to the left, keeping the car from slipping, and because it is the only horizontal force acting on the car, the friction is the centripetal force in this case.

**Discussion**

We could also solve part (a) using the first expression in [latex]\begin{array}{l} \boldsymbol{F_{\textbf{c}}=m\frac{v^2}{r}} \\ \boldsymbol{F_{\textbf{c}}=mr\omega^2} \end{array}[/latex][latex]\rbrace[/latex], because ** m**,

*, and*

**v***are given.*

**r****Figure 2.**This car on level ground is moving away and turning to the left. The centripetal force causing the car to turn in a circular path is due to friction between the tires and the road. A minimum coefficient of friction is needed, or the car will move in a larger-radius curve and leave the roadway.

### TAKE-HOME EXPERIMENT

Ask a friend or relative to swing a golf club or a tennis racquet. Take appropriate measurements to estimate the centripetal acceleration of the end of the club or racquet. You may choose to do this in slow motion.

### PHET EXPLORATIONS: GRAVITY AND ORBITS

Move the sun, earth, moon and space station to see how it affects their gravitational forces and orbital paths. Visualize the sizes and distances between different heavenly bodies, and turn off gravity to see what would happen without it!

**Figure 4.**Gravity and Orbits

# Section Summary

- Centripetal force
is any force causing uniform circular motion. It is a “centre-seeking” force that always points toward the center of rotation. It is perpendicular to linear velocity*F*_{c}and has magnitude**v**

[latex]\boldsymbol{F_{\textbf{c}}=ma_{\textbf{c}}},[/latex]

which can also be expressed as

[latex]\boldsymbol{F_{\textbf{c}}=m\frac{v^2}{r}}[/latex]

or

[latex]\boldsymbol{F_{\textbf{c}}=mr\omega^2}[/latex]

# Conceptual Questions

### Conceptual Questions

**1: **If you wish to reduce the stress (which is related to centripetal force) on high-speed tires, would you use large- or small-diameter tires? Explain.

**2: **Define centripetal force. Can any type of force (for example, tension, gravitational force, friction, and so on) be a centripetal force? Can any combination of forces be a centripetal force?

**3: **If centripetal force is directed toward the centre, why do you feel that you are ‘thrown’ away from the centre as a car goes around a curve? Explain.

**4: **Race car drivers routinely cut corners as shown in Figure 5. Explain how this allows the curve to be taken at the greatest speed.

**Figure 5.**Two paths around a race track curve are shown. Race car drivers will take the inside path (called cutting the corner) whenever possible because it allows them to take the curve at the highest speed.

**5: **A number of amusement parks have rides that make vertical loops like the one shown in Figure 6. For safety, the cars are attached to the rails in such a way that they cannot fall off. If the car goes over the top at just the right speed, gravity alone will supply the centripetal force. What other force acts and what is its direction if:

(a) The car goes over the top at faster than this speed?

(b)The car goes over the top at slower than this speed?

**Figure 6.**Amusement rides with a vertical loop are an example of a form of curved motion.

**7: **What is the direction of the force exerted by the car on the passenger as the car goes over the top of the amusement ride pictured in Figure 6 under the following circumstances:

(a) The car goes over the top at such a speed that the gravitational force is the only force acting?

(b) The car goes over the top faster than this speed?

(c) The car goes over the top slower than this speed?

**8: **As a skater forms a circle, what force is responsible for making her turn? Use a free body diagram in your answer.

**9: **Suppose a child is riding on a merry-go-round at a distance about halfway between its centre and edge. She has a lunch box resting on wax paper, so that there is very little friction between it and the merry-go-round. Which path shown in Figure 7 will the lunch box take when she lets go? The lunch box leaves a trail in the dust on the merry-go-round. Is that trail straight, curved to the left, or curved to the right? Explain your answer.

**Figure 7.**A child riding on a merry-go-round releases her lunch box at point P. This is a view from above the clockwise rotation. Assuming it slides with negligible friction, will it follow path A, B, or C, as viewed from Earth’s frame of reference? What will be the shape of the path it leaves in the dust on the merry-go-round?

**11: **Suppose a mass is moving in a circular path on a frictionless table as shown in figure. In the Earth’s frame of reference, there is no centrifugal force pulling the mass away from the centre of rotation, yet there is a very real force stretching the string attaching the mass to the nail. Using concepts related to centripetal force and Newton’s third law, explain what force stretches the string, identifying its physical origin.

**Figure 8.**A mass attached to a nail on a frictionless table moves in a circular path. The force stretching the string is real and not fictional. What is the physical origin of the force on the string?

### Problems & Exercises

**1: **(a) A 22.0 kg child is riding a playground merry-go-round that is rotating at 40.0 rev/min. What centripetal force must she exert to stay on if she is 1.25 m from its centre?

(b) What centripetal force does she need to stay on an amusement park merry-go-round that rotates at 3.00 rev/min if she is 8.00 m from its center?

(c) Compare each force with her weight.

**2: **Calculate the centripetal force on the end of a 100 m (radius) wind turbine blade that is rotating at 0.5 rev/s. Assume the mass is 4 kg.

**7: **A large centrifuge, like the one shown in Figure 10(a), is used to expose aspiring astronauts to accelerations similar to those experienced in rocket launches and atmospheric reentries.

(a) At what angular velocity is the centripetal acceleration 10 *g* if the rider is 15.0 m from the centre of rotation?

(b) The rider’s cage hangs on a pivot at the end of the arm, allowing it to swing outward during rotation as shown in Figure 10(b). At what angle θ below the horizontal will the cage hang when the centripetal acceleration is 10 *g*? (Hint: The arm supplies centripetal force and supports the weight of the cage. Draw a free body diagram of the forces to see what the angle θ should be.)

**Figure 10.**(a) NASA centrifuge used to subject trainees to accelerations similar to those experienced in rocket launches and reentries. (credit: NASA) (b) Rider in cage showing how the cage pivots outward during rotation. This allows the total force exerted on the rider by the cage to be along its axis at all times.

**9: **Modern roller coasters have vertical loops like the one shown in Figure 11. The radius of curvature is smaller at the top than on the sides so that the downward centripetal acceleration at the top will be greater than the acceleration due to gravity, keeping the passengers pressed firmly into their seats. What is the speed of the roller coaster at the top of the loop if the radius of curvature there is 15.0 m and the downward acceleration of the car is 1.50 g?

**Figure 11.**Teardrop-shaped loops are used in the latest roller coasters so that the radius of curvature gradually decreases to a minimum at the top. This means that the centripetal acceleration builds from zero to a maximum at the top and gradually decreases again. A circular loop would cause a jolting change in acceleration at entry, a disadvantage discovered long ago in railroad curve design. With a small radius of curvature at the top, the centripetal acceleration can more easily be kept greater than

*so that the passengers do not lose contact with their seats nor do they need seat belts to keep them in place.*

**g**## Glossary

- centripetal force
- any net force causing uniform circular motion

- ideal banking
- the sloping of a curve in a road, where the angle of the slope allows the vehicle to negotiate the curve at a certain speed without the aid of friction between the tires and the road; the net external force on the vehicle equals the horizontal centripetal force in the absence of friction

### Solutions

**Problems & Exercises**

**1: **(a) 483 N b) 17.4 N (c) 2.24 times her weight, 0.0807 times her weight

**3: **4.14^

**5: **(a) $$\boldsymbol{24.6\textbf{ m}}$$ (b) [latex]\boldsymbol{36.6\textbf{ m/s}^2}[/latex] (c) [latex]\boldsymbol{a_{\textbf{c}}=3.73\textbf{ g}.}[/latex]

This does not seem too large, but it is clear that bobsledders feel a lot of force on them going through sharply banked turns.

**7: **(a) 2.56 rad/s (b) 5.71^{o}