Do now:

The propeller blades of an aircraft rotate with a frequency of 40Hz. If the blades have a radius of 85cm how fast do the tips of the propeller blades move?

Wednesday, April 19, 2023

Homework reminder:Set 1 and 2 should now be finished

This week: work on Set 3 (forces and circles)

and Set 4 (gravitation and satellites)

• Recall: period (T) - time it takes to make one full rotation (revolution, cycle), measured in s

• Frequency (f) – number of rotations per second (Hz or s-1)

fT

1

• Distance travelled in one rotation is the circumference:

• Speed of an object moving in a circle of radius r is:

T

r

t

dSpeed

2

rd 2

During a hammer throw, a steel ball isswung horizontally with a constantspeed of 10ms-1 in a circle of radius 2.0m.

1) Calculate the frequency of rotation

2) Sketch a vector diagram to show the change in velocity as the steel ball moves from point P to point Q

P

Q

10ms-1

10ms-1

2.0m2.0m

To keep an object moving in a circular motion, a force must act to push it toward the centre of the circle.

a

This force is called a centripetal force.

F

FC keeps the ball on a circular orbit - otherwise it flies off at a tangent.

Circular Motion

• Centripetal force and acceleration can be calculated using the following formulae:

• v=speed(ms-1)• r=radius of motion

2

c

va

r

r

vmFc

2

During a hammer throw, a steel ball isswung horizontally with a constantspeed of 10ms-1 in a circle of radius 2.0m.

The steel ball has a mass of 7.0kg

3) Calculate the force required to keep the ball moving in a circle

P

Q

10ms-1

10ms-1

2.0m2.0m

1) Centripetal force is not magic. It must come from somewhere. Explain what supplies the centripetal force for each of these circular motions:

a) The earth around the sunb) An electron around a protonc) A car travelling around a cornerd) A stone being swung on the end of a string

2) A car travels around a corner. Why does loose change on the dashboard slide around?

3) Explain what happens in circular motion if the centripetal force is removed.