2.5: Non constant forces/Circular motion

Which of the following is true for an object in uniform circular motion?

It has constant speed

Which statement best describes Kepler’s 3^rd Law of Planetary Motion?​

The closer a planet is to the sun, the shorter the orbital period is.

Which statement best describes Kepler’s 2^nd Law of Planetary Motion?​

Fast & Furious: Close = fast racing, far = slow cruising, but equal areas in equal times.

Which statement best describes Kepler’s 1^st Law of Planetary Motion?​

Planetary orbits are in the shape of an ellipse

A man weighing 800 Newtons is standing in an elevator. If the elevator rises with an acceleration of 0.5 meters per second², the force exerted by the elevator on the man will be

1200N

The weight of an object_______as it moves away from the center of Earth

decreases

The force of gravity on the surface of the Moon is about one-sixth the force of gravity on the surface of Earth. Which describes the relationship of mass and weight of an object on the Moon compared to that on Earth?

Mass is the same, but weight is less on the Moon.

If the radius of the string is .3 meters long and the stopper makes 5 revolutions per second, what is the speed of the stopper? (Hint: C=2πr)

9.4 m/s

What object experiences uniform circular motion?

A car traveling around a roundabout at a constant speed

At the instant shown in the diagram, the car's centripetal acceleration is directed-

toward E

What is the difference between centripetal and centrifugal force? Which occurs in a non-rotating frame of reference? Which occurs in a rotating frame of reference?

Centripetal force is real and occurs in a non-rotating frame; centrifugal is fictitious and occurs in a rotating frame.

Centripetal acceleration

Change in velocity direction, points inward, pulls direction toward center (a=v2/ra = v^2/ra=v2/r)

Centripetal force

Real inward force that causes circular motion (F=mv2/rF = mv^2/rF=mv2/r).

Centrifugal force

The “fake” outward force you feel due to inertia when something forces you into circular motion.

Calculate the centripetal force required to make a 5 kg ball move 4 m/s in a circle with a radius of 2 m.

40N

Bruno the bat flies at a speed of 0.5 m/s in circle of radius 1 m. What is his acceleration?

0.25 ms^-2

A certain neutron star has a mass of 1.0x10^31 kg packed into a sphere of radius 10000m. What would be the weight of a 100kg person on the star’s surface?

9.8 x 10^12 N

A man standing on a scale in an elevator notices that the scale reads 40 newtons greater than his normal weight. Which type of movement of the elevator could cause this greater-than-normal reading?

accelerating upward

An astronaut has a mass of 100 kg and has a weight of 370 N on Mars. What is the gravitational strength on Mars?

3.7 m/s²

What is the effect of gravitational mass on an object?

It defines the gravitational force between objects

Planets orbit the Sun in a shape called a(n)

ellipse

Captain America and the Avengers follow the Chitauri back to their home planet, Chitauri Prime. If gravity on Chitauri Prime is rumored to be 8.76 m/s², how much will Captain America weigh there? (Remember, his mass is 86.18 kg)

754.94 N

At the position shown in the diagram, which arrow indicates the direction the instantaneous (tangential) velocity of the cart?

C

Elevator accelerating up

fn is greater than objects weight making the person feel heavier (f=ma) (f=fn(up)-fn(rest))

Elevator accelerating down

fn i s smaller, person feels lighter (f=ma)

Uniform circular motion

Constant speed in perfect circle

v and a makes a right angle

Change in direction

v and a make an angle

Causes speed up and change in direction (not uniform circular motion)

Tangential acceleration

• It’s how quickly an object speeds up or slows down as it moves along a circle.

• It points along the edge (tangent) of the circle, not inward.

👉 Example: A car on a curved racetrack. If the driver hits the gas or brake, that change in speed is tangential acceleration.