Circular Motion
What must always point toward the center in uniform circular motion?
Centripetal force and centripetal acceleration (both point toward the center).
What happens to gravitational force if the mass of one object doubles?
The gravitational force doubles.
What force holds electrons in orbit around the nucleus?
The electromagnetic force.
Why does a charged balloon stick to a wall?
The charged balloon polarizes the wall molecules, leading to electrostatic attraction.
What two conditions must be true for work to be done on an object?
1) A force must be applied.
2) The object must move in the direction of the force (or at least have a component in that direction).
A car takes a turn at constant speed. Does its velocity change? Why?
Yes. Its velocity changes direction, even if speed stays constant.
Why would you weigh more on Jupiter than on Earth?
Jupiter has greater gravitational acceleration (≈ 2.5 × Earth’s), so your weight increases.
Which fundamental force must be overcome for radioactive decay?
The strong nuclear force must be overcome.
What type of force causes your foot to feel the floor when you stand?
A repulsive electric force between the atoms of your foot and the floor.
What is the difference between work and power?
Power is measuring the rate at which work is done.
A ball whirled on a string moves in a circle. Which way does the acceleration vector point?
Toward the center of the circle (along the string).
State Newton’s Law of Universal Gravitation in words.
Every two masses attract each other with a force proportional to the product of their masses and inversely proportional to the square of the distance between them.
Name two forces described by the Standard Model.
Electromagnetic, strong nuclear, weak nuclear.
How are cell phones and refrigerator magnets alike in terms of forces?
Both rely on the electromagnetic force.
Two machines do the same work, but one does it faster. What can you say about their power?
The machine that finishes faster has the higher power output.
A 2.0 kg object moves at 4.0 m/s in a circle of radius 3.0 m. Calculate the centripetal force.
F = mv2/r = (2.0)(4.02)/3.0 = 10.7N.
Calculate the weight of a 50 kg person on Earth (g = 9.8 m/s²).
F = mg = (50)(9.81) = 490N.
What makes the atomic nucleus stable despite protons repelling each other?
The strong nuclear force binds protons and neutrons together.
What is plasma made of?
A mixture of ions and electrons.
A machine exerts a force of 150 N over 25.0 cm. How much work is done?
Distance = 25.0 cm = 0.25 m
Work: W = Fd = (150)(0.25) = 37.5 J
Two objects move at the same speed but different radii. Which requires a greater centripetal acceleration?
The one with the smaller radius, because ac = v2/r increases as r decreases.
A 200 kg and 150 kg object are 0.5 m apart. Estimate the gravitational force between them (use G = 6.67 x 10-11).
F = G(m1m2) = (6.67×10-11)(200)(150)/(0.52) ≈ 8.0×10-6 N
Which force dominates interactions between planets and stars?
The gravitational force.
A plastic rod attracts small foam pieces after being rubbed. What force explains this?
The electric force between the charged rod and polarized foam molecules.
A motor does 12,000 J of work in 2.5 minutes. What is its power in watts?
Time = 2.5 min = 150 s
Power: P = W/t = 12000/150 = 80 W