Force and Motion, Gravitation

Question 1

The center of mass

Answer 1

The center of mass is the average distance, weighted by mass

Question 2

Newton’s first law, inertia

Answer 2

The law of inertia basically states the following: without an external force acting on an object, nothing will change about that object in terms of speed and direction.

Question 3

Newton’s second law

Answer 3

Net force = mass * acceleration

Question 4

Units of a Newton

Answer 4

N = kg·m/s²

Question 5

Newton’s third law

Answer 5

Every action has an equal and opposite reaction. That’s why rockets work in space.

Question 6

Law of Gravitation

Question 7

Compare the strength of gravitational force to electric force and how you know which is stronger.

Answer 7

• Gravity is the weakest of the four universal forces.
• This weakness is reflected in the universal gravitational constant, G, which is orders of magnitude smaller than the Coulomb’s constant.

Question 8

Acceleration of Uniform Circular Motion

Question 9

Force of Uniform Circular Motion

Question 10

Circumference of Uniform Circular Motion

Question 11

Arc of Uniform Circular Motion

Question 12

Sector of Uniform Circular Motion

Question 13

What is the direction of uniform circular motion?

Answer 13

The direction of both the acceleration and the force is toward the center of the circle.

Question 14

What is weight?

Answer 14

Weight is the force that acts on a mass

Question 15

What is the weight at the surface of the earth?

Answer 15

W = mg

Question 16

What do you weigh on an elevator going up?

Answer 16

W = mg + ma

Question 17

What do you weigh on an elevator going down?

Answer 17

W = mg - ma

Question 18

When something is laying still on a horizontal plane, what is the normal force equal to?

Answer 18

When something is laying still on a horizontal surface, the normal force is equal and opposite to the weight.

Question 19

When something is laying still on an inclined plane, what is the normal force equal to?

Answer 19

When something is laying still on an inclined plane, the normal force and friction force adds up in a vector fashion to equal the weight.

Question 20

What is friction?

Answer 20

Friction is a force that is always in the direction to impede the sliding of surfaces.

Question 21

What are the equations of static and kinetic friction?

Answer 21

Static Friction:

Kinetic Friction:

Question 22

What is the difference between static and kinetic friction?

Answer 22

• Static friction pertains to objects sitting still. An object can sit still on an inclined plane because of static friction.
• Kinetic friction pertains to objects in motion. A key sliding across the table eventually comes to a stop because of kinetic friction.

Question 23

What is the direction of the friction vector?

Answer 23

Always opposite the motion of the surface involved

Question 24

Motion on an inclined plane

Question 25

On an inclined plane, what does the normal force do?

Answer 25

To prevent the object from crashing through the surface of the inclined plane, the surface provides a normal force that is equal and opposite to the normal component of gravity.

Question 26

In an object that begins to slip on the inclined plane, what is the parallel component in relation to static friction

Answer 26

parallel component of gravity > static friction.

Question 27

In an object that accelerates down the inclined plane, what is the parallel component of gravity in relation to friction

Answer 27

parallel component of gravity > kinetic friction.

Question 28

When you push an object up an inclined plane, what do you need to overcome?

Answer 28

you need to overcome both the parallel component of gravity and friction.

Question 29

What is the purpose of pulleys on a system?

Answer 29

Pulleys reduce the force you need to lift an object. The catch - it increases the required pulling distance.

Question 31

What are the four forces?

Answer 31

• The strong force: also called the nuclear force. It is the strongest of all four forces, but it only acts at subatomic distances. It binds nucleons together.
• Electromagnetic force: about one order of magnitude weaker than the strong force, but it can act at observable distances. Binds atoms together. Allows magnets to stick to your refrigerators. It is responsible for the fact that you are not falling through your chair right now (MCAT people love to throw you quirky examples like this one).
• Weak force: roughly 10 orders of magnitude weaker than the strong force. Responsible for radioactive decay.
• Gravity: roughly 50 orders of magnitude weaker than the strong force. Responsible for weight (not mass!). Also, responsible for planet orbits.

Question 32

Given a number of ropes contributing to a pulley, n, how much force do you have input to move the mass m?

Answer 32

You have to input: m / n