Force and Newton's Laws

Question 1

What is force? Are they vectors? What is force measured in?

Answer 1

Any influence capable of causing a mass to accelerate.

• -forces are vectors and can, therefore, sum to ZERO. This is called NO net force, which is the same as if no force existed at all.
• -if one of those forces is suddenly changes, a net force is instantly created.
• -force is measured in NEWTONS.

Question 2

Examples of force:

Answer 2

1. the force due to gravity
2. contact forces
3. electrostatic forces
4. torque
5. tension
6. magnetic forces
   etc.

Question 3

Newton’s First Law is what/says?

—some general statements:

Answer 3

The Law of Inertia.

This law states a few things.

1. If the new force on an object is zero, then that object will not have a change in velocity.
2. An unbalanced force on a body will always change it’s VELOCITY.
3. The reason why initially moving objects tend to come to a rest in our “everyday” lives is because they are being acted upon by UNBALANCED forces.
4. An unbalanced force will SOMETIMES, but NOT always, change an object’s direction.

Question 4

Newton’s First Law

–what is inertia?

Answer 4

The ability of an object to resist a change to its velocity.

Question 5

Newton’s First Law

–what is mass?

Answer 5

A measure of an object’s inertia.

Question 6

Newton’s First Law

–what is the center of mass?

–How do you calculate center of mass?

Answer 6

A weighted average of mass distribution.

Cmass = (r1m1 + r2m2 + r3m3 …) / mtotal

Question 7

Newton’s First Law

–where is the center of gravity?

Answer 7

Located exactly at the center of mass.

Question 8

Newton’s First Law

–Where is the center of buoyancy?

Answer 8

Located exactly at the center of mass of the fluid dispersed by the submerged object

–NOT at the center of mass of the submerged object itself.

Question 9

Newton’s Second Law

–Equation?

Answer 9

Fnet = ma

Fnet = the vector sum of all forces acting on the object

Question 10

What is important to know about Newton’s Second Law?

–Why is it important for the MCAT?

Answer 10

** Whenever force, mass, or acceleration are asked for, first try to apply this law:

** All objects follow Fnet = ma

Question 11

Newton’s Second Law

–What simple change causes a change in acceleration?

Answer 11

A changing force can cause a changing acceleration.

Question 12

Newton’s Second Law

–A constant force can cause an object to move faster and faster. Here, what is changing and what is constant?

Answer 12

• Displacement is changing non-linearly
• Velocity is changing linearly
• Acceleration is not changing at all

Question 13

Newton’s Second Law

–True or False: A constant force applied to an object will NOT cause it to travel at a constant velocity.

Answer 13

TRUE!

–A constant net force will always cause a constant acceleration and therefore a changing VELOCITY.

Question 14

Newton’s Second Law

–What is required for an object to accelerate?

Answer 14

An object must be either:

1. in contact with the object creating the force
2. be under the influence of a field force at that exact moment

–the field force could be gravitational or electrical!

Question 15

Newton’s Second Law

• -what is acceleration?
• -Units?

Answer 15

Change in velocity over time.

Units: m/s2

Question 16

Newton’s Second Law

–a loaded spring shoots a ball across a flat table. After the ball is no longer in contact with the spring, the acceleration of the ball will:

Answer 16

Remain constant.

–Explanation: once the ball is released, if no friction or other horizontal forces act on the ball (like friction), it will maintain constant velocity. The ball accelerated while it was in contact with the spring, but after being released by the spring, the ball has no acceleration. It remains constant at zero value.

Question 17

Newton’s Third Law

• -is?
• -says?

Answer 17

–Action-reaction (equal and opposite reactions)

–Whenever one object exerts a force (action) on a second object, the second object always exerts an equal and opposite force (reactions) on the first object.

Question 18

Newton’s Third Law

–Examples of action-reaction pairs:

Answer 18

1. Constant force between two objects
2. Magnetic force between two objects
3. Gravitational force between two objects
4. Electrostatic force between two charged particles

Question 19

The moon is approx. 300x smaller than the sun. If the sun exerts a gravitational force of F on the moon, what is the approximate force exerted on the sun by the moon, in terms of F?

Answer 19

The force exerted on the sun by the moon will be the same amount of force exerted by the moon on the sun.

Newton’s Third Law