 Newton’s Second Law of Motion - dummies

Force saves you from the monotony of everything moving at the same speed and direction forever. Force can act on objects, changing their direction and/or speed. The relationship between force, mass, and acceleration is primary in physics classes.

To start, you need to know Newton’s Second Law of Motion, which is a big one in physics: “When a net force sigmaF acts on an object of mass m, the acceleration of that mass can be calculated by sigmaF = ma.” The sigma sign stands for “sum,” so sigmaF = ma can be read as “the vector sum of all forces on an object (the net force) equals mass times acceleration.” It is important to note that this is a vector equation and that both force and acceleration are vectors in this equation. Mass, like time, is a scalar.

Newton’s second law is often just abbreviated as F = ma. It’s important to note that sigmaF refers to the vector sum of all the forces on the object and not to just a single force.

What is the unit of force? This table gives you a rundown for the three measurement systems most commonly used in physics:

Units of Force
System Unit Name Abbreviation
MKS kg-m/s2 newton N
CGS g-m/s2 dyne dyn
FPS pound lb

Here’s how you relate the three different units of force:

1 lb = 4.448 N

1 N = 105 dyn

## Sample question

1. You’re at rest on an ice rink when you get hit from behind with a force of 50.0N as someone bumps you. If your mass is 70.0 kg, what is your acceleration?

The correct answer is 0.714 m/s2.

1. Use the equation F = ma: Solving for a gives you a = F/m.

2. Plug in the numbers: a = F/m = (50.0 N)/(70.0 kg) = 0.714 m/s2.

## Practice questions

1. You come home to find a delivered package with a mass of 100 kg blocking the door. If you push it with a force of 100 N, what will its acceleration be if no friction is involved?

2. You’re gliding across a frictionless lake in a sailboat. If your mass is 70 kg and the boat’s mass is 200 kg, with what force does the wind need to blow you to give you an acceleration of 0.30 m/s2?

3. You have control of a space station, which has a mass of 400,000 kg. To give it an acceleration of 2.0 m/s2, what force do you need to apply with the rockets?

4. You find a stone in the forest and give it a push of 50.0 N. It accelerates at 2.0 m/s2. What is its mass?

5. You’re applying a force of 17 N to a hockey puck with a mass of 0.17 kg. Starting from rest, how far has the puck gone in 0.10 seconds?

6. You push a rowboat on a calm lake (assuming no friction) with a force of 40.0 N. If the rowboat has a mass of 80.0 kg, how far has it gone in 10.0seconds?

7. A space station with a mass of 1.0 x 105 kg is moving toward a satellite at 5.0 m/s. If you want to avoid crashing them together and have only 1.0 x 103 seconds in which to act, what force do you need to apply to stop the space station from colliding with the satellite?

8. Your 1,000-kg car needs a push. Starting at rest, how hard do you have to push to get it up to 5.0 m/s in 1.0 x 102 seconds?

Following are answers to the practice questions:

1. 1 m/s2

1. Solving F = ma for a gives you a = F/m.

2. Plug in the numbers: a = F/m = 100 N/100 kg = 1 m/s2.

2. 81 N

1. Use the equation F = ma.

2. Plug in the numbers: F = ma = (70 kg + 200 kg)(0.30 m/s2) = 81 N.

3. 800,000 N

1. Use the equation F = ma.

2. Plug in the numbers: F = ma = (400,000 kg)(2.0 m/s2) = 800,000 N.

4. 25 kg

1. Use the equation F = ma, and solve for the mass, giving you m = F/a.

2. Plug in the numbers: m = F/a = 50 N/2.0 m/s2 = 25 kg.

5. 50 m

1. Use the equation F = ma, and solve for the acceleration, giving you a = F/m.

2. Use the equation and substitute F/m for a: 3. Plug in the numbers: 6. 25 m

1. Use the equation F = ma, and solve for the acceleration, giving you a = F/m.

2. Use the equation and substitute F/m for a: 3. Plug in the numbers: 7. –5.0 x 102 N

1. Use the equation F = ma, and solve for the acceleration, giving you a = F/m.

2. Use the equation vf = vo + at. In this question, vf = 0, so at = –vo.

3. This becomes solving for F gives you 4. Plug in the numbers, and you get F = –500 N = –5.0 x102 N (it’s negative because it’s opposite to the direction of travel).

8. 50 N

1. Use the equation F = ma, and solve for the acceleration, giving you a = F/m.

2. Use the equation vf = vo + at. In this question, vf = 10 and vo = 0, so at = vf.

3. This becomes solving for F gives you 4. Plug in the numbers, and you get F = 50 N.