Newton's laws of motion: flashcards, quiz and summary

Flashcards

1. 1What does Newton's first law say happens to an object when the net force on it is zero?

   It stays at rest if it was at rest, or keeps moving at the same speed in a straight line if it was moving. Its motion does not change.

2. 2Why does a passenger keep moving forward when a car stops suddenly?

   Inertia: the passenger's body resists a change in motion, so it keeps moving forward until a force, such as a seat belt, stops it.

3. 3What does F = ma tell you about the relationship between net force, mass, and acceleration?

   Net force equals mass times acceleration. For the same force, a larger mass gives a smaller acceleration, and acceleration points in the direction of the net force.

4. 4Why don't action and reaction forces cancel each other out?

   Because they act on different objects. Cancellation only happens when two forces act on the same object.

5. 5Why does Earth's acceleration from your pull seem so small?

   Earth exerts and receives the same size force as you, but Earth's huge mass means a = F/m is tiny for Earth.

6. 6How is weight different from mass, and how does it change on the Moon?

   Mass is the amount of matter in kilograms and stays the same everywhere. Weight is the gravitational force, W = mg, so on the Moon it is about one sixth of its Earth value because g is about 1.6 m/s² instead of 9.8 m/s².

7. 7What is the difference between static and kinetic friction?

   Static friction keeps an object from starting to slide. Kinetic friction acts while the object is sliding and is usually smaller than static friction.

8. 8Why does a falling object reach terminal velocity, and what is its net force then?

   Air resistance increases as speed increases until it equals weight. At that point the net force is zero, so the object moves at constant speed.

Practice quiz

Pick an answer to check it.

1. 1A box has a net force of 0 N acting on it. Which statement is correct?

   1. The box may be at rest or moving at constant velocity.
   2. The box cannot be moving.
   3. The box must be at rest.
   4. The box is accelerating in the direction of its motion.
   Show the answer

   A. The box may be at rest or moving at constant velocity. Zero net force means no change in motion. An object at rest stays at rest, and an object moving at constant velocity keeps moving that way.

2. 2A 2 kg cart is pushed with a net force of 10 N. What is its acceleration?

   1. 5 m/s²
   2. 0.2 m/s²
   3. 8 m/s²
   4. 20 m/s²
   Show the answer

   A. 5 m/s² a = F / m = 10 N / 2 kg = 5 m/s², in the direction of the net force.

3. 3You push on a wall, and the wall pushes back on you. Why don't these two forces cancel?

   1. The forces happen at different times.
   2. The wall's force is always smaller than yours.
   3. Friction removes the reaction force.
   4. The forces act on different objects.
   Show the answer

   D. The forces act on different objects. Cancellation requires two forces acting on the same object. Action and reaction forces act on different objects, so they never cancel.

4. 4A 10 kg box is pulled right with 50 N, and friction is 20 N to the left. What is the box's acceleration?

   1. 7 m/s² to the left
   2. 3 m/s² to the right
   3. 2 m/s² to the right
   4. 5 m/s² to the right
   Show the answer

   B. 3 m/s² to the right Net force is 50 N minus 20 N = 30 N to the right. Then a = 30 N / 10 kg = 3 m/s² to the right.

5. 5Ignoring air resistance, how do a heavy ball and a light ball compare when dropped from the same height?

   1. The heavy ball accelerates more because it has more mass.
   2. The heavy ball falls faster because it has more weight.
   3. They accelerate at the same rate, g, and reach the ground together.
   4. The light ball falls faster because gravity pulls it harder.
   Show the answer

   C. They accelerate at the same rate, g, and reach the ground together. With air resistance ignored, all objects accelerate at g regardless of mass. Heavier objects have more weight but also more mass, so the acceleration is the same.

6. 6A 50 kg student is on the Moon, where g is about 1.6 m/s². What is the student's mass and weight?

   1. Mass 50 kg, weight about 490 N
   2. Mass about 8 kg, weight about 80 N
   3. Mass about 8 kg, weight about 490 N
   4. Mass 50 kg, weight about 80 N
   Show the answer

   D. Mass 50 kg, weight about 80 N Mass stays 50 kg everywhere. Weight is W = mg = 50 kg × 1.6 m/s² = 80 N on the Moon.

7. 7A falling skydiver reaches terminal velocity. Which statement describes the forces at that point?

   1. Gravity has stopped acting on the skydiver.
   2. Air resistance equals weight, so the net force is zero.
   3. Weight is greater than air resistance, so the skydiver keeps speeding up.
   4. Air resistance is greater than weight, so the skydiver slows down.
   Show the answer

   B. Air resistance equals weight, so the net force is zero. Terminal velocity is reached when drag equals weight. The net force is zero, so the speed stays constant.

Test traps

What teachers like to test on this, and the mistakes students make.

• Confusing zero net force with zero forces: a moving object at constant speed has forces acting on it, but they balance to zero net force.
• Thinking action and reaction forces cancel out. They are equal and opposite but act on different objects, so they never cancel.
• Using a single force instead of the net force in F = ma. Add all forces with their directions first.
• Forgetting direction in net force problems, such as subtracting friction when it acts opposite to the pull.
• Saying a heavier object falls faster in a vacuum. With air resistance ignored, all objects accelerate at g.
• Thinking mass changes on the Moon. Only weight changes, because weight depends on g and mass does not.

Summary

• A force is a push or a pull, and it is a vector with both size and direction, measured in newtons (1 N = 1 kg·m/s²).
• The net force is the sum of all forces on an object, and only a nonzero net force changes an object's motion.
• Newton's first law says an object stays at rest or keeps moving at constant velocity unless a net force acts on it, and inertia is the resistance to changes in motion that grows with mass.
• Newton's second law, F = ma, means acceleration points in the direction of the net force and is smaller for a larger mass under the same force.
• Newton's third law says action and reaction forces are equal in size and opposite in direction, but they act on different objects, so they never cancel.
• Mass is the amount of matter and stays the same everywhere, while weight is the gravitational force (W = mg) and changes with the value of g.
• Common forces include the normal force, friction (static and kinetic), tension, and air resistance, which reaches terminal velocity when it equals weight.
• Free-body diagrams show every force on an object as an arrow, and adding those forces gives the net force used in F = ma.

Key terms

Force

A push or a pull on an object. It has both size and direction, so it is a vector.

Net force

The sum of all forces acting on an object, taking direction into account. Only a nonzero net force changes motion.

Inertia

An object's resistance to a change in its motion. More mass means more inertia.

Newton (N)

The unit of force. 1 N = 1 kg·m/s².

Weight

The force of gravity on an object, calculated as W = mg. It changes with the value of g.

Terminal velocity

The constant speed reached when air resistance (drag) equals weight, so the net force is zero.

How to study it

The notes this pack came from

Questions