Newton's Laws of Motion Flashcards

All 26 cards in this deck

State Newton's First Law of motion.

If the resultant force on an object is zero, a stationary object stays stationary and a moving object keeps moving at the same speed in the same direction (uniform velocity).

What must act on an object for its velocity (speed and/or direction) to change?

A resultant force.

What is true about the forces on a vehicle travelling at a steady speed?

The resistive forces balance the driving force, so the resultant force is zero.

A child is standing still on the floor. What is the resultant vertical force on the child, and why?

$0$ N — the upward forces equal the downward forces (the child is not accelerating).

True or false? An object moving in a straight line at a steady speed must have a resultant force acting on it.

False. Uniform velocity means the resultant force is zero.

True or false? An object moving at constant speed in a circle has a resultant force acting on it.

True. The resultant force changes its direction of motion, so its velocity changes, even though the speed does not.

State Newton's Second Law of motion (in words, as a proportionality).

The acceleration of an object is proportional to the resultant force acting on it, and inversely proportional to the mass of the object.

Write down the equation that links resultant force $F$, mass $m$ and acceleration $a$, with the units of each quantity.

resultant force = mass × acceleration, $F = m\,a$ — $F$ in newtons (N), $m$ in kilograms (kg), $a$ in metres per second squared (m/s²).

What are the steps to calculate the acceleration of an object from the resultant force on it?

e.g. a resultant force of $12$ N acts on a mass of $4$ kg

1. Write the equation: $F = m\,a$, so $12 = 4 \times a$
2. Rearrange: $a = F \div m = 12 \div 4$
3. Evaluate: $a = 3$ m/s²

What are the steps to find the resultant force when you are given a change in velocity and a time?

e.g. a $1000$ kg car speeds up from rest to $10$ m/s in $5$ s

1. Find the acceleration: $a = \dfrac{\Delta v}{t} = \dfrac{10}{5} = 2$ m/s²
2. Write $F = m\,a = 1000 \times 2$
3. Evaluate: $F = 2000$ N

What typical values would you use for the mass and acceleration of an ordinary car when estimating the force involved in road transport?

Mass about $1000$ kg and acceleration a few m/s² (roughly $2$ m/s²), giving a force of around $2000$ N.

True or false? In $F = m\,a$, $F$ can be any one of the forces acting on the object.

False. $F$ must be the resultant (net) force of all the forces acting.

In the acceleration practical, how is friction on the runway compensated for?

Raise one end of the runway (e.g. with a wooden block) until the trolley moves at a constant speed after a small push.

Name one variable that must be controlled when investigating how force affects acceleration in the trolley practical.

The combined mass of the trolley and mass hanger.

How can the effect of random errors be reduced in the force and acceleration practical?

Repeat the measurements, ignore any anomalies and calculate a mean.

In the required practical, what relationship do the results show between the resultant force on the trolley and its acceleration (constant mass)?

They are directly proportional — the graph is a straight line through the origin.

State Newton's Third Law of motion.

Whenever two objects interact, the forces they exert on each other are equal and opposite.

A bumper car collides with a barrier. How does Newton's Third Law apply?

The force of the car on the barrier is equal in size to the force of the barrier on the car, and in the opposite direction.

True or false? The weight of a book resting on a table and the upward contact force from the table are a Newton's third law pair.

False. Both act on the book, so they are balanced forces (Newton's First Law); the third-law partner of the book's weight is the book's gravitational pull on the Earth.

True or false? The two forces of a Newton's third law pair cancel each other out.

False. They act on different objects, so they never cancel on a single object.

What is inertia?

The tendency of objects to continue in their state of rest or of uniform motion.

What is inertial mass a measure of?

How difficult it is to change the velocity of an object.

How is inertial mass defined as a ratio?

Inertial mass = force ÷ acceleration, $m = \dfrac{F}{a}$.

True or false? Inertia is a force that acts on a moving object.

False. Inertia is a property (tendency) of the object, not a force.

True or false? An object with a larger inertial mass needs a larger force to give it the same acceleration.

True. Inertial mass is the ratio of force to acceleration, so a larger mass needs a larger force for the same acceleration.

In required practical 7, what apparatus is used to measure the acceleration of the trolley?

A light gate (with a card on the trolley) connected to a data logger, which records the velocities and calculates the acceleration.