Newton's Laws in Daily Life

A bus brakes and you lean forward. A football flies away when you kick it. A balloon shoots across the room when you let go of it. All three are explained by three laws. Here they are side by side.

The three laws in plain words

• First law. If the net force on an object is zero, its velocity does not change. A body at rest stays at rest. A body in motion keeps its speed and its direction. The tendency to keep its state is called inertia. A bigger mass has more inertia.
• Second law. A net force changes the velocity of an object. The bigger the net force, the bigger the acceleration. The bigger the mass, the smaller the acceleration. The law is $F = ma$.
• Third law. When object A pushes object B, then B pushes A back with a force of the same size in the opposite direction. The two forces act on different objects.

See it in numbers

A car of mass 1200 kg has a net force of 3000 N on it. What is its acceleration?

We want $a$, so $a = F/m$. Then $a = \dfrac{3000}{1200} = 2.5$ m/s².

Check with the same program that you can run here:

Mistakes to avoid

• "A moving object needs a force to keep moving." It does not. A moving object keeps moving when the net force is zero. Friction is the force that stops everyday objects.
• "Action and reaction cancel each other." They do not, because they act on different objects. Forces cancel only when they act on the same object.
• "The heavier object pushes harder in a collision." In a collision the two forces are always equal in size. The effects differ because the masses differ.

Think it through

Write your answers in full sentences. Try each one before you read the pointers.

1. A book rests on a table. Its weight pulls it down, and the table pushes it up. The two forces are equal and opposite. A student says that they are a third-law pair. Explain why this is not correct, and name the third-law partner of the weight of the book.
2. A passenger leans forward when a bus brakes suddenly. Explain this using inertia. Do not use the words "thrown" or "pushed forward".
3. A heavy truck and a small car collide head on. The force on the truck is equal to the force on the car. Yet the car is damaged much more. Explain how both statements can be true.
4. The same net force acts on a 2 kg trolley and on a 4 kg trolley. Predict the ratio of their accelerations and give your reason. Then say what happens if the net force on the 4 kg trolley is doubled.
5. Design an experiment with a trolley, a string, a pulley and small masses to test $F = ma$. Say what you would change, what you would keep the same, and what graph you would draw. What would a straight line through the origin tell you?

Pointers for your answers

1. The weight and the push of the table act on the same object (the book). A third-law pair acts on two objects. The partner of the weight is the pull of the book on the Earth.
2. The passenger was moving with the bus. The bus slows down, but the passenger's body has inertia and tends to keep its velocity. So the body moves forward relative to the bus.
3. The forces are equal, but $a = F/m$. The car has a smaller mass, so it gets a much larger acceleration, and it is more badly damaged.
4. $a = F/m$, so the 2 kg trolley has twice the acceleration of the 4 kg trolley. If the force on the 4 kg trolley is doubled, its acceleration doubles, and then both accelerations are equal.
5. Change the pulling force. Keep the mass the same. Plot acceleration against force. A straight line through the origin means that acceleration is proportional to the net force.