Forces and the First Law: Quick Revision

A brief note on Forces and the First Law. Read it before a test. Try each question at the end before you open its answer.

The big ideas

• A force is a push or a pull, in newtons (N). It can start, stop, speed up, slow down or turn something, or change its shape.
• The net force adds the forces with their directions. Zero: balanced; not zero: unbalanced.
• First law: with no unbalanced force, an object stays at rest or keeps moving steadily in a straight line.
• Inertia is that tendency to keep the same motion; more mass, more inertia.
• Friction acts against sliding. It matches a push up to a limit, and is a little less once the object slides.

The facts at a glance

A worked example

A child pulls a $25\ \text{kg}$ sledge with $90\ \text{N}$; friction is $40\ \text{N}$. Find the net force and the acceleration.

1. Net force. $90\ \text{N} - 40\ \text{N} = 50\ \text{N}$ forward.
2. Acceleration. $\dfrac{50\ \text{N}}{25\ \text{kg}} = 2\ \text{m/s}^2$.
3. So the forces are unbalanced and the sledge speeds up.

Practice with new numbers

A program to try

The program works out the sledge.

Mistakes to avoid

• 'Moving means a force keeps it going.' A force changes motion; it is not needed to keep it.
• Balanced means at rest. It means no change: a steady speed is balanced too.
• Friction always points backwards. It points against sliding; when you walk it pushes your foot forward.
• Adding forces without signs. Opposite directions subtract.

Check yourself

1. A car moves at a steady $60\ \text{km/h}$ on a straight road. What is the net force on it?

2. Why do passengers fall forward when a bus brakes?

3. Forces of $70\ \text{N}$ right and $95\ \text{N}$ left act on a box. What is the net force?

4. A heavy table does not move when you push it with $50\ \text{N}$. How big is the friction?

5. Which has more inertia, an empty or a full water bottle?