Learning objectives
- Identify contact and non-contact forces.
- Explain balanced and unbalanced forces.
- Use the idea of pressure as force per unit area.
- Apply pressure concepts to daily life.
What a force can do
A force is a push or pull. It can start or stop motion, change speed, change direction or alter the shape of an object. Force has both magnitude and direction.
Balanced forces do not change an object's state of motion. Unbalanced forces produce acceleration, meaning the object's velocity changes.
Contact and non-contact forces
Muscular force, friction and normal reaction require contact. Gravitational, magnetic and electrostatic forces can act without direct contact.
Friction opposes relative motion between surfaces. It can be useful for walking and braking, but it can also cause wear and energy loss.
Pressure and area
Pressure describes how concentrated a force is on a surface. For the same force, reducing the contact area increases pressure.
A sharp knife cuts more easily because the same force acts over a very small area. Snowshoes spread a person's weight over a larger area and reduce pressure on snow.
Same force, different pressure
Snowshoes, wide tyres and building foundations spread force over a larger area. A pin or knife edge concentrates force over a small area. The force can be similar while the resulting pressure is very different.
Design problem: protect a soft surface
Imagine placing a heavy model on soft clay without leaving a deep mark. You cannot reduce the model's weight. Sketch two base designs that would reduce pressure, then explain which variable in the pressure relationship your design changes.
Worked example: the same force on different areas
Pressure is force divided by the area over which that force acts. Suppose a downward force of 60 N acts evenly over an area of 0.03 m². The pressure is 60 ÷ 0.03 = 2,000 Pa. If the same force is spread over 0.06 m², the pressure is 60 ÷ 0.06 = 1,000 Pa. Doubling the contact area halves the pressure when the force stays the same.
| Area | Calculation | Pressure |
|---|---|---|
| 0.03 m² | 60 ÷ 0.03 | 2,000 Pa |
| 0.06 m² | 60 ÷ 0.06 | 1,000 Pa |
This explains why a wide schoolbag strap can feel more comfortable than a narrow strap under similar conditions. It distributes a similar load across a larger area. A sharp pin works in the opposite way: a small tip area produces high pressure for the applied force.
Balanced and unbalanced forces
If a stationary book experiences an upward support force equal to its downward weight, its forces are balanced and its velocity stays unchanged. If an unbalanced horizontal force acts on a trolley, the trolley's velocity changes. Balanced forces do not mean that no forces are present.
Test your understanding
A force of 100 N acts over 0.5 m². Calculate the pressure.
Pressure = 100 N ÷ 0.5 m² = 200 Pa.
Two people push a box equally hard in opposite directions. What is its net horizontal force?
If the forces have equal magnitudes and exactly opposite directions, the net horizontal force is 0 N. That alone does not describe any vertical forces or friction that may also act.
Sort the force before calculating pressure
Classify a magnet attracting a paper clip, a hand pushing a desk, and Earth pulling a dropped object. Then name the visible effect: change in speed, direction, shape, or equilibrium. Only after that introduce pressure, because pressure is a force-per-area relationship rather than a synonym for force.
For every pressure problem, name the force, name the contact area, write the relationship, and predict the direction of change before using numbers.
Pressure test
Same force plus smaller area means greater pressure. Same area plus greater force means greater pressure. If a design spreads the load, explain how the contact area changed and why the surface experiences less pressure.
Practice questions with explanations
Try each question before opening the answer. The explanation shows the reasoning, not only the final response.
Q1. What is a force?
Answer: A push or a pull.
Explanation: A force can affect motion, direction or shape.
Q2. Give one example of a contact force.
Answer: Friction.
Explanation: Friction acts when surfaces touch.
Q3. Give one example of a non-contact force.
Answer: Gravity.
Explanation: Gravity acts between masses without direct contact.
Q4. What do balanced forces do to motion?
Answer: They cause no change in the state of motion.
Explanation: Their effects cancel because the net force is zero.
Q5. What do unbalanced forces cause?
Answer: A change in velocity.
Explanation: The object may speed up, slow down or change direction.
Q6. Why are school-bag straps made broad?
Answer: Broad straps reduce pressure on the shoulder.
Explanation: The same weight is spread over a larger area.
Q7. Why does a nail have a pointed tip?
Answer: To produce high pressure over a small area.
Explanation: This makes it easier to penetrate wood.
Q8. Does a liquid exert pressure only downward?
Answer: No, it exerts pressure in all directions.
Explanation: Fluid particles push on the walls and base of a container.
Q9. Why is friction necessary for walking?
Answer: It provides grip between the foot and ground.
Explanation: Without friction, the foot would slide backward.
Q10. How can friction be reduced in machines?
Answer: By lubrication or using bearings.
Explanation: These methods reduce direct rubbing between surfaces.