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.
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.