Forces
1. A force is a push or pull that one object exerts on another object. It produces or tends to produce motion, and stops or tends to stop motion.
| Type of force | Nature of force |
| Contact force | The push experienced when two objects are pressed together. |
| Weight | The gravitational pull of the earth on an object. |
| Friction | This is a type of force that opposes motion. It exists between the surfaces of two objects in contact. |
| Tension | The pull at both ends of a stretched spring, string or rope. |
| resistance | This force is also known as viscous force. It is a type of force that is found in fluids such as oil or air. |
| Electric force | The push or pull between electric charges. |
| Magnetic force | The push or pull between magnets only or magnets or magnetic materials. |
2. Friction and resistance are types of contact forces that oppose motion; they cause energy loss in the form of thermal energy.
3. The SI unit of force is Newton (N).
4. A force always acts in a particular direction.
5. Scalar quantities are physical quantities that have magnitude only,e.g. mass and distance
Vector quantities are physical quantities that possess both magnitude and direction, e.g. force and velocity.
6. Scalar quantities are simply adding or subtracting positive numbers together. For instance, a mass of 100g added to 200g gives a total mass of 300g.
7. When we add two vectors, we are trying to find a single vector that will produce the same effect as the two vectors added together. The single vector is called the resultant vector.
8. Different methods of adding vectors:
Addition of parallel vectors:
State of equilibrium
When two equal forces acting on the object is parallel, both has a force of 3N and acting in opposite directions. The resultant force is zero and the object is said to be in a state of equilibrium.
Addition of non- parallel vectors :
The two vector’s resultant force can be found using the:
Parallelogram method
Tip-to-tail method
9. What is the effect of a force on motion?
| Observation | Effect of force |
| When a stationary soccer ball is kicked, it moves off in the direction it is kicked. | The ball accelerates from rest(i.e. zero velocity) Acceleration is positive, and in the same direction as the force exerted by the kick. A force can cause a stationary object to start moving. |
| A person rollerblading gives a push to move even faster. | The rollerblading accelerates and moves faster after the push. Acceleration is positive, and velocity increases. A force can cause a moving object to increase speed. |
| When cycling down a slope, the cyclist applies the brake to slow down the bicycle. | The bicycle decelerates Acceleration is negative. A force can cause a moving object to decrease speed. |
| During a game of badminton, a player intercepts and hits the shuttlecock to change its direction and motion. | The direction and magnitude of the velocity of the shuttlecock change. Acceleration of the shuttlecock is non-zero. A force can cause a moving object to change its direction of motion. |
10. We can see that a force can cause:
1. A stationary object to start moving.
2. A moving object to increase speed.
3. A moving object to decrease speed.
4. A moving object to change its direction of motion.
11. For an object with zero acceleration, the different forces acting on it are balanced or add up to zero- i.e the resultant or net force is zero.
12. Every object will continue in its state of rest or uniform motion in a straight
Line unless a resultant force acts on it to change its state.
13. If the resultant force acting on an object is not zero, we say that the forces are
Unbalanced.
14. When a resultant force acts on a object of constant mass, the object will accelerate and move in the direction of the resultant force. The product of the mass and acceleration of the object is equal to the resultant force.
15.Formula
F=ma
F =resultant force
m= mass of object
a= acceleration of object
16. This equation tells us that:
1. A resultant force F on a object will produce an acceleration a.
2. Doubling the resultant force F on an object will double the acceleration a.
3. With the same resultant force F, doubling the mass will double halve the acceleration a.
17. For every action, there is an equal and opposite reaction, and these forces act
On mutually opposite bodies.
18. Four characteristics of forces:
1. Forces always occur in pairs. Each pair is made of an action force
And a reaction force.
2. Action and reaction forces are equal in magnitude.
3. Action and reaction forces act in opposite directions.
4. Action and reaction forces act on different bodies.
19. How does friction affect motion?
1. Friction always opposes motion between two surfaces in contact.
| Negative effects of friction | Positve effects of friction |
| Reduces the efficiency of cars by upto 20% | Needed for walking or holding a pair of chopsticks |
| Causes wear and tear of moving parts in engines, motors and machines | Used in braking pads to slow down cars |
20. Friction as a negative force:
- Wheels of a shopping trolley
- Ball bearings
- Lubricants and polishing surfaces
- Air cushion
21. Friction as a useful force:
-car tyres
- Parachutes
- Rock climbing
