1. What Is a Force?
A force is a push or a pull that can change the motion or shape of an object.
Examples:
Pushing a door (push)
Pulling a drawer (pull)
Gravity pulling an apple down
A magnet pulling a piece of iron
2. Effects of a Force
A force can:
1. Start a motion
– Pushing a stationary toy car makes it move.
2. Stop a motion
– Applying brakes stops a moving bicycle.
3. Change speed
– Pedaling harder increases speed.
4. Change direction
– A football player kicking the ball changes its direction.
5. Change shape
– Squeezing a sponge or stretching a rubber band.
3. Types of Forces
A. Contact Forces (touching required)
1. Push and Pull forces
2. Frictional force — opposes motion
3. Tension force — in ropes or strings
4. Normal reaction — support force from surfaces
5. Applied force — any force you apply by pushing or pulling
B. Non-Contact Forces (no touching required)
1. Gravitational force — pulls objects toward Earth
2. Magnetic force — magnets attract or repel
3. Electrostatic force — charges attract or repel
4. Units of Force
The SI unit of force is the Newton (N).
It is named after Sir Isaac Newton.
Example:
A force of 10 N pushes a box.
5. Measuring Force
We measure force using a spring balance (or Newton meter).
It works by stretching a spring when a force is applied.
6. Weight vs Mass
These two are often confused!
Mass
- Amount of matter in an object
- Measured in kilograms (kg)
- Does not change anywhere
Weight
- Force with which gravity pulls on an object
- Measured in Newtons (N)
- Changes with location (Earth vs Moon)
Formula: Weight = mass × gravitational field strength
W = m × g
On Earth,
g = 10 m/s2 (approx.)
Example:
A mass of 5 kg has a weight of:
W = 5 × 10 = 50 N
7. Friction
Friction is a force that opposes motion.
Advantages of friction
- Helps us walk
- Helps cars grip the road
- Helps hold things (no slipping)
Disadvantages of friction
- Causes wear and tear
- Produces unwanted heat
- Slows moving parts (machines)
Reducing friction
- Lubrication (oil, grease)
- Smooth surfaces
- Using ball bearings
- Streamlined shapes
8. Balanced and Unbalanced Forces
Balanced Forces
- Two equal and opposite forces
- No change in motion
Example: A book resting on a table.
Unbalanced Forces
- Forces are not equal
- Causes motion, speeding up, slowing down, or changing direction
Example: Kicking a ball makes it move.
9. Newton’s Laws (Simplified)
Newton's 1st Law – Law of Inertia
Objects stay at rest or move at constant speed unless a force acts on them.
Newton's 2nd Law – Acceleration
The bigger the force, the bigger the acceleration.
F = m × a
Newton's 3rd Law – Action and Reaction
For every action, there is an equal and opposite reaction.
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