Forces and effects of forces Notes

What Is a Force and What Does It Explain?

1. Forces are how objects interact.
2. In everyday language, a force is a push or pull, but in physics, it also includes "invisible" interactions such as gravity, magnetism, and electrostatic attraction/repulsion.
3. The key idea is that a force can change an object's velocity (speed or direction) or change its shape.

How Forces Change Motion

1. A force can make an object start moving from rest.
2. A force can make a moving object speed up.
3. A force can make a moving object slow down or stop.
4. A force can make an object change direction, even if its speed stays the same.

How Forces Change Shape (Deformation)

1. Forces can stretch objects, such as pulling on an elastic band.
2. Forces can compress objects, such as squeezing a sponge.
3. Forces can bend or squash objects, causing permanent or temporary deformation.

Non-Contact Forces

1. Non-contact forces act through fields and do not require physical contact.
2. Objects experience these forces when they enter a region of influence around another object.

1. Gravitational Force (Weight)

1. Earth pulls objects toward its centre.
2. This pulling effect is responsible for objects falling when dropped.
3. The gravitational force acting on an object is called its weight.
4. Weight depends on both the object’s mass and the strength of the gravitational field.

2. Electrostatic Force

1. Electrostatic forces act between electrically charged objects.
2. These forces can attract or repel depending on the type of charge.

3. Magnetic Force

1. Magnetic forces act between magnets or magnetic materials.
2. Opposite poles attract, while like poles repel.
3. Magnetic forces can act through air and other materials.

Contact Forces

1. Contact forces only act when objects are touching.
2. These forces often occur in pairs where objects push or pull on each other.

1. Normal Force

1. When an object rests on a surface, the surface pushes back on the object.
2. This force prevents objects from falling through solid surfaces.
3. The force always acts perpendicular to the surface.

2. Friction

1. Friction acts when two surfaces move or try to move past each other.
2. Friction always opposes motion.
3. Rough surfaces produce more friction than smooth surfaces.
4. Friction can be reduced using lubrication but not completely removed unless contact is removed.

3. Air Resistance (Drag)

1. Air resistance increases with speed and surface area.
2. Air resistance acts opposite to the direction of motion.

4. Tension and Compression

1. Tension occurs when objects such as ropes or elastic bands are stretched.
2. Compression occurs when objects such as springs are squashed.
3. These forces help explain how objects store energy elastically.

5. Upthrust (Buoyancy)

1. Objects in fluids experience an upward force.
2. This force helps objects float or reduces their apparent weight.
3. Upthrust acts in both liquids and gases, though it is more noticeable in liquids.

6. Lift

1. Lift is an upward force that helps aircraft fly.
2. Lift is generated when air moves around wings.
3. Lift works together with other forces acting on an aircraft.

How Forces Are Measured

1. Forces are measured using a standard unit so they can be compared fairly.
2. The unit used to measure force is the newton (N).

Why Direction Matters

1. A force is not fully described unless its direction is known.
2. Two forces of the same size can have very different effects if they act in different directions.
3. Because forces include direction, they are classified as vector quantities.

Understanding Resultant Force

1. Objects often experience several forces at the same time.
2. These forces can work together or oppose each other.
3. The combined effect of all forces is called the resultant force.
4. If the forces on an object cancel out, the object is in equilibrium.
5. If the net force is zero, then the object's velocity does not change. It may be:
   1. at rest (velocity is zero), or
   2. moving at constant velocity (constant speed in a straight line).

Free-Body Diagrams Help You Identify All Forces Acting On An Object

1. The object is represented as a simple shape, such as a box or dot.
2. Forces are shown using arrows that start from the object.
3. The arrow direction shows the direction of the force.
4. The arrow length shows the relative size of the force.

How To Draw A Free-Body Diagram

1. Choose the object of interest and draw it as a simple shape.
2. Draw an arrow for every force acting on the object.
3. Label each force with a clear name or symbol (for example $W$ for weight, $N$ for normal reaction).
4. Make arrow directions realistic, and (when possible) arrow lengths proportional to force size.

Identifying Forces in Diagrams

1. Weight always acts vertically downward.
2. Normal reaction acts perpendicular to the surface.
3. Friction acts parallel to the surface and opposite to motion.
4. Tension acts along the rope or string pulling the object.
5. Air resistance acts opposite to the direction of motion.

Forces Can Change Direction Without Changing Speed (Centripetal Force)

1. Any time an object follows a curved path, there must be a net force towards the center of curvature.
2. For a planet orbiting the Sun, gravity provides this inward (centripetal) force.
3. Without gravity, the planet would move off in a straight line.