Forces

Defining Forces

• A force is a push or pull upon an object resulting from the object’s interaction with another object.
• Forces are described quantitatively by the standard metric unit known as the Newton (N).
• Forces are vector quantities as they have both magnitude and direction.

Types of Forces

• Weight is the force of gravity acting on an object. Its direction is towards the centre of the Earth.
• Tension is the force which is transmitted through a string, rope, cable or wire when it is pulled tight by forces acting from opposite ends.
• Friction is the force that opposes the sliding or rolling of one solid object over another.
• Normal force, also known as the contact force, is the support force exerted upon an object that is in contact with a stable object.
• Air resistance is a special type of frictional force that acts upon objects as they travel through the air.

Free Body Diagrams

• A free body diagram is a diagram used to show the relative magnitude and direction of all forces acting upon an object in a given situation.
• Free body diagrams can be used to solve problems in mechanics.

Applying Newton’s Laws of Motion

• Newton’s first law (Inertia): An object at rest tends to stay at rest, and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force.
• Newton’s second law (F = ma): The acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object.
• Newton’s third law (Action and Reaction): For every action, there is an equal and opposite reaction.

Effects of Forces

• When a force acts on an object, it will cause the object to change its state of motion resulting in an acceleration.
• Multiple forces acting upon an object may cancel each other out, resulting in zero net force, or balanced forces. This results in no change to the object’s state of motion.
• If the forces acting upon an object do not cancel out, they are described as unbalanced forces. This results in a change in the object’s state of motion.

Calculating the Resultant Force

• To find the resultant force, add or subtract the products of the mass and acceleration for each individual force.

Friction and Motion

• Friction acts to oppose motion. Friction depends on both the nature of the surfaces in contact and the amount of force pressing the surfaces together.
• Static friction prevents an object from moving initially. It is the frictional force experienced by a stationary object.
• Sliding friction is the frictional force that opposes an object’s motion. It acts once the object is in motion.

Gravitational Fields and Forces

• Gravitational force is a force of attraction that exists between any two masses, any two bodies, any two particles.
• The gravitational force decreases with distance, described mathematically by Newton’s Law of Universal Gravitation.
• The gravitational field around a body is represented by field lines, field strength is greatest where these lines are closest together..