Effects of forces

Forces can produce changes in the size and shape of an object, and they can also produce changes in its motion.

• When forces act along the same straight line, the resultant force is found by adding forces acting in the same direction and subtracting forces acting in opposite directions.
• An object remains at rest, or continues to move at constant speed in a straight line, unless a resultant force acts on it.
• A resultant force changes the velocity of an object — either by changing its speed, its direction, or both.

Solid Friction

• Solid friction is the force between two surfaces that may impede motion and produce heating, transferring energy from the kinetic store to the thermal store.

Drag

• Drag is the friction that acts on an object moving through a liquid or a gas (e.g. air resistance), opposing its motion and increasing with speed.

Extended Extended students must also understand the following:

Spring Constant

k = F / x

Key Definition The spring constant (k = F / x) is defined as the force per unit extension.

Key Definition The limit of proportionality is the point on a load–extension graph beyond which the extension is no longer directly proportional to the load.

Newton's Second Law

F = ma

• Newton's second law (F = ma) states that resultant force equals mass times acceleration, with the force and the acceleration always acting in the same direction.
• At constant mass, acceleration is directly proportional to the resultant force.
• At constant force, acceleration is inversely proportional to the mass.

Circular Motion

• Circular motion arises when a force acts perpendicular to the direction of motion, directed towards the centre of the circle.
• For constant mass and radius, increasing the force increases the speed.
• For constant mass and speed, increasing the force decreases the radius.
• A greater mass requires a greater force to maintain the same speed and radius.