Forces and Their Effects

What is a force?

A force is a push or pull that can change an object's motion, shape or direction.

How can objects interact at a distance?

Objects can interact without contact through fields, including gravitational, electrostatic and magnetic fields.

What is a gravitational field?

A region around a mass where another mass experiences a gravitational force.

What is an electrostatic field?

A region around a charged object where another charged object experiences an electrostatic force.

What is a magnetic field?

A region around a magnet or current-carrying conductor where another magnetic material, magnet or current experiences a magnetic force.

How can objects interact by contact?

Contact interactions include normal contact forces and friction.

What is the normal contact force?

The force exerted perpendicular to a surface when an object is in contact with it.

What is friction?

A contact force that opposes motion or attempted motion between surfaces.

What happens when interacting objects produce forces?

They produce pairs of forces acting on the two interacting objects.

How are forces represented?

Forces are represented as vectors, using arrows showing both magnitude and direction.

What is the difference between a scalar and a vector quantity?

A scalar has magnitude only, whereas a vector has both magnitude and direction.

Give examples of scalar quantities.

Distance, speed, mass, time and energy.

Give examples of vector quantities.

Displacement, velocity, acceleration, force, weight and momentum.

What is a resultant/net force?

The single force that has the same effect as all the forces acting on an object combined.

What does it mean if forces are balanced?

The resultant force is zero, so there is no change in velocity. The object remains at rest or continues at constant velocity.

What does it mean if forces are unbalanced?

The resultant force is not zero, so the object's velocity changes.

What is a free-body force diagram?

A diagram showing all the forces acting on an isolated object, represented by labelled arrows.

How should forces be shown on a free-body diagram?

As arrows pointing in the direction the forces act, with labels identifying the forces.

What is force resolution?

Resolving a force means splitting one force into two or more components, usually perpendicular components.

What is a vector diagram used for?

To represent forces and show their magnitude and direction, including resultant forces, resolved forces and equilibrium.

What is equilibrium?

A state in which the resultant force on an object is zero.

What happens to an object when the resultant force is zero?

It remains at rest or continues moving at constant velocity.

What happens when several forces act on an isolated solid object?

The forces combine to produce a resultant force, which determines whether the object's velocity changes.

What is a turning effect of a force?

The tendency of a force to cause an object to rotate about a pivot or axis.

Give examples of forces causing rotation.

Pushing a door, turning a spanner, using scissors or applying a force to a lever.

What is the moment of a force?

The turning effect of a force about a pivot.

What is the equation for the moment of a force?

Moment = force × perpendicular distance from the pivot to the line of action of the force. M = F × d.

What is the unit of moment?

The newton metre (N m).

What distance is used when calculating a moment?

The perpendicular distance from the pivot to the line of action of the force.

How can the moment of a force be increased?

Increase the force or increase the perpendicular distance from the pivot.

What is the principle of moments?

For an object in rotational equilibrium, the sum of clockwise moments equals the sum of anticlockwise moments.

What is rotational equilibrium?

When the total clockwise moment equals the total anticlockwise moment, so there is no resultant turning effect.

What equation represents the principle of moments?

Sum of clockwise moments = sum of anticlockwise moments.

How do levers transmit the rotational effects of forces?

A lever uses a pivot and a force acting at a distance from the pivot to produce a turning effect, allowing a force to move a load.

What is the mechanical advantage of a lever?

It is the ratio of the output force to the input force.

How do gears transmit rotational effects of forces?

Gears transfer rotational motion and forces between shafts; changing gear sizes changes the turning effect and rotational speed.

What happens when a small gear drives a larger gear?

The larger gear rotates more slowly but produces a greater turning effect.

What happens when a large gear drives a smaller gear?

The smaller gear rotates faster but produces a smaller turning effect.

Why is lubrication used to reduce unwanted energy transfer?

Lubrication reduces friction between moving surfaces, reducing energy dissipated as thermal energy.

How does lubrication make mechanical systems more efficient?

It reduces friction and therefore reduces unwanted energy transfer to the surroundings as heating.