P6.5. Forces

What is a scalar quantity?

A quantity with magnitude only.

What is a vector quantity?

A quantity with magnitude and direction.

How can a vector be represented?

By an arrow: its length represents magnitude and its direction represents the direction of the vector.

What is a force?

A push or pull that acts on an object due to an interaction with another object.

What are contact forces?

Forces where the objects are physically touching.

What are examples of contact forces?

Friction, air resistance, tension and normal contact force.

What are non-contact forces?

Forces where the objects are physically separated.

What are examples of non-contact forces?

Gravitational, electrostatic and magnetic forces.

What type of quantity is force?

A vector quantity.

How should forces between interacting objects be represented?

As vectors showing their magnitude and direction.

What is weight?

The force acting on an object due to gravity.

What causes gravity close to Earth?

The gravitational field around Earth.

What does an object's weight depend on?

The gravitational field strength at its location.

What is the equation for weight?

W = mg.

What is weight measured in?

Newtons (N).

What is mass measured in?

Kilograms (kg).

What is gravitational field strength measured in?

Newtons per kilogram (N/kg).

Where can an object's weight be considered to act?

At its centre of mass.

How is weight measured?

Using a calibrated spring balance (newtonmeter).

What is the relationship between weight and mass?

They are directly proportional.

What is a resultant force?

A single force that has the same effect as all the original forces acting together.

How do you calculate the resultant of two forces acting in a straight line?

Add forces acting in the same direction and subtract forces acting in opposite directions.

What is a free-body diagram?

A diagram showing the forces acting on an isolated object or system.

What does a zero resultant force mean?

The forces are balanced.

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

If stationary, it remains stationary; if moving, it continues at the same velocity.

What is force resolution?

Splitting a single force into two components acting at right angles to each other.

What do the two components of a resolved force have together?

The same effect as the original single force.

What should you be able to use to resolve forces?

Vector diagrams.

What can scale drawings of force vectors be used to determine?

The magnitude and direction of a resultant force.

What is work done?

Energy transferred when a force causes an object to move through a distance in the direction of the force.

What is the equation for work done?

W = Fs.

What is work done measured in?

Joules (J).

What is one joule equal to?

One newton metre (1 J = 1 Nm).

What happens to the temperature of an object when work is done against friction?

Its temperature increases.

What forces can stretch, bend or compress an object?

Forces that change its shape.

Why are more than one force needed to change the shape of a stationary object?

Forces must act to deform the object rather than simply move it.

What is elastic deformation?

Deformation where an object returns to its original shape when the force is removed.

What is inelastic deformation?

Deformation where an object does not return completely to its original shape when the force is removed.

What is the relationship between force and extension before the limit of proportionality?

Extension is directly proportional to force.

What is the equation linking force, spring constant and extension?

F = ke.

What is the unit of spring constant?

N/m.

What is the limit of proportionality?

The point beyond which force and extension are no longer directly proportional.

Does the force-extension relationship also apply to compression?

Yes, where e represents the compression.

What is a linear force-extension relationship?

Force and extension are directly proportional.

What is a non-linear force-extension relationship?

Force and extension are not directly proportional.

How can you calculate the spring constant in a linear case?

Use k = F/e, or find the gradient of a force-extension graph.

What happens when a spring is stretched or compressed?

Work is done and elastic potential energy is stored.

When are work done and elastic potential energy stored equal?

When the spring has not been inelastically deformed.

What is the equation for elastic potential energy?

Eₑ = ½ke².

What can the elastic potential energy equation be used to calculate?

Work done in stretching or compressing a spring up to the limit of proportionality.

What is the aim of the spring practical?

To investigate the relationship between force and extension for a spring.

What equipment could be used in the spring practical?

A spring, clamp stand, masses and a ruler or other suitable length-measuring equipment.

What is the independent variable in the spring practical?

The force applied to the spring.

What is the dependent variable in the spring practical?

The extension of the spring.

What should be controlled in the spring practical?

The spring and the method/position used to measure its extension.

How can you calculate the extension of a spring?

Extension = stretched length − original length.

How should you collect results in the spring practical?

Add increasing forces and measure the corresponding extension each time.

Why should you take several measurements rather than just one?

To identify the relationship and make the results more reliable.

What graph should you plot for the spring practical?

Force against extension.

What does a straight-line section through the origin show on a force-extension graph?

Force and extension are directly proportional.

What does the gradient of a force-extension graph represent?

The spring constant.

What does a curve on a force-extension graph show?

The relationship is no longer directly proportional.

What is the limit of proportionality on a force-extension graph?

The point where the graph stops being a straight line through the origin.

What should you do if a result is anomalous?

Check for errors and repeat the measurement if possible.

What is distance?

How far an object moves.

What type of quantity is distance?

A scalar quantity.

What is displacement?

The distance an object moves in a straight line from its starting point to its finishing point, together with the direction.

What type of quantity is displacement?

A vector quantity.

What is speed?

The distance travelled per unit time.

What type of quantity is speed?

A scalar quantity.

What is the equation for distance travelled at constant speed?

s = vt.

What is average speed?

Total distance travelled ÷ total time taken.

What is velocity?

Speed in a given direction.

What type of quantity is velocity?

A vector quantity.

What is the difference between speed and velocity?

Speed has magnitude only; velocity has magnitude and direction.

What is the Higher-only relationship between circular motion and velocity?

An object moving in a circle can have constant speed but changing velocity because its direction changes.

What are typical speeds for walking, running and cycling?

Walking ≈ 1.5 m/s, running ≈ 3 m/s and cycling ≈ 6 m/s.

What is the typical speed of sound in air?

About 330 m/s.

What does the gradient of a distance-time graph represent?

Speed.

What does a horizontal section on a distance-time graph mean?

The object is stationary.

What does a steeper gradient on a distance-time graph mean?

A greater speed.

What should you be able to do with distance-time graphs?

Draw them from measurements and interpret lines and slopes to determine speed.

How can you find the speed of an accelerating object at a particular time from a distance-time graph?

Draw a tangent at that point and calculate its gradient.

What is acceleration?

The rate of change of velocity.

What is the equation for average acceleration?

a = Δv/t.

What is acceleration measured in?

m/s².

What is deceleration?

An object slowing down.

What does the gradient of a velocity-time graph represent?

Acceleration.

What should you be able to do with velocity-time graphs?

Draw them from measurements and interpret lines and slopes to determine acceleration.

What does the area under a velocity-time graph represent?

Distance travelled or displacement.

How can you measure an area under a velocity-time graph when appropriate?

By counting squares.

What is the equation for uniform acceleration?

v² − u² = 2as.

What is the acceleration of a freely falling object near Earth's surface?

About 9.8 m/s².

What happens to an object falling through a fluid?

It initially accelerates due to gravity.

What happens to the resistive force as a falling object speeds up?

It increases.

What happens when the resistive force becomes equal to the object's weight?

The resultant force becomes zero.

What is terminal velocity?

The constant velocity reached when the resultant force on a falling object is zero.

What is Newton's First Law?

If the resultant force is zero, a stationary object remains stationary and a moving object continues at the same velocity.

When does an object's velocity change?

When a resultant force acts on it.

What happens to the forces on a vehicle travelling at steady speed?

The driving force and resistive forces are balanced.