Forces & Elasticity Flashcards

All 28 cards in this deck

Name the three ways a force can change the shape of an object.

Stretching, bending and compressing.

What forces are involved in stretching a spring?

e.g. a spring hanging from a clamp with a weight on the end

Two forces pulling in opposite directions — the weight pulling down and the clamp pulling up.

How many forces must act to change the shape of a stationary object, and why?

More than one — a single force would simply make the object move (accelerate) instead of deforming it.

What is meant by elastic deformation?

The object returns to its original shape/length when the force is removed.

What is meant by inelastic deformation?

The object does not return to its original shape/length when the force is removed.

True or false? A single force applied to a stationary object can stretch it.

False. One force would make it move; more than one force is needed to deform it.

State Hooke's law.

The extension of an elastic object is directly proportional to the force applied, provided the limit of proportionality is not exceeded.

Which equation links force, spring constant and extension?

$F = k e$ (force = spring constant × extension), with $F$ in N, $k$ in N/m and $e$ in m.

What is the unit of the spring constant $k$, and what does a large value of $k$ mean?

Newtons per metre (N/m); a large $k$ means a stiffer spring (more force needed per metre of extension).

What are the steps to calculate a spring constant from a force and an extension?

e.g. a force of $2.88$ N extends a spring by $1.50$ cm

1. Convert extension to metres: $1.50$ cm $= 0.015$ m
2. Rearrange $F = ke$ to $k = F/e$
3. $k = 2.88 / 0.015 = 192$ N/m

Does $F = k e$ apply to a spring being compressed as well as stretched?

Yes — $e$ is then the compression of the object instead of the extension.

True or false? The extension of a spring is the length of the spring while the load is attached.

False. Extension = loaded length − original (natural) length.

What does a force–extension graph look like when force and extension are directly proportional?

A straight line that passes through the origin (constant gradient).

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

The point beyond which extension is no longer directly proportional to force — the line stops being straight and begins to curve.

What is meant by a non-linear relationship between force and extension?

Extension is not directly proportional to force — equal increases in force give different increases in extension, so the graph curves.

On a force–extension graph with force on the vertical axis, what does the gradient of the straight section represent?

The spring constant $k$ (in N/m).

What are the steps to determine a spring constant from a force–extension graph?

e.g. the straight line passes through force $= 5.00$ N at extension $= 0.125$ m

1. Read a correct pair of values from the straight part of the line: $5.00$ N and $0.125$ m
2. Use $k = F/e$ (the gradient): $k = 5.00 / 0.125$
3. $k = 40$ N/m

Which equation gives the elastic potential energy stored in a stretched spring?

$E_e = \tfrac{1}{2} k e^2$, with $E_e$ in J, $k$ in N/m and $e$ in m.

How does the work done on a spring compare with the elastic potential energy stored in it?

They are equal, provided the spring is not inelastically deformed.

What are the steps to calculate the elastic potential energy stored in a stretched spring?

e.g. $k = 13$ N/m and extension $= 20$ cm

1. Convert extension to metres: $e = 0.20$ m
2. Substitute into $E_e = \tfrac{1}{2} k e^2$: $E_e = 0.5 \times 13 \times 0.20^2$
3. $E_e = 0.26$ J

On a force–extension graph, what does the area under the line represent?

The work done on the spring, which equals the elastic potential energy stored.

True or false? Doubling the extension of a spring doubles the elastic potential energy stored.

False. Energy depends on $e^2$, so doubling the extension stores four times the energy.

What are the steps of the required practical investigating the relationship between force and extension for a spring?

e.g. using 1 N weights hung from a clamped spring

1. Clamp the spring and measure its original length with a ruler.
2. Add weights one at a time, recording the force and the new length each time.
3. Work out extension = new length − original length and plot force against extension.

In the required practical on force and extension, what are the independent, dependent and control variables?

Independent: force (weight) applied; dependent: extension of the spring; control: the same spring throughout.

Name one hazard in the force–extension practical and how to reduce the risk.

Falling masses (or the spring snapping) — wear eye protection and place a mat/tray below, keeping feet clear.

What are the steps to find the extension of a spring from measured lengths?

e.g. original length $5.0$ cm, length with load $7.5$ cm

1. Measure the original (unloaded) length: $5.0$ cm
2. Measure the length with the load attached: $7.5$ cm
3. Extension = loaded length − original length $= 2.5$ cm

What is an anomalous result in a force–extension investigation, and what should you do about it?

A result that does not fit the pattern (lies far from the line of best fit); repeat that measurement and exclude it from any mean.

Name one way to improve the accuracy of the length measurements in the force–extension practical.

Read the ruler at eye level, using a pointer or set square, to avoid parallax error.