Grade by Grade
D. Structures and Mechanisms · Forces Acting on Structures

External Forces: Static and Dynamic Loads

D2.2 A1.2 A1.4About 45 minutes
Learning goals
  • I can explain what an external force is and give examples.
  • I can tell a static load from a dynamic load.
  • I can describe the effects external forces can have on a structure.
  • I can carry out a fair test to compare a static load and a dynamic load.

Words to know

external force
A force that acts on a structure from the outside, like gravity, wind, snow, moving water or people.
load
The external forces a structure must hold up or resist.
static load
A load that stays the same or changes very slowly, like the weight of the structure itself or snow sitting on a roof.
dynamic load
A load that moves or changes, often quickly, like wind gusts, people jumping, cars driving over a bridge or the ground shaking.
sway
To move slowly back and forth from side to side, like a tall tree in the wind.
fair test
An experiment where you change only one thing and keep everything else the same.

Sit or jump?

Sit gently on your bed. Now imagine jumping onto it from a chair. (Don't!) You are the same weight both times, but the jump makes the mattress squish much more and the bed frame creak.

Why? Today you will find out how the forces outside a structure act on it, and why a moving load can do more than a load that sits still.

Remember from Grade 3

In Grade 3 you sorted loads into dead loads (the weight of the structure itself) and live loads (loads that come and go, like people, cars and snow). In Grade 5 you will learn another way engineers sort loads: static and dynamic.

Transport trucks and cars driving across a concrete highway bridge over a wide river.
Moving trucks and cars are dynamic loads. The bridge's own weight is a static load that is always there. AI-generated image

External forces

An external force acts on a structure from the outside. The external forces a structure must hold up are called its load.

  • Gravity pulls every structure and everything on it down. It never stops.
  • Wind pushes on the sides of structures, and can even lift roofs.
  • Snow and ice press down on roofs, trees and power lines.
  • Moving water and waves push on docks, dams and bridges.
  • People, animals and vehicles push and pull when they stand, walk, drive or lean.
  • In an earthquake, the ground shakes structures back and forth.

External forces cause the internal forces you learned about: compression, tension, torsion and shear.

snowwindgravity
Some external forces on a house: gravity pulls down, snow presses down on the roof, and wind pushes on the side.

Static loads and dynamic loads

A static load stays the same or changes very slowly. It does not move suddenly.

  • the weight of the structure itself (its dead load)
  • a heavy bookcase standing on a floor
  • snow that has piled up on a roof
  • a car parked on a bridge

A dynamic load moves or changes, often quickly.

  • gusts of wind pushing on a tall building
  • people jumping or dancing on a floor
  • cars and trucks driving over a bridge
  • waves crashing on a dock
  • the ground shaking in an earthquake

A dynamic load can push much harder than the same weight sitting still. A box dropped onto a table pushes harder at the moment it lands than the same box set down gently. Dynamic loads can also make structures shake or sway, which can loosen joints over time.

Static loadplaced gently: small sagDynamic loaddropped: much bigger sag
The same beanbag on the same paper bridge. Dropped, it is a dynamic load and pushes much harder for a moment, so the bridge sags more. (The sag is drawn bigger than in real life.)
Static loadDynamic load
a sleeping cat on a coucha dog jumping on the couch
a full bookshelf against a walla truck rumbling over a bridge
snow resting on a roofwind gusting against a roof
water sitting in a full rain barrelwaves crashing on a dock
Static or dynamic?

What can external forces do to a structure?

If a structure is not built to handle its loads, external forces can make it:

  • sag or bend (a shelf loaded with books)
  • tip over (a tall bookcase pushed from the side)
  • slide (a shed pushed by strong wind if it is not fastened down)
  • shake or sway (a tall tower in the wind)
  • crack, break or collapse (a roof under very deep, wet snow)

Engineers add up every load a structure might feel, static and dynamic, and then make it stronger than it needs to be, just to be safe.

Worked example

Let's sort the loads on a hockey arena

Sort these loads on a community hockey arena as static or dynamic: (a) the weight of the steel roof; (b) fans jumping up and cheering in the stands; (c) snow piled on the roof; (d) a windstorm; (e) the bleachers bolted to the floor.

  1. Ask: does the load move or change quickly? If yes, it is dynamic.
  2. (a) The roof's weight never changes: static.
  3. (b) Jumping fans move up and down quickly: dynamic.
  4. (c) Snow piles up slowly and then sits there: static (it changes very slowly).
  5. (d) Wind gusts push and let go again and again: dynamic.
  6. (e) The bleachers' weight stays the same: static.

Static: (a), (c), (e). Dynamic: (b), (d).

Worked example

Let's plan a fair test

Question: does a beanbag push harder on a paper bridge when it is dropped than when it is placed gently?

  1. Change only one thing: how the beanbag arrives (placed or dropped).
  2. Keep the same: the same beanbag, the same paper bridge, the same gap between the book stacks, the same spot in the middle.
  3. Measure: how far the middle of the bridge sags, using a ruler held upright beside it.
  4. Repeat each test 3 times so one odd result doesn't fool you.

A fair test changes only how the load arrives and measures the sag. We predict the dropped beanbag (a dynamic load) will make the bridge sag more.

Just for fun

A trampoline is a dynamic-load machine! Stand still on it (a static load) and it just sags a little. Jump, and it stretches much farther, then springs you back up. Same you, much bigger push!

Hands-on activity

Static and dynamic load test

You need: two equal stacks of books (about 10 cm high), a sheet of thin cardboard (like a cereal box panel) or a file folder, a ruler, a small beanbag or a sock filled with dry rice and tied closed, paper and a pencil.

  1. Lay the cardboard across the gap between the two book stacks to make a bridge. Make the gap about 20 cm.
  2. Stand the ruler upright on the table beside the middle of the bridge. Record the height of the bridge's middle.
  3. Static load: gently place the beanbag in the middle. Record the new height. Work out the sag (how far it went down). Do this 3 times.
  4. Dynamic load: hold the beanbag 10 cm above the middle and let it drop. Watch the ruler closely at the moment it lands (a helper can watch or film in slow motion). Do this 3 times.
  5. Make a table of your results. Which made the bridge sag more at the moment of landing?

What to look for: the dropped beanbag makes the bridge dip lower for a moment than the placed beanbag, even though it weighs the same. The bridge may also bounce. That bounce is a dynamic load making the structure move.

Safety: use something soft like a beanbag, not anything hard or breakable. Keep the drop height low and keep fingers away from under the bridge.

Your turn

1

What is an external force?

2

Name three external forces that can act on a bridge.

3

Is a parked car on a bridge a static or a dynamic load?

4

Is a crowd of people dancing on a floor a static or a dynamic load?

5

Why can a dynamic load push harder than a static load of the same weight?

6

Name two things external forces can do to a structure that is not built to handle them.

More practice: warm-up

1

Which external force pulls every structure down?

2

Static or dynamic: a refrigerator standing in a kitchen?

3

Static or dynamic: waves splashing against a pier?

4

Which external force pushes on the side of a tall building?

5

True or false: the weight of a structure itself is a load.

More practice: on your own

1

Snow on a roof is a static load. But it is also a live load (from Grade 3). How can it be both?

2

Why do you think gym floors are built to hold more than the weight of the people standing on them?

3

A tall bookcase can tip over if someone climbs it. Which external force is the person adding, and why is the bookcase likely to tip?

4

In the beanbag test, why did you do each test 3 times?

5

Give one static load and one dynamic load on a school bus.

Challenge

1

Engineers say some dynamic loads are dangerous because they are repeated over and over, like trucks crossing a bridge every day. Why might a load that comes again and again be a problem, even if each truck is safe on its own?

2

A 50 kg person stands on a bathroom scale and it reads 50 kg. If they jump onto the scale, the reading jumps for a moment to more than 50 kg. Explain what this shows about dynamic loads.

3

A wooden deck is built for a family barbecue. List two static loads and two dynamic loads it must hold, then say which one you think the builder should worry about most and why.

Check your understanding

Quiz

1.

Which is an external force on a structure?

2.

Which is a dynamic load?

3.

Which is a static load?

4.

A box is dropped onto a table instead of being placed on it gently. At the moment it lands, the box pushes on the table...

5.

Which is not a likely effect of an external force on a weak structure?

6.

In a fair test comparing a placed beanbag and a dropped beanbag, what should you change?

For parents and teachers

Big idea (D2.2): External forces (gravity, wind, snow, water, people, vehicles, earthquakes) act on a structure from the outside and cause internal forces inside it. Grade 5 adds the idea of static loads (unchanging or slowly changing) and dynamic loads (moving or quickly changing). Dynamic loads can push harder than the same weight at rest, and can make structures sway or shake. The load test practises A1.2 (fair testing) and A1.4 (safe procedures).

How to help: Name loads as you go about your day: "Is the snow on the garage static or dynamic? What about the wind?" Ask your child to predict before they test.

Common difficulties:

  • Mixing up static/dynamic with dead/live. Dead/live is about whether the load is part of the structure; static/dynamic is about whether the load moves or changes quickly. Snow is live and static.
  • Changing more than one thing in a test. Ask, "What did you keep the same?"
  • Safety: use only soft objects for drop tests and keep drop heights low. Bookcases and dressers should be anchored to the wall, which is a good real-life example to discuss.