Force
Forces and Motion

Introduction to forces

for 14-16

A variety of forces are investigated to show that forces can change either the shape of an object or its motion. Both balanced and unbalanced forces are considered.

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Stretching and compressing materials

Elastic/Inelastic Behaviour
Forces and Motion

Stretching and compressing materials

Practical Activity for 14-16

Class practical

Working with a range of materials provides opportunity for observation and discussion of the effects of forces.

Apparatus and Materials

For each pair of students

  • String
  • Rubbber bands, or length of elastic cord
  • Steel springs which can be compressed or extended
  • Latex foam block or cylinder

Health & Safety and Technical Notes

Eye protection should be worn.

Read our standard health & safety guidance


The latex foam could be of the type used for lagging piping, taped to hold the seam closed if necessary. Draw a square onto it to show its changes in shape when deforming forces are applied, for example by bending or twisting it.

Procedure

  1. Pull the string from one end only. Then pull the string from both ends at the same time. Compare the very different behaviours.
  2. Repeat this with the rubber band or elastic cord, and with the spring. Again, compare what happens.
  3. Exert pairs of forces on the foam cylinder. Notice the different changes in shape shown by the square.

Teaching Notes

  • It is incorrect to say that force produces motion. Motion is possible without force, as on a slippery surface or in Outer Space where there is no resistance to motion.
  • Force produces change in motion (acceleration), unless it is balanced by other forces. So when the string is pulled from one end only it accelerates. When it is pulled from both ends at once, the forces may balance, so that there is no acceleration. Instead the string is in tension.
  • The rubber band, the elastic cord and the steel spring change their shape when subject to tension. The changes can include extension, compression, twisting and bending. Larger forces are needed to deform materials by larger amounts. If the force is big enough, the material may become permanently deformed, or snap.
  • Additional activities
  • Vivid effects: stretch strips of clear adhesive tape, stick them in criss-cross patterns onto an overhead projector transparency, and view them through crossed polarising filters.
  • Compare elastic behaviour, as shown by the cord and the springs, with plastic behaviour, in which the change in shape remains after the force is removed. A rubber ball could be compared with a ball of Plasticine, for example. A gas-filled balloon can be squashed and released.
  • Connect two similar elastic bands or springs in parallel. Students will find that they need twice the force to extend the pair compared with a single spring, for the same extension.
  • Two similar elastic bands, or springs, connected in series will extend by twice the amount of a single spring for the same force.
  • A comparison between series and parallel elastic bands and series and parallel electric circuits could be made.
  • Parallel Circuit: currents add at junction.
  • Parallel Elastic bands: tensions add at junction.
  • Series Circuit: current the same all the way through.
  • Series Elastic bands: tension the same all the way through.

This experiment was safety-checked in February 2005

Up next

Pushes, pulls and muscles

Force
Forces and Motion

Pushes, pulls and muscles

Practical Activity for 14-16

Class practical

We exert force by contraction, in length, of muscles.

Apparatus and Materials

For each student group

  • Mass, 1 kg
  • Mass, 100 g
  • Clamp and stand
  • Plywood or strong card (about 25 cm long), 2 lengths
  • Split pins (stationery variety), 3
  • Plywood or card to act as hand, small piece
  • String or other cord, approx 20 cm long

Health & Safety and Technical Notes

Read our standard health & safety guidance


Prepare the materials before the lesson. Cut the plywood or heavy card into lengths and drill a hole in each of them. Students will fit a split pin through these holes. Drill an additional hole in each of them to fit the other two split pins. Students will fit the ends of the string to these, to act as the "biceps muscle". Cut a slot into one end of one of the lengths. Students can use the slot for fitting the "hand".

Procedure

  1. Line up the holes that are close to the ends of the two lengths of board. Push a split pin through both holes, to make the elbow pivot.
  2. Push split pins, in the same direction, through the other two holes.
  3. Fix the string to these two pins, so that when the string is tight the two halves of the arm are roughly at 90° to each other.
  4. Push the smaller piece of board into the slot, to make a hand.
  5. Clamp the 'upper arm' so that it is vertical.
  6. Put the mass onto the hand.
  7. Find out what you have to do to the string to lift the mass.

Teaching Notes

  • Before you set up the model, ask each student to hold a kilogram mass with their arm extended horizontally. Tell them to feel their biceps muscle and tendons as the load is raised.
  • Students must effectively shorten the string in the model to raise a load. When we raise a load in a real arm the biceps muscle contracts in length and also bulges. When the load is lowered the triceps muscle contracts.
  • A more elaborate alternative: use a piece (about 20 cm) of bicycle inner tube as the biceps muscle. Tie it tightly with string or wire to close it near each end. A rubber tube connected to a bicycle pump enters through the lower tie, so that this muscle can be inflated - which will make it contract. Pieces of cord, representing tendons, run from the upper and lower ties (the upper arm and forearm). When the muscle is inflated it contracts and pulls the forearm up. If the ties are slightly leaky, the model imitates a characteristic of real muscles - the pumping has to be repeated to maintain tension.

This experiment was safety-tested in September 2004

Up next

Experiments with magnets

Magnet
Electricity and Magnetism | Forces and Motion

Experiments with magnets

Practical Activity for 14-16

Class practical

Magnets provide an introduction to attraction and repulsion, and to action at a distance.

Apparatus and Materials

For each student group

  • Magnets, different types (at least 2 pairs)
  • Iron filings in a pepper pot


  • Nails
  • Other materials for testing magnetic behaviour, including small scraps of paper
  • Compasses
  • Sheets of paper

Health & Safety and Technical Notes

Iron filings must be kept out of eyes (and sinks). It is worth warning the class to keep fingers away from faces when iron filings are around.

Read our standard health & safety guidance


You could use cylindrical, horseshoe, flat, ceramic, or strong (Eclipse major) magnets. A large permanent magnet should be used with teacher supervision.

One of the pairs of magnets should be strong enough so that, when separated by a few centimetres, students can feel attraction and repulsion.

Procedure

  1. Hold pairs of magnets and feel the forces between them, repulsions as well as attractions.
  2. Use the magnets to try to attract nails and other materials. Some of them (such as small scraps of paper) cannot be attracted by a magnet.
  3. Place a magnet underneath a piece of paper and scatter iron filings on top to reveal a magnetic field pattern. The purpose of the sheet of paper is to prevent direct contact between magnets and filings, since they can be hard to separate. Tap the paper gently to ensure the filings do not stick together.
  4. Place compass needles tip-to-tail near to a magnet. Record their orientations, to plot the maget's field as continuous field lines.
  5. Suspend a bar magnet and show it aligns roughly North and South. The pole which points North is the "North-seeking pole" of the magnet.

Teaching Notes

  • The observation that a single magnet can experience and exert both attractive and repulsive forces with other magnets is important. It allows introduction of the idea that magnets have two different ends or faces, called poles.
  • To show a magnetic field pattern to a whole class, you could place a magnet on an overhead projector, with a piece of transparent material on top of it, and sprinkle the iron filings on to this.
  • You can use a compass to develop the idea of magnetic polarity. The compass points towards the Earth's North. The arrowhead end of the needle is a North-seeking pole. All magnets have a North-seeking pole and a South-seeking pole. Poles that are the same always repel each other. Poles that are different always attract each other. Show this with pairs of compass needles.
  • Sometimes students get into a tangle about North-seeking and South-seeking poles when they learn that the Earth is a big magnet, and that the pole that is geographically to the North must be a South-seeking pole. So at this stage it is unhelpful to shorten 'North-seeking pole' and 'South-seeking pole' into plain North and South poles.

This experiment was safety-tested in July 2005

Up next

Introduction to electric forces

Electrostatic Force
Forces and Motion

Introduction to electric forces

Practical Activity for 14-16

Demonstration

Electrostatic forces are an example, like gravitational and magnetic forces, of "action at a distance".

Apparatus and Materials

  • Balloons, 4
  • Nylon thread, e.g. fishing line, 1 reel

Health & Safety and Technical Notes

Read our standard health & safety guidance


Procedure

  1. Hang up two inflated balloons by long nylon threads. The balloons must be far from any metal supports.
  2. Charge them with like charges by rubbing them against clothing. Then let them hang freely to show repulsion.
  3. Take two more balloons and rub them together. This produces unlike charges on the two balloons. They show attraction.

Teaching Notes

  • Two similarly charged balloons repel each other. Two differently charged balloons attract each other. This observation is sufficient for us to conclude that there are two types of electric charge. You could invite students to suggest titles for the two types. Then point out that the established titles are positive and negative. These are good names because they suggest oppositeness.
  • Another demonstration
  • Charge a balloon by rubbing it on clothing and 'stick' it to a glass fronted cupboard. Repeat with a second balloon. If the balloons are close enough they will repel each other but remain attracted to the glass.
  • Alternative method of charging balloons
  • The balloons can be made to conduct by various means. Paint them with graphite. Spray them with some aluminium paint. Dip them into a strong detergent solution, which is allowed to dry.
  • Hang the two balloons up by insulating threads. Charge one of them. Bring the other balloon, uncharged, near to the charged balloon. Interpose a very thin sheet of plastic (such as polythene) between them. Touch the uncharged balloon. Separate the balloons and they will be oppositely charged.

This experiment was safety-checked in February 2005

Up next

The Earth's gravitational pull

Gravitational Force (Weight)
Forces and Motion

The Earth's gravitational pull

Practical Activity for 14-16

Class practical

Gravitational force can act at a distance; it shows little variation over short distances, but does vary over larger distances.

Apparatus and Materials

For each student group

  • Load with mass up to 1 kg , e.g. bag of sand, brick, heavy book, or 1 kg mass
  • Forcemeter
  • Loop and hook, to hang load from forcemeter
  • Small object for studens to drop, e.g. bag of sand

Health & Safety and Technical Notes

Use a box or tray lined with bubble wrap (or similar) under heavy objects being lifted. This will prevent toes or fingers from being in the danger zone.

Read our standard health & safety guidance


Procedure

  1. Stand with your feet apart and hold the load. Feel the force pulling it down. The force is its weight, the force of gravity.
  2. Imagine that the force is the pull of a long spring stretching down into the ground.
  3. Lift the load higher. Imagine the spring stretching. Decide whether its force gets bigger.
  4. Use a forcemeter to check your idea about whether the force got bigger. Find the weight, in newtons, of the load when it is close to the ground, and when it is high up.
  5. Decide what force is balancing the weight when you hang it from the forcemeter, so that it does not accelerate.
  6. Drop an object so that it accelerates to the ground. This is to see what happens when the downwards force acts alone, and is not balanced by the force provided by the stretched spring in the forcemeter.

Teaching Notes

  • Every object attracts every other object with a gravitational force, but the attractions are small unless one of the objects is big. You could compare this with electric (or electrostatic) forces where not only can the force be either attractive or repulsive, but forces between small objects can be large.
  • Some students may apply their knowledge of springs, so that they expect the imaginary spring to exert a larger force as it stretches. If so, you need to explain that this spring goes all the way to the centre of the Earth. It is so long that small stretches would not make a noticeable difference to the force it exerts.
  • Explain that with extremely precise measuring equipment the weight of the load might, in fact, be seen to vary. It would get smaller as it moves away from the centre of the Earth. In the laboratory the change in its position is much too small to be detected by a forcemeter. For spacecraft far from the Earth's centre the effect is significant.
  • In everyday affairs we measure weight in kilograms. While everyday affairs are confined to the Earth, physics is not. We have to recognize that mass and weight are different things. The quantity of material of an object, its mass, stays the same wherever it goes. Its weight depends on the local pull of gravity, and it changes. We measure mass in kilograms and weight in the units of force, newtons.
  • By allowing a load to fall, this activity also demonstrates that unbalanced force produces a change in velocity (acceleration), always and inevitably.

This experiment was safety-tested in October 2004

Up next

An example of balanced and unbalanced forces

Force
Forces and Motion

An example of balanced and unbalanced forces

Practical Activity for 14-16

Demonstration

Force applied to a load can produce "balanced" and "unbalanced" force scenarios.

Apparatus and Materials

  • Retort stands and bosses, 2
  • G-clamps, 2
  • Pulley, single, on clamp
  • Sand (1 small bag) or block of wood
  • Forcemeter, reading up to 10 N
  • String or other cord (not elastic)
  • Beam, horizontal, 1 m

Health & Safety and Technical Notes

If tall stands are used, G-clamps will be essential to prevent toppling by enthusiastic student demonstrators.

Read our standard health & safety guidance


A pulley fixed to a beam on the ceiling of the laboratory is ideal if such a beam is available. A pulley fixed to a clamp system that cannot topple is adequate. You can prevent toppling by using G-clamps to secure the clamp bases to the benchtop.

Procedure

  1. Raise a small bag of sand or block of wood steadily and quite slowly, using the length of string that goes up and over the pulley. Let students pull it in the same way, and feel for themselves whether the force is the same however far the cord is pulled.
  2. Pull the cord with the forcemeter. Show that the force is constant.
  3. Allow the force to vary. This produces unbalanced force, so that the load speeds up (it accelerates) or slows down (it decelerates, or has negative acceleration).

Teaching Notes

  • You applied a constant force to the moving load through the string. This can balance its weight and the forces of friction that act at the pulley. When the forces are balanced there is no acceleration (though it does have velocity).
  • You could hold the load in a fixed position, and compare the force with that required for steady motion. You should find that the force is smaller. This is only because no effort is now needed to overcome frictional forces. The force, as before, is constant. Again the combined forces acting on the load are balanced. The load has no acceleration (or velocity). The common feature of the balanced forces is the absence of any change in velocity (acceleration).

This experiment was safety-tested in October 2004

Up next

Constant and varying forces between trucks

Force
Forces and Motion

Constant and varying forces between trucks

Practical Activity for 14-16

Demonstration

This is an interesting way to show some balanced and unbalanced forces and the effects on motion.

Apparatus and Materials

  • Model railway track (1.5 m)
  • Flat trucks, 4
  • Magnets, 2, horseshoe or flat-faced ceramic (suitably mounted)
  • blocks, wooden, with spring buffers attached, 2
  • Rubber cord (or extending spring)

Health & Safety and Technical Notes

Consider fitting buffer stops to the ends of the track if there is a risk that loaded trucks may fall onto observers.

Read our standard health & safety guidance


In advance of the lesson, mount the magnets on the trucks, such as to repel each other. It may be necessary to counterbalance the magnets with masses taped to the other ends of the trucks.

Attach buffer springs to the wooden block, for example with sticky tape. They should be short compressible springs. Each of them should have a disc attached, so that they can make reasonably "clean" contact with each other. The blocks can then be attached to the trucks using sticky tape or rubber bands.

Alternatively, the demonstration can be done using dynamics trolleys, but the low mass and low friction of the trucks produces more effective results.

Procedure

  1. Push one of the trucks with a mounted magnet towards the other one, which should be stationary to start with. Release it. A collision can be made to take place without contact.
  2. Repeat this with both trucks being given an initial push.
  3. Now repeat the actions, using the pair of trucks with the buffer springs.
  4. Compare these varying forces acting on the trucks with a situation in which a force is constant. Such a constant force can be applied to a truck by pulling it with a cord (or spring) which is carefully held with a constant extension.

Teaching Notes

  • A truck moving along a level track experiences only a small force of friction. This is quite similar to balanced forces, and acceleration (or deceleration) is small. If the wheels and track were perfectly smooth, the truck's motion would not change at all.
  • As the magnets approach each other (or the springs contact each other) the varying forces produce varying changes in motion. There is both acceleration and deceleration (or negative acceleration).
  • The trucks decelerate as they approach and accelerate again when they move apart. The velocity is zero at maximum compression of the spring. This can be used as an analogy for the magnetic case.
  • An entertaining variant involves hiding one of the trucks, initially stationary, behind a large screen. This initially stationary truck can be first of all free to move and then fixed. This produces different effects on the pushed truck.
  • For older students or able younger students it is worth inviting comparison of the observations with magnets with what we imagine must take place when a pair of repelling particles, such as electrons, collide. As with the trucks and their magnets and springs, the sizes of the forces depend on their distance apart. More sophisticated again are interactions between atoms, where there is longer range attraction and shorter range, strong repulsion.
  • If the trucks have the same mass it is useful to point out what happens when the moving one hits the stationary one. (The moving one stops and the stationary one moves off at the same velocity).

This experiment was safety-tested in October 2004

Up next

Solving problems – force or energy?

Force
Forces and Motion

Solving problems – force or energy?

Teaching Guidance for 14-16

Physicists aim to understand, and sometimes to predict, physical interactions. They use two abstract concepts a lot: force and energy.

Force

A force is something that can change an object's shape or how it is moving (or not moving). A force can have any size and acts in a particular direction. Forces are something to think about when analyzing things such as:

  • pressure
  • turning effect
  • momentum
  • acceleration

The concept of force is used to explain what causes things to happen. You can analyze the forces acting on macroscopic objects or systems, but also on microscopic objects such as the particles in a gas. When forces acting on an object are in equilibrium (balanced), the velocity of the object does not change. If the object is at rest, it stays there. If the object is moving it doesn’t speed up, slow down or change direction.

Energy

Energy is a numerical value that we calculate for an object, or a system, that quantifies the amount of change that has taken, or will take, place. What is important about this quantity is that, in every event or process, there is the same amount of it at the end as there was at the beginning. Energy does not explain why things happen, though we can use it to explain why some things do NOT happen.

The concept of energy has much wider use than the concept of force. The concept of energy can provide insights into movement and materials. It can also be used to analyze electrical and magnetic behaviour, wave behaviour, changes inside the atom, engines that burn fuels … and anything else.

Solving problems

In some situations, you can think in terms of either energy or force. For example, when trying to improve vehicle safety in the event of a crash, you could calculate the energy absorbed in the collision, or calculate the forces acting on the vehicle’s occupants.

Physicists are resourceful and will draw on whatever thinking tools help them understand a particular situation. They prefer to understand things quantitatively.

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