Gravitational Force (Weight)
Forces and Motion

Gravitational force and free fall

for 14-16

These experiments run from the 'feel' of a newton of force to the phenomenon of weightlessness.

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Feeling a force of 10 N

Gravitational Force (Weight)
Forces and Motion

Feeling a force of 10 N

Practical Activity for 14-16

Demonstration

This is a fairly elementary demonstration, but it will help students to develop a ‘feel’ for a newton of force, together with the concept of field strength.

Apparatus and Materials

  • Masses, 1 kg , 2 kg , 3 kg , 4 kg
  • Forcemeter, 0-50 N

Health & Safety and Technical Notes

Read our standard health & safety guidance


Procedure

  1. Get a student to hold a kilogram in one hand.
  2. Ask, "Can you feel the force of the Earth pulling down on it?" And "How big a force is it?" If the student cannot answer the latter question, attach the mass to a forcemeter.
  3. Prepare a table with 3 columns: label the first column ‘mass’ and the second column ‘gravitational force on the mass’. Leave the third column unlabelled for the moment.
  4. Complete the table for 2, 3, and 4 kg masses.
  5. Now head the third column ‘force on a unit mass of 1 kilogram’ or ‘gravitational field strength’. Ask, "What is the force on one kilogram of each of the masses?" Complete the third column together.

Teaching Notes

  • Students may have heard the story of Newton discovering gravity when an apple fell on his head as he sat thinking under an apple tree. The weight of an apple is about 1 newton. Newton's remarkable thought about gravity is that the Moon too is falling.
  • Students might find the question in part 5 puzzling because it is so obvious. They need only imagine the mass cut up into 1-kg lumps. The force on each kilogram is 10 N whether its mass comes in 1-kg lumps or 4-kg lumps. This is a very important demonstration because it leads to the concept of gravitational field strength. Its symbol is g and its units are N/kg.
  • Physicists picture the gravitational field of force spreading out from the Earth with a ‘readiness to pull’ another mass, radially, towards the centre of the Earth. There is no actual force at a place near the Earth until you put some stuff there for the field to pull on. The field is always there.
  • The force F that acts on a mass m in a gravitational field is F = mg newtons. This is the weight of the body. When you hold an object and feel the pull of the Earth’s gravitational field on it, that object is not falling. It is not accelerating downwards with an acceleration g and it is therefore nonsense to talk of finding its weight by multiplying its mass by an acceleration, g. Here g represents the Earth’s field strength, 9.8 N/kg.
  • See also the apparatus item:

    Forces and energy demonstration box


This experiment was safety-tested in April 2005.

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Falling objects: a demonstration

Gravitational Force (Weight)
Forces and Motion

Falling objects: a demonstration

Practical Activity for 14-16

Demonstration

It may seem surprising that the motion of all objects falling freely under gravity is the same. Click here for a video showing free fall from which you can make measurements (please note this runs only in Internet Explorer 4+).

Apparatus and Materials

  • Objects, 2, larger and smaller (e.g. pair of smallish stones, or sets of keys
  • Vacuum pump
  • Hoffman clip

Health & Safety and Technical Notes

Read our standard health & safety guidance


The objects should be dense enough and the length of fall small enough to make the effect of air resistance insignificant.

Procedure

  1. Simultaneously release the two objects side by side and watch what happens.
  2. A multiflash photograph could be taken of the falling objects. See the guidance note:

    Multiflash photography


Teaching Notes

  • Your accompanying chat might run like this: "Why doesn’t the heavier object fall faster? I can feel the Earth pulling it with a bigger force."
  • The gravitational force acting on each object is found as F = mg, where g is the gravitational field strength. In other words, the pull of gravity on an object is proportional to its mass.
  • Ask: "What happens to an object when an unbalanced force acts on it?" Newton’s Second Law will apply and the object’s acceleration will be a = F/m. In other words, with a bigger mass, a greater force must be applied to cause the same acceleration.
  • Putting the two equations together, a = F/m = mg/m
  • As a result, the acceleration of free fall a = g, is independent of an object’s mass. All masses fall in the same way. The units of acceleration and gravitational field strength look different but are really the same.
  • This means that you can measure the strength of the gravity field by finding the acceleration of free fall.
  • Many students will puzzle over this result because the same symbol g is used for both acceleration of free fall and gravitational field strength. Some students will understand the argument here better if you use a different symbol for the acceleration of free fall, perhaps ag .
  • With more advanced students, you could point out that Newton’s second law refers to inertial mass m i , and write a = F/m i .
  • Likewise, the force of gravity on an object depends on gravitational mass mg and F = m g g.
  • Einstein showed that inertial mass and gravitational mass are the same, i.e. mi = mg . So a = F/m i = mg g/m i = g . In other words, mass as measured by accelerations is exactly the same as mass measured by gravitational forces.

This experiment was safety-tested in May 2005

Up next

Apparent weightlessness

Gravitational Force (Weight)
Forces and Motion

Apparent weightlessness

Practical Activity for 14-16

Demonstration

This experiment shows that it is not possible to measure the weight of a body in free fall using a forcemeter.

Apparatus and Materials

  • Demonstration forcemeter, reading at least 1 0N
  • Mass, 1 kg
  • Digital camera or videa camera
  • Lamp

Health & Safety and Technical Notes

If it is necessary for the student dropping the forcemeter to stand on something, a 'kick-stool' as used in libraries or store rooms would be safest. Do not allow other students to stand close to the stool.

Read our standard health & safety guidance


It is a good idea to highlight the pointer so that it shows brightly in photographs. Paint it white or cover it in foil.

This activity is suitable for multiflash treatment. Or you could make a video and play it back frame by frame. If a jump from a diving board is possible, then the student could hold a balance.

Procedure

  1. Hang the kilogram mass from the forcemeter, and get a student to drop it onto a cushion or a blanket held by other students.
  2. Take a photograph of the falling apparatus. Also, ask students whether they can see what the balance reads whilst dropping. Repeat this several times, so that students can look again, and so that you have a collection of photographs.
  3. Use the forcemeter readings in the photographs as the starting points for discussion.
  4. A simple demonstration of free fall is to offer a student a string on which a ring has been threaded. The string has loops at each end which can be put over the student's wrists. Without throwing the ring up into the air ask the student to bring his or her hands together so that the ring is trapped. This is clearly impossible. Ask the students for solutions to the problem before revealing that if the student jumps (safely with bent knees) off the bench then it is indeed possible to grasp the ring.

Teaching Notes

  • The forcemeter shows a reading, equal to the weight of the mass, when the mass simply hangs from it. The extended spring exerts an upwards force which balances the downwards weight.
  • The forcemeter reads zero during the fall (ignoring oscillations of the spring due to disturbance on release). Gravity has not been switched off, however, and the mass is still subject to a downwards force. It still has weight, which is the unbalanced force resulting in its acceleration throughout its fall.
  • When you release the system, oscillation of the spring can produce unwanted motion of the pointer. Try to take photographs during the later part of the fall, and take photographs of several falls. This will show that there is little pattern to these motions.

This experiment was safety-tested in March 2005

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Guinea and feather

Gravitational Force (Weight)
Forces and Motion

Guinea and feather

Practical Activity for 14-16

Demonstration

American astronauts on the Moon repeated Galileo's classic experiment.

Apparatus and Materials

  • Vacuum pump
  • Hoffman clip
  • Rubber pressure tubing, approx 30cm length
  • Rubber disc, small
  • Paper, small piece
  • Tubing, Perspex or glass, approximately 5cm in diameter, 60cm long
  • Safety screen

Health & Safety and Technical Notes

Glass tubing should have ends which are carefully annealed to withstand small shocks. Before evacuating the tube, ensure that there are no cracks and the outside should be covered in a spiral of Sellotape to help if the tube should shatter.

Use a safety screen between the demonstration and the class, and eye protection for the demonstrator.

Read our standard health & safety guidance


One end of the tube is sealed with a solid bung. The other has a one-hole bung with a glass tube through it. The glass is connected with pressure tubing to the vacuum pump.

Procedure

  1. Place the rubber disc and piece of paper in the tube, letting them fall onto the solid bung. Seal the tube and close the Hoffman clip on the pressure tubing.
  2. Invert the tube rapidly and watch the motion of both paper and rubber disc.
  3. Repeat he same procedure after evacuating the tube and compare the motions with those when the tube was full of air.

Teaching Notes

  • With air in the tube, the paper flutters down and arrives after the rubber disc. Once the tube has been evacuated, the paper and disc will arrive together.
  • The conclusion: in the absence of air resistance, objects of quite different mass fall in exactly the same way. Acceleration does not depend on the mass of a falling body. For an explanation, see the Teaching notes to the experiment:

    Falling objects


  • It is possible to hear when the tube is evacuated, as the pump becomes more and more quiet in its action.
  • If students ask for evidence that there is a vacuum in the tube, then once the experiment is completed, place the pressure tube under coloured water. As soon as you open the Hoffman clip water will rush into the tube, propelled by atmospheric pressure. The air left in the tube with the water is air that the pump was unable to remove.
  • There is a fable that Galileo gave a wonderful demonstration from the Leaning Tower of Pisa. The story says that he dropped a little iron ball and a big cannon ball side by side. Everyone was astonished, and some even angry, to see that they arrived at the ground together. They expected the cannon ball, which weighed ten times as much, to fall ten times faster. For this is what Aristotle had taught: heavier objects fall faster.
  • Galileo could not produce a vacuum to prove that the removal of air would allow objects of different mass to fall exactly together but he was sure it would be the case.
  • Instead, he invented a thought experiment to prove Aristotle wrong. Galileo said: "Suppose I let three equal bricks fall to the ground, starting together neck and neck. They are all the same. They all fall with the same motion and all arrive together." Followers of Aristotle agreed.
  • "Now suppose I repeat the experiment but first tie together two of the bricks with a light, invisible chain, so light that it isn't really there. Then I suppose I have a brick and a double brick. According to Aristotle, the double brick will fall twice as fast. Is that likely, just because there is a little chain there?"
  • Ah yes, one of Aristotle's followers might say, one of the pair of bricks gets a little ahead and drags the other one down faster than the single brick.
  • "Oh I see," Galileo would reply, "one of the pair gets a little behind and drags the other backwards making it fall slower!"
  • Galileo had shown that Aristotle's thinking produced two different predictions. He made his opponents furious by making their arguments look foolish.

This experiment was safety-tested in April 2005

Up next

Weightless?

Gravitational Force (Weight)
Forces and Motion

Weightless?

Teaching Guidance for 14-16

An object in free fall is said to be ‘weightless’ but is better described as ‘apparently weightless’.

Questions about weightlessness are likely to come up when discussing satellite motion. In free fall, no forces other than gravity act. To someone in a satellite, or (hypothetically) in an ordinary lift after the cables have been cut, objects appear to have no weight. Something placed in mid-air will just float there. Video clips of astronauts show this vividly.

A stationary observer watching from outside the satellite (or lift) will see all objects falling in exactly the same way. They are all in free fall.

An object can only be truly weightless if there is no gravitational field. This would have to be infinitely far away from any other body. Or it could be at a point between two bodies, such the Earth and the Sun, where the pull of the Earth exactly balances the pull of the Sun. Or at the centre of the Earth, where an object would be pulled equally in all directions.

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Multiflash photography

Conservation of Energy
Energy and Thermal Physics | Forces and Motion

Multiflash photography

Teaching Guidance for 14-16

Multiflash photography creates successive images at regular time intervals on a single frame.

Method 1: Using a digital camera in multiflash mode

You can transfer the image produced direct to a computer.

Method 2: Using a video camera

Play back the video frame by frame and place a transparent acetate sheet over the TV screen to record object positions.

Method 3: Using a camera and motor-driven disc stroboscope

You need a camera that will focus on images for objects as near as 1 metre away. The camera will need a B setting, which holds the shutter open, for continuous exposure. Use a large aperture setting, such as f3.5. Digital cameras provide an immediate image for analysis. With some cameras it may be necessary to cover the photocell to keep the shutter open.

Set up the stroboscope in front of the camera so that slits in the disc allow light from the object to reach the camera lens at regular intervals as the disc rotates.

Lens to disc distance could be as little as 1 cm. The slotted disc should be motor-driven, using a synchronous motor, so that the time intervals between exposures are constant.

You can vary the frequency of ‘exposure’ by covering unwanted slits with black tape. Do this symmetrically. For example, a disc with 2 slits open running at 300 rpm gives 10 exposures per second.

The narrower the slit, the sharper but dimmer the image. Strongly illuminating the objects, or using a light source as the moving object, allows a narrower slit to be used.

Illuminate the object as brightly as possible, but the matt black background as little as possible. A slide projector is a good light source for this purpose.

Method 4: Using a xenon stroboscope

This provides sharper pictures than with a disc stroboscope, provided that you have a good blackout. General guidance is as for Method 3. Direct the light from the stroboscope along the pathway of the object.

In multiflash photography, avoid flash frequencies in the range 15-20 Hz, and avoid red flickering light. Some people can feel unwell as a result of the flicker. Rarely, some people have photosensitive epilepsy.

General hints for success

You need to arrange partial blackout. See guidance note

Classroom management in semi-darkness


Use a white or silver object, such as a large, highly polished steel ball or a golf ball, against a dark background. Alternatively, use a moving source of light such as a lamp fixed to a cell, with suitable electrical connections. In this case, place cushioning on the floor to prevent breakage.

Use the viewfinder to check that the object is in focus throughout its motion, and that a sufficient range of its motion is within the camera’s field of view.

Place a measured grid in the background to allow measurement. A black card with strips of white insulating tape at, say, 10 cm spacing provides strong contrast and allows the illuminated moving object to stand out.

As an alternative to the grid, you can use a metre rule. Its scale will not usually be visible on the final image, but you can project a photograph onto a screen. Move the projector until the metre rule in the image is the same size as a metre rule held alongside the screen. You can then make measurements directly from the screen.

Use a tripod and/or a system of clamps and stands to hold the equipment. Make sure that any system is as rigid and stable as possible.

Teamwork matters, especially in Method 3. One person could control the camera, another the stroboscope system as necessary, and a third the object to be photographed.

  • Switch on lamp and darken room.
  • Check camera focus, f 3.5, B setting.
  • Check field of view to ensure that whole experiment will be recorded.
  • Line up stroboscope.
  • Count down 3-2-1-0. Open shutter just before experiment starts and close it as experiment ends.
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