Revision as of 05:22, 16 December 2009

Outcomes

• appreciate that there is a force on a charged particle when it moves in a magnetic field as long as its motion is not parallel to the field
• recall that a force is exerted on a current-carrying wire in a magnetic field, and how this effect is applied in simple d.c.electric motors and loudspeakers

Specification References

• 6.11 appreciate that there is a force on a charged particle when it moves in a magnetic field as long as its motion is not parallel to the field
• 6.12 recall that a force is exerted on a current-carrying wire in a magnetic field, and how this effect is applied in simple d.c.electric motors and loudspeakers

Starter

Main Body of Lesson

• Use the Force vectors powerpoint. Discuss the images and have the class help label the forces using arrows. Guide them towards the correct direction of arrow and type of force as revision from the 3rd year. Begin looking for pairs of forces which are balanced and result in zero force on the object.
• Explain that force vectors are depicted as arrows to show the direction, with the length of the arrow representing the magnitude of the force. You could introduce the 'head' and 'tail' of the force vector.
• Show that the direction of a force is important by carrying out this demo. Have two pupils carry out a 'tug o war' with the rope and two forcemeters. Take your forcemeter and attach it to one end and 'help' one of the pupils by pulling with an extra 10N. Now, 'help' the other pupil in the same way. Ask the pupils what happens when you 'help'. Now hold up the 1kg mass. Ask the class how much it weighs. Recall the formula W=mg from the 3rd year. Hang the mass on the forcemeter to show that it is 10N. Now hang the 1lke mass on the middle of the rope. The students should experience a lot more of a 'tug' than before. Get them to describe how different it feels compared to the original senario. Ask them what they could do to make it easier to hold the mass. They should get the idea that they take a step towards each other.
• It may help to draw a diagram to show the tension forces and how a vertical force needs a very large force if the angle is small. Show what happens to the tension needed to balance the vertical force if the angle is increased.
• Use the last few images (the canal boat and the loop) in the powerpoint to introduce the idea that the resultant of two given forces is an imaginary force and is equivalent to the actual forces. It would have the same effect as the real forces combined.
• Go through the examples on how to add vectors from the power point slide. Emphasis how to add vectors 'head to tail'. Make them redraw the arrows in 'head to tail' formation.
• If there is time and you wish to extend to 2-D vectors a nice activity is to use the trio of ropes tied to a loop. In groups of four, have three pupils pull on the three ropes using forcemeters. They should pull and create a 'balanced loop' which is resting on the A3 paper. The fourth member uses a ruler to trace the three ropes with a pencil (on both sides) and label the force from the forcemeter. The centre of the loop should also be marked. Now have them take the loop and rope away and using the pencil draw the force vectors leading from the centre of the loop. The vectors should be in the correct direction and be drawn to scale. (i.e 2cm = 1N) Now using the acetate sheet they should trace the vectors one at a time 'adding them' head to tail. The three force vectors should crate a triangle showing that they are balanced or the resultant is zero.

Plenary

• Use this Adding Forces sheet. Student should add forces along a line and may go onto an extension activity where they both label and add up forces for a number of situations.

Homework

• Students could research the difference between distance and displacement, speed and velocity and how the change in velocity can occur by the velocity only changing direction and not magnitude. This could lead to acceleration and circular motion.

Additional Information

Resources Required

• Class set of quantity cards in envelopes + whiteboards
• Set of 'ring with three ropes' systems, 3 Newton meters x 3, A3 sheet x 1, acetate sheet x 1 , rulers x 1 and OHP pen x 1.
• Large rope for tug o war with a loop at each end, large newtonmeters x 3, 1 kg weight with hook x 1

Textbook References

• None

Website References

• None

Skills Addressed

Safety/Hazards

• None

Notes

• None