P5 In Revision Guide - Forces

Question 1

Is force scalar or vector?

Answer 1

Force is a vector quantity

Question 2

What is a vector quantity?

Answer 2

A quantity which has magnitude and direction.
• lots of physical quantities are vectors:
- force
- velocity
- displacement
- acceleration
- momentum

Question 3

What is a scalar quantity?

Answer 3

• only magnitude, no direction
physical quantities include:
e.g. speed, distance, mass, temperature, time

Question 4

How are vector quantities usually represented?

Answer 4

• by an arrow
• the length of the arrow shows the magnitude, and the direction of the arrow shows the direction of the quantity

Question 5

What is a force?

Answer 5

• a push or pull on an object that is caused by it interacting with something
• either contact or non-contact

Question 6

What is a contact force?

Answer 6

• when the two objects have to be touching for a force to act

• friction, air resistance, tension in ropes, normal contact force

Question 7

What is a non-contact force?

Answer 7

• if the objects do not need to be touching for the force to act

• magnetic force, gravitational force, electrostatic force

Question 8

What is an interaction pair?

Answer 8

• a pair of forces that are equal and opposite and act on two interacting objects (Newton’s Third Law)

Question 9

Parallelogram of forces (method)

Answer 9

1. choose a scale
2. Adjust your compass to the length of a force arrow
3. Place the compass on the end of the other force arrow and sketch a curve
4. Do the same for the other force arrow
5. Draw a resultant force line from the original point to where they cross
6. Measure the length of the resultant force and calculate the magnitude using the scale

Question 10

An object is at equilibrium if the forces on it are balanced

Answer 10

• resultant force of zero = object at equilibrium
• on a scale diagram, this means that the tip of the last force you draw should end where the tail of the first force you drew begins
E.g. for three forces, it forms a triangle

Question 11

You can split a force into components

Answer 11

1) Not all forces act horizontally or vertically - some act at awkward angles
2) to make these easier to deal with, they can be split into two components at right angles to each other (usually horizontal and vertical)
3) acting together, these components have the same effect as the single force
4) you can resolve a force (split it into components) by drawing it on a scale grid. Draw the force to scale, and then add horizontal and vertical components along the grid lines. Then you can just measure them.

Question 12

Forces and elasticity - stretching, compressing or bending transfers energy

Answer 12

1) when you apply a force to an object you may cause it to stretch, compress or bend
2) to do this, you need more than one force acting on the object (otherwise the object would simply move in the direction of the applied force, instead of changing shape)
3) an object has been elastically deformed if it can go back to its original shape and length after the force has been removed
4) objects that can be elastically deformed are called elastic objects (eg a spring)
5) an object has been inelastically deformed if it doesn’t return to its original shape and length after the force has been removed
6) work is done when a force stretches or compresses an object and causes energy to be transferred to the elastic potential energy store of the object. If it is elastically deformed, all this energy is transferred to the objects elastic potential energy store

Question 13

Extension is directly proportional to force

Answer 13

F = ke
Force (N) = spring constant (N/m) x extension (m)
• spring constant depends on the material that you are stretching - a stiffer spring has a greater string constant
• the equation also works for compression (where e is just the difference between the natural and compressed lengths - the compression)

Question 14

Limit of proportionality on a graph

Answer 14

• force = y axis
• extension = x axis
• where the line begins to curve is the limit of proportionality
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Question 15

Investigating link between force and extension practical (method)

Answer 15

1) measure the natural length of the spring (when no load is applied) with a millimetre ruler clamped to the stand. Make sure you take the reading at eye level and add a marker (e.g. a thin strip of tape) to the bottom of the spring to make the reading more accurate
2) Add a mass to the spring and allow it to come to rest. Record the mass and measure the new length of the spring. The extension is the change in length.
3) repeat this process until you have enough measurements (no fewer than 6)
4) Plot a force-extension graph of your results. It will only start to curve if you excess the limit of proportionality, but don’t worry if yours doesn’t (as long as you’ve got the straight line bit)

• when the line of best fit is a straight line it means there is a linear relationship between force and extension (directly proportional). Gradient of the line is equal to k, the spring constant
• when the line begins to bend, the relationship is now non-linear between the force and extension - the spring stretches more for each unit increase in force

Question 16

Work done in stretching or compressing a spring in a linear relationship

Answer 16

E = 1/2 k esquared

Elastic potential energy (J) = 1/2 x spring constant (N/m) x extension (m)