Contact and Non-Contact Forces
Brook Edgar & Hannah Shuter
Teachers
Explainer Video
Contact & Non-Contact Forces
Forces can be divided into contact and non-contact forces.
Contact forces act between objects that are physically touching, for example:
Friction - Opposes motion between surfaces
Air resistance/Drag - A frictional force acting against motion through a fluid
Normal reaction force - acts perpendicular to a surface
Tension - exists in ropes, cables or strings
Non-Contact forces act between objects that are are not touching. Examples include:
Gravity - the attraction between masses
Electrostatic forces - exist between charged objects
Magnetic forces - occur between magnets or magnetic materials
Scalars and Vectors
Forces are an example of a vector quantity - this means they have both a magnitude (size) and a direction. Examples of a vector quantity include:
Velocity
Force
Acceleration
The opposite of a vector is a scalar quantity, these only have magnitude. Examples of a scalar quantity include:
Speed
Distance
Mass
Time
Worked example
Which statement is true?
Gravity is a contact force.
Gravity is a force that acts on objects with mass.
Weight is the downwards force of gravity measured in kg.
Answer:
Gravity is a force that acts on objects with mass. B is correct.
This is incorrect as Gravity is a non-contact force.
This is incorrect because weight is measured in Newtons, not kg.
Worked example
Which statement is true?
Friction is a non-contact force.
Friction is a contact force that opposes motion.
Friction is measured in kg.
Answer:
Friction is a contact force that opposes motion. B is correct.
This is incorrect because friction is a contact force.
This is incorrect because friction is measured in Newtons.
Work Done
We can calculate the amount of energy transferred by a force by calculating the work done:
Formula:
Work done is the same as the energy transferred.
Example: A horse pulls a cart with a force of , and I want to find out how much energy they are transferring. I can do this by calculating the work done to pull the cart:
To find the amount of energy transferred to the cart, it would be the same as the work done, .
Note that the distance is in the direction of the force - for example if a cyclist was moving North but the wind was blowing East, the wind would not do any work on the cyclist unless they started moving in the easterly direction.
Worked Example:
Calculate the work done on the box below if it is pushed for .
Answer:
Worked Example:
If I push my car with all the force I have, but it doesn't move, am I doing any work?
If my car moves north and the wind is pushing the car east with a force of , does the wind do any work on the car?
Answer:
No, because the car doesn't move, no energy is being transferred to the car.
, where the is distance parallel to motion. The wind is acting at a right angle to the motion of the car, so the distance in the direction of the wind is . Therefore no work is being done on the car by the wind.
Worked Example:
Calculate the work done on the box below if it is pushed North for .
Answer:
The box is moving North, so we need to use the force that in that direction to calculate the work done. The force pointing Northwards is the force:
Practice Questions
A book rests on a table.
insert image
Identify one contact force and one non-contact force acting on the book.
State the difference between scalar and vector quantities.
-> Check out Brook's video explanation for more help.
Answer:
Contact: normal reaction / friction. Non-contact: weight / gravitational force.
Scalars have magnitude only; vectors have magnitude and direction.
A cyclist experiences several forces while riding up a hill.
insert image
Name two contact forces that act on the cyclist and bicycle.
Name one non-contact force acting on the cyclist.
-> Check out Brook's video explanation for more help.
Answer:
Friction (tyres on road), air resistance, normal reaction from the ground.
Weight / gravitational force.