Friction

The idea

Friction is the force that comes into play when an object moves or tries to move over another surface; it always acts opposite to the direction of motion or attempted motion and is a contact force.

A ball that stops

A ball rolling on flat ground stops after some time. A bicycle stops if you stop pedalling.

What stops them?

• A force that acts against the motion
• Nothing, they just get tired
• The ball's colour

Push the block

Pick a surface and push the block. Try at least two surfaces.

A force that resists

In each case the block stopped. A force acted between the block and the surface, opposite to the motion. That force is friction.

The force that comes into play when an object moves or tries to move over another surface is called friction. It always acts opposite to the direction in which the object is moving or trying to move. It is a contact force.

Notes

Friction comes into play when an object moves or tries to move over another surface. It acts opposite to the motion. It is a contact force.

Check yourself

A force acts between a sliding box and the floor, against its motion. It is called …

Friction acts …

A box is pushed to the right along the floor. Friction acts …

Friction is a …

• magnetic force. Magnetic force comes from magnets. The force against sliding is friction.
• friction — correct. Yes!
• muscular force. Muscular force comes from muscles. The force against sliding is friction.
• in the same direction as the motion. Friction acts opposite to the motion.
• always upwards. Friction acts opposite to the direction the object moves or tries to move.
• opposite to the direction of motion — correct. Yes!
• towards the left — correct. Yes!
• towards the right. Friction acts opposite to the direction of motion.
• downwards only. On a sliding object, friction acts opposite to its motion.
• non-contact force. Friction arises when two surfaces are in contact.
• contact force — correct. Yes!
• force that needs no surface. It needs two surfaces in contact.