Contact and Non-contact Force

Contact and
Non-contact Forces

A contact force is defined as the force between two objects that are in physical contact. For example, kicking a soccer ball is a contact force since physical contact is between the foot and the ball.

Contact Force

Multiple Choice

Closing a door by pushing it is an example of a contact force.

• True

• False

Friction is the resistance of motion when one object rubs against another. Anytime two objects rub against each other, they cause friction. Friction works against the motion and acts in the opposite direction.

Friction

Multiple Choice

The direction of the force of friction is the same as the direction of motion.

• True

• False

Multiple Choice

Friction is a kind of contact force.

• True

• False

Types of friction

• Dry Friction - This is what we've been mostly talking about here. Dry friction occurs when two solid objects touch each other. If they are not moving, it is called static friction. If they are moving, it is called kinetic or sliding friction.

• Fluid Friction - Fluid friction involves a fluid or air. The air resistance on an airplane or water resistance on a boat is fluid friction.

• Rolling Friction - Rolling friction occurs when a round surface rolls over a surface, like a ball or wheel.

Multiple Choice

Which of the following shows dry friction?

• dragging a sack of rice on the floor

• swimming in the pool

• an airplane flying overhead

• rolling wheels of the bus on the road

Multiple Select

Which of the following show/s fluid friction?

• dragging a sack of rice on the floor

• swimming in the pool

• an airplane flying overhead

• rolling wheels of the bus on the road

Multiple Choice

Which of the following shows rolling friction?

• dragging a sack of rice on the floor

• swimming in the pool

• an airplane flying overhead

• rolling wheels of the bus on the road

Multiple Choice

• The friction between two non-moving objects is called static friction.

• True

• False

Fun facts about Friction

• Although wheels are great for rolling and reducing friction, they couldn't work without friction.

• It would be really tough just to stand up without friction.

• Friction can generate static electricity.

• The harder two surfaces are pressed together, the more force it takes to overcome the friction and get them to slide.

• Fluid friction is used a lot in water parks so we can slide smoothly and fast down giant slides.

Multiple Choice

• The harder two surfaces are pressed together, the more force it takes to overcome the friction and get them to slide.

• True

• False

A non-contact force is defined as the force between two objects that are not in physical contact. For example, the attraction between the Earth and the Moon is a non-contact force since the two never come in contact. A non-contact force is also called a field force.

Non-contact Force

Magnetic forces are non contact forces; they pull or push on objects without touching them.

Magnetic Force

Multiple Choice

The force of a magnet is an example of a non-contact force.

• True

• False

The biggest non-contact force is gravity. Even if you jump, gravity pulls us back down. Gravity from the Earth even keeps our moon in place, all without touching.

Gravity

Multiple Choice

Gravity is the biggest non-contact force

• True

• False

Electrostatic forces are non-contact forces; they pull or push on objects without touching them. Rubbing some materials together can result in something called 'charge' being moved from one surface to the other. Charged objects pull on other uncharged objects and may either push or pull on other charged objects.

Electrostatic Force

Rubbing the items quickly, like when you rub a balloon fast over something or your feet on the carpet, will build up a large charge. Items with different charges (positive and negative) will attract, while items with similar charges (positive and positive) will push away from each other. Sort of like a magnet.

Multiple Choice

Objects with similar charge (positive and positive) will attract each other

• True

• False

​One example of this is when you slide down a slide and all of your hair stands up straight. This is because the friction of sliding has caused a positive charge to be built up on each hair. Since each hair has the same charge, they all try to push away from each other and end up standing up straight.

Multiple Choice

Your hair may stand when going down the slide because each hair strand has a positive charge and pushes each other away

• True

• False

Likewise, when your skin is charged with static electricity and you touch something metal, like a door handle, the metal is very conductive and will quickly discharge the static electricity, creating a zap or small spark.