Intro to Forces

• Mechanical principles
• Further Education (Key Stage 5)

What is a force?

Forces

• A force is a push or a pull that impacts the motion of an object.
  
• Force is measured in newtons (N).
• To fully define a force, you need to know both its magnitude in Newtons and its direction

A bullet is shot straight up into the air. Why doesn't it continue out to space?

A constant force is needed to maintain constant movement.

True

False

A ball is travelling through the air after being thrown. What forces act on the ball?

1. The force that was used to throw the ball
2. Air resistance
3. The weight of the ball
4. Thrust

A stone slides across a horizontal sheet of ice after being pushed with a stick. What forces are acting on the stone?

1. A frictional force
2. The force from the stick that was used to push it
3. Its weight
4. A vertical force from the ice.

A book is resting on a horizontal table. What forces are acting on the book?

1. Its weight
2. Friction
3. A reaction force from the table
4. Tension

An object is hanging from a string. What forces act on the object?

1. Its weight
2. Air resistance
3. A reaction force
4. Tension from the string

Weight vs Mass

• Mass is measured in kilograms. It is a measure of how much matter there is in an object.
• Weight is measured in Newtons. It is the amount of force an object exerts due to the pull of gravity.
• Mass x Gravity = Weight
• Weight ÷ Gravity = Mass

The weight of an object is the same on the Moon as on Earth.

1. True
2. False

The mass of an object is the same on the Moon as on Earth.

1. True
2. False

Which of the following statements is true?

1. The force on an object from a string attached to the object can be either tension or thrust.
2. The force on an object from a string attached to the object can be tension but not thrust.
3. The force on an object from a string attached to the object can be thrust but not tension.

Which of the following statements is true?

1. The force on a trailer from a towbar can be thrust but not tension.
2. The force on a trailer from a towbar can be tension but not thrust.
3. The force on a trailer from a towbar can be either tension or thrust.

An object is placed on a rough surface which has a slight slope, but not enough for the object to move, so friction does not act.

1. True
2. False

What forces are acting?

What forces are acting?

What forces are acting?

Representation of forces

Forces can be represented by space diagrams or free body diagrams. 

The box to the left is sliding down a slope at a constant speed. Draw a free body diagram showing the box with 5N friction, 6.7N reaction force, and 10N weight.

The boat has weight 5000N, upthrust 5000N, thrust 800N and air resistance 200N. Draw a free body diagram showing the forces and determine if the boat is in motion.