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Force and motion

Force and motion. Learning outcomes In this section, you will learn: What a force is and what it does. The unit of force. How to name different forces and their effects. What friction is. What lubrication is and how it is used. The force of weight and how it is calculated.

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Force and motion

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  1. Force and motion Learning outcomes In this section, you will learn: What a force is and what it does. The unit of force. How to name different forces and their effects. What friction is. What lubrication is and how it is used. The force of weight and how it is calculated. The relationship between the stretch in a spring and the force causing it.

  2. What is a force? A force gives velocity to an object that is at rest or it changes the velocity of a moving object. This is the same as saying that a force causes a mass to accelerate. Remember: An acceleration can be a change in speed OR a change in direction OR both!

  3. Measuring force The force on an object is measured by multiplying the mass of the object by the acceleration.

  4. The unit of force If you look at the right hand side of the formula, mass has the unit of kg and acceleration has the unit of m/s2, so the unit of force is kg.m/s2. This unit is given the name of Newton, in honour of the famous scientist Isaac Newton. The symbol for the Newton is N.

  5. Different kinds of force and their effects

  6. The force of friction Friction is a force that tries to stop the motion of an object when the object is in contact with another object. Friction between our shoes and the ground stops our feet slipping backwards. Friction between car tyres and the road stops the car skidding. Friction is useful in these cases, but friction between objects can also cause wear and damage.

  7. Lubrication • When friction causes wear and tear a material can be placed between the surfaces of the two objects to reduce friction. The material is called a lubricant. • Examples: putting oil in a car engine or putting grease on the hinge of a door or gate.

  8. The force of weight and how to calculate it The weight of an object on the earth is the force of attraction between the earth and the object. The weight of an object is found using the force formula F = m.a The acceleration that an object gets from the earth is the same for all objects and is approximately 10m/s2.

  9. Calculation of weight To find the weight of an object, use the force formula with the acceleration 10m/s2. Example: What is the weight of a book which has a mass of 1.5kg? Solution:

  10. The relationship between the stretch in a spring and the stretching force When a spring or an elastic is stretched, its length increases. The increase in the length of an elastic or spring is called the extension. The bigger the pulling force, the bigger the extension or, in the language of physics, the extension increases as the force increases.

  11. Summary: Key points A force causes an object that is not moving to move or changes the velocity of an object. The change in velocity can be a change in its speed or its direction or both. The unit of force is the Newton, symbol N. Friction is a force that opposes the motion of objects when they are in contact.

  12. Summary (continued) Lubrication is used to reduce friction between moving objects in contact. Weight is a force caused by the attraction between an object and the earth. Weight is calculated by multiplying mass by the earth’s acceleration (10m/s2). The extension in an elastic or spring increases with the stretching force.

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