Key Concepts Rewrite the following sentences in your copybook and fill in the blanks. 1. A force is anything which causes an object to ____________ or change its ____________. 2. Forces are measured in ____________. 3. The force which pulls objects towards the ground is called ____________. 4. The force which prevents easy movement between two objects in contact is ____________. 5. Oil and grease are ________________ because they reduce ____________.
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Knowledge and Understanding 1. Which law of motion explains why a ball eventually stops moving after it is kicked?
2. Which law of motion explains why we need to wear seatbelts? 3. Which law of motion explains how rockets are launched into space? 4. Which law of motion explains why a 50 kg bag on earth weighs more than a 50 kg bag in space? 5. How does mass affect the force needed to move an object? 6. Use forces to explain the following (a) Hands get warmer when rubbed together. (b) A gun recoils when it is fired. (c) Racing cars have wide tyres. (d) Swimmers shave their legs. (e) A car engine requires oil. (f) One person alone can lift an entire car using a jack. (g) Because of its shape a double-decker bus has a much slower maximum speed than a car has. 7. Determine the force when an object of mass 3 kg is given an acceleration of 2 m s-2 8. A force of 15 N is exerted on an object to cause it to accelerate at a rate of 5 m s-2 Determine the mass of the object.
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9. Suppose that a 10 kg skateboard is accelerating at a rate of 2 m s-2 the mass is doubled, then what is the new acceleration of the skateboard?
T I . If the force is tripled and
Thinking and Investigating 1. Explain how a spring balance works. 2. Describe one way to show that every force has an equal but opposite force. 3. Give a reason for the following: (a) A car has broken down. The driver must ask for help to push it. (b) Trolleys are used to move heavy furniture. (c) Buses need more powerful engines than cars. (d) There is a maximum weight or number of people allowed in a lift. (e) Accelerating quickly in a car uses more fuel than accelerating slowly. (f) It is more difficult for a runner to increase his/her speed than to run at a constant speed.
4. Find out about the life and work of Isaac Newton. 321
