1. Forces are vector quantities. Forces are measured in Newtons (N). Types of forces forces acting at a distance – gravitational, electrostatic, electromagnetic contact forces – between two solids, friction, viscous forces in fluids tension – a stretched solid Gravitational force (weight) The gravitational field of the Earth acts on every object with a force proportional to the mass of the object (m) and the gravitational field strength (g). This is known as the weight of the object. At the surface of the Earth g has a value of 9.81 N kg −1. Gravitational force or weight (W) = mg 5 ForcesRevision summary © Hodder & Stoughton 2015

2. Newton’s laws of motion Newton’s first law of motion A body remains at rest or in a state of uniform motion unless acted on by a resultant force. The resultant force on a body is the vector sum of all the forces acting on that body. Resultant force on a body =  F 5 ForcesRevision summary © Hodder & Stoughton 2015

3. 5 ForcesRevision summary © Hodder & Stoughton 2015 Newton’s second law of motion Large force small mass gives large acceleration Force = mass × acceleration F = ma One Newton (N) is the force that will give a mass of one kilogram an acceleration of 1 m s −2. Newton actually stated his second law of motion as: The rate of change of momentum of a body is directly proportional to the applied force and in the direction of that force. See chapter 4 of the student book for a full explanation of momentum.

4. 5 ForcesRevision summary © Hodder & Stoughton 2015 Newton’s third law of motion If a force acts on one body an equal and opposite force acts on another body or Action and reaction are equal and opposite. A Newton pair of forces A Newton pair of forces must: Force of caravan on car Force of car on caravan act on two separate bodies be of the same type act along the same line be equal and opposite act in opposite directions.

5. 1.The weight of an object is dependent on the gravitational field acting on the object. Discuss and explain the variation in the mass and weight of an object at different places on the surface of the Earth. 2.‘Driving at high speed on the roads is always dangerous’. Discuss this statement. Your answer should include a consideration of the driver, other road users, road conditions, acceleration and type of road. 3.A heavy load of shopping may be supported in a plastic bag. However if the bag is lifted upwards suddenly the bag may split. Explain why this happens. 4.A skydiver jumps from a plane and opens his parachute after falling for 3 s. This causes him to decelerate. At this moment, is the force on him greater than, equal to or less than his own weight. 5 ForcesRevision summary © Hodder & Stoughton 2015 Review questions

6. 5. The apparatus in the diagram is designed to verify Newton’s second law of motion. Give a careful account of how this experiment should be carried out. 5 ForcesRevision summary © Hodder & Stoughton 2015 Review questions Accelerating washers washers trolley Mask size 1 cm light gates The mass of each trolley is 60 g and that of each washer 10 g.

7. 6. The diagram shows a uniform box at rest on a slope. The weight of the box has been shown. Copy the diagram adding any additional forces acting on the box. Explain your additions. 7.Write a short, but well explained, account of the forces between colliding objects in sport. You could include a cricket ball and bat, a tennis ball and tennis racquet, a high jumper’s foot and the ground, two stock cars and a football and a boot. 8.When training in a plane that is moving in ‘free fall’ an astronaut is said to be weightless. Comment on this statement. Explain your answer. 5 ForcesRevision summary © Hodder & Stoughton 2015 Review questions mg 