Cambridge International AS and A level Physics
a Determine the resultant force acting on each object. Which object has balanced forces acting on it? b For each of the other two objects, draw a diagram and add an arrow to represent the resultant force. c Describe how each of the objects will move as a result of the forces acting on it. 2 These free body diagrams represent the forces acting on two cars: 7000 N 850 N
1200 N
7000 N 800 N
7000 N Diagram 1
7000 N Diagram 2
a Which diagram represents the forces acting on a car that is accelerating to the right? b Calculate the resultant force acting on this car. The other diagram represents the forces on a car that is braking. c Name the forces acting on this car. d Calculate the resultant force acting on this car. 16
Exercise 3.3 Force, mass and acceleration Force, mass and acceleration are linked by the equation F = ma. In this equation, F represents the resultant force acting on mass m. This exercise gives you practice using and rearranging this equation. 1 a A truck of mass 40 000 kg accelerates at 1.20 m s −2. Calculate the resultant force acting on the truck. Give your answer in kilonewtons (kN). b Calculate the acceleration of a ball of mass 2.8 kg when a force of 48 N acts on it. c A spacecraft accelerates at 0.40 m s −2 when a force of 200 N acts on it. Calculate the mass of the spacecraft. 2 A parachutist has a mass of 95 kg. She is acted on by an upward force of 1200 N caused by her parachute. (Acceleration due to gravity g = 9.81 m s −2.) a Calculate the parachutist’s weight. b Calculate the resultant force acting on her and give its direction. c Calculate her acceleration and give its direction. (To find the direction, it can help to draw a simple free body diagram.) TIP You may find it simplest to calculate the car’s average speed.
3 A car of mass 680 kg is moving at 12 m s −1. When the driver presses harder on the accelerator pedal, there is a resultant forward force of 510 N on the car. This force acts for 20 s. Calculate: a the car’s speed after this time b the distance travelled by the car in this time.