CHAPTER 12. FORCE,MOMENTUM AND IMPULSE 12.6
tool box
The driver brakes suddenly, bringing the car rapidly to a halt. There is negligible
friction between the carseat and the box.
(a) Draw a labelled free-body diagram of the forces acting on the child during the
time that the car is being braked.
(b) Draw a labelled free-body diagram of the forces acting on the box during the
time that the car is being braked.
(c) What is the rate ofchange of the child’s momentum as the car isbraked to a
standstill from a speed of 72 km.h−^1 in 4 s.
Modern cars are designed with safety features(besides seat belts) to protect
drivers and passengersduring collisions e.g. the crumple zones on the car’s
body. Rather than remaining rigid during a collision, the crumple zones allow
the car’s body to collapse steadily.
(d) State Newton’s second law of motion.
(e) Explain how the crumple zone on a car reduces the force of impact on it during
a collision.
- [SC 2003/11 HG1]The total mass of a lifttogether with its load is1 200 kg. It is
moving downwards at aconstant velocity of 9 m·s−^1.
9 m·s−^1
1 200 kg
(a) What will be the magnitude of the force exerted by the cable on thelift while it
is moving downwards atconstant velocity? Givean explanation for youranswer.
The lift is now uniformlybrought to rest over a distance of 18 m.
(b) Calculate the magnitude of the accelerationof the lift.
(c) Calculate the magnitude of the force exertedby the cable while the lift is being
brought to rest.
- A driving force of 800 N acts on a car of mass 600 kg.
(a) Calculate the car’s acceleration.
(b) Calculate the car’s speed after 20 s.
(c) Calculate the new acceleration if a frictionalforce of 50 N starts to act on the car
after 20 s.
(d) Calculate the speedof the car after another 20 s (i.e. a total of 40 s after the start).
- [IEB 2002/11 HG1- A Crate on an InclinedPlane]
Elephants are being moved from the Kruger National Park to the EasternCape. They
are loaded into crates that are pulled up a ramp (an inclined plane) onfrictionless
rollers.
